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MG MGB Technical - 1973 MGB Over Heating Question

My car is a 1973 MGB. The engine was completely rebuild approximately 2000 miles ago. While the engine was being rebuilt, the radiator was cleaned and checked; and a new water pump was also installed.
From the time the engine was rebuilt until now, once the car was warmed up, the temperature gage has always registered in the dead center, right on the "N" for normal. During the winter I use a 180F thermostat and during the summer, I use a 165F thermostat. Today, in Atlanta, it was approximately 85F, and I have a 165F installed.
I was driving the car today and it was running fine, then after about 20 miles on the expressway at 65 mph, the temperature gage shot up to just below the "H". It was never at the "H" but was very close. It made no difference if I was moving at 65 mph, or sitting still.
When I got home, I changed the thermostat to another new one I had in my parts storage, also 165F. Now the temperature gage needle is registering about half way between the "N" and the "H".
WHY? Why would it have changed like this? What would cause this? Do the temperature sending units on the block go bad? Would a faulty sending unit cause this?
When I checked the thermostat that I removed, with hot water and a temperature gage, it open perfectly at the correct temperature.
Suggestion? I do not know what else to check.
By the way, the engine timing has not been changed. I am also running an anti-freeze water mix, with the correct mixture ratio, and the radiator was full.
Robert Browning

I was reading some of the MG Archives for Overheating and I just want to add a few things to my thread.
Yes, my fan is on correctly - sucking and not blowing. The fan belt is good and tight. The wire to the sending unit is secure and not melted or grounded anywhere.
One thread said something about the alternator/voltage regulator could cause this problem - or cause the gage to give a false reading. I do have an intermittent problem in that my alternator light will come on sometimes, very faintly. It may stay on for a while, then go out. The alternator is new (standard Lucas MG alternator), installed when the engine was rebuilt, and all the connections are clean and secure. I have had this alternator issue for some time now, and it has never effected the temperature gage before.
Also, when the temperature gage first started increasing today, the alternator light was not and had not been on.
Also, turning on the heater did not have any effect.
Any suggestion on the alternator light would also be appreciated?
I am baffled?
Robert Browning

Robert - First of all, as long as the cooling system was not boiling (steam coming out while the car is moving) and the engine is running smoothly, you are NOT overheating. The N on the gauge is not necessarily where the gauge should register, it mearly indicates a range that is "normal" and it extends from very close to Cold to very close to Hot. If I am not mistaken, the temperature gauge and the fuel gauge get their power from the voltage stabilizer, which could be sticking and causing the temperature gauge to read high (check the voltage at the output of the stabalizer, it should read an average of 10 volts). Did you happen to notice if the fuel gauge was acting flakey at the same time that the temperature gauge was reading high? A candy thermometer in the filler neck of the radiator will give you some indication if the gauge is lying to you.

Next thing to consider is that the temperature marked on a thermostat is the minimum temperature it will maintain the system at. The general health of the cooling system and the engine, along with a whole bunch of other things will determine how hot the engine will run. I run a 195° thermostat in our MGB year around (the car doesn't even run well without the choke at 165°) and have never had the car overheat on me regardles of the climate I am driving in. On our recent trip to California, we hit some really hot weather in Northern California/Southern Oregon (temp into the 90s) and even climing over the passes, the temperature never went above 210°.

I don't know what temperature the Hot range equates to on the newer MGBs, but with a 7 psi cap on the radiator and plain water in the system the boiling temperature (and therefore the point of overheating) would be 233° and with a 50% antifreeze solution that is kicked up to 248°. On the early MGBs with the temperature marked on the gauge, the highest it goes is 230° (that is just before the needle goes into the oil pressure), so with antifreeze it still has a ways to go before it would boil.

You may want to established what your coolant temperature actually is and what the temperature gauge is actually reading at, say the bottom and top of the Normal range (put the sensor in a pan of water with a thermometer and start heating it up while monitoring the thermometer and the temperature gauge). Once you have extablished these points, the needle just below the H on the temperature gauge should not be as terrifying as it first seems. I remember the time I was driving our new (to us) Magnette home from Portland and looked down to see the temperature gauge pegged ont he hot side. There was a panic exit from the freeway to see what I had done to our "new" car, only to find that it was running just fine and the under hood temperature was fine. It was only after I got the car home and did a bit of research that I found when the sensor was disconnected from the gauge (or when the ignition was off) the gauge went to the Hot side. Turns out that MG played some silly games with the temperature gauge/sensor on that particular model and the gauge works just the opposit from the way one would expect. Good luck - Dave
David DuBois

I agree with David in that if it is not losing coolant or steam it is not overheating, although I would say that anywhere between N and H can be 'normal' depending on ambient temperatures and load i.e. slogging up Stelvio in high summer, it would have to be exceptionally cold for it to register much below N in my experience.

It is the instrument voltage regulator that could be a cause of reading high, not the alternator regulator, although that would similarly affect the fuel gauge, again as David says. Although if the battery became disconnected the alternator voltage could rise above what the instrument voltage stabiliser was designed to cope with, and cause high temp and fuel gauge readings. I have seen reports that this happening at night caused every illuminated bulb on the car to blow.

I would suspect the temperature sensor, or a partial short on the wiring to it, a very faint possibility is the gauge. I'm 99.9% certain that the MGB didn't have a temperature gauge like David's Magnette, although very early Bs did have the fuel gauge like that, but on those cars the temp gauge was capilliary.

You say 'shot up'. Did you see it? Would you say it moved as fast as the fuel gauge when turning on the ignition with a full tank? Faster? Or slower? If about the same then I'd suspect the previously mentioned wiring or sender. If slower then possibly actual engine temperature, although in my experience turning the heater from Off to Hot does momentarily reduce coolant temperature as that slug of 'cold' coolant is released into the system.

I'm not sure why two 165 stats should give different readings unless one of them is off, which could be checked in a pan of hot water. But the critical thing is just what *is* the coolant temperature in the header tank.
Paul Hunt

I will now try to add to my comments and try to clarify a few things and ask some additional questions.
To begin, I agree that the car was not “over heating”. It was not boiling over. It is just that the car started running hotter then it ever had before. The temperature needle was just below the hot on the temperature gage, when it has always been directly in the middle at the “N”. It has never been this hot before.
As for the voltage stabilizer, if I am not mistaken, this is the small unit mounted to the firewall, under the dash, above the drivers feet. If this correct, then this is new - less then a year old. As for a connection between the temperature gage and the fuel gage, the fuel gage was/is acting totally normal, and nothing “flakey” as David asked.
David suggested to put the “sensor in a pan of water” and monitor it while heating the water. Does this mean to take the temperature sensor out of the engine block and put it into a pan of water and heat it? This would be easy to do if this is what he meant. What would be the highest operating temperature the engine can run? At what temperature should I be concerned?
As for Paul’s question, when I said the temperature gage “shot up”, the gage actually increased from the “N” to just below the “H” in a period of about one to two minutes. To me, this was quick, or shooting up.
Also, as Paul suggested, I did check the thermostat that I took out of the car yesterday, in a pan of hot water with a temperature gage, and it opened at approximately 165F.
So my question now is, should I replace the voltage stabilizer? Should I replace the sending unit? Or replace both? What is the maximum operating temperature?
I will try to run the tests David and Paul suggested tomorrow.
Thank you.
Robert Browning

The sending unit could be bad, and is the
cheapest to replace -- about 9 bucks.
Since the fuel gauge
reading didn't change, the problem isn't in the
voltage stabilizer.

Other possible problems are

1. Loose impeller in the water pump. This is
an outside possibility, but is mentioned in the
archives.

2. Worst case is a head gasket leak or crack
in the cylinder head/block. If the sending
unit fix doesn't solve the problem, and if the
water pump is ok, you might want to get a
"Block Tester" from a NAPA or equivalent store.
This is a tube filled with blue fluid that goes
into the radiator neck. If the fluid turns yellow,
there is a head gasket leak, or worse crack in
the head/block. I think this is unlikely -- your
problem really sounds like a bad sender or bad
gauge.

A neat trick is to get or borrow an infrared
thermometer. You just point this at
the radiator and cylinder head and see what
the actual running temperature is.

Good luck and please
let us know if you find out more.
Ronald

Robert - The sensor would have to be removed from the head and put in a pan of water on some kind of stove (I use a camp stove) along side the car. You would need to run a jumper to extend the wire to the sensor in the pan and another ground wire to the body of the sensor (I use this ground wire to suspend the sensor from a stiff wire across the top of the pan. Put a thermometer in the pan with the sensor ans start heating everything up while monitoring the temperature on the gauge in the car. as soon as the gauge registers into the lower side of the normal range, note the temperature of the water in the pan. Let everything continue heating and note the temperature when the gauge in the car reads right at the top end of the normal range and again when the gauge goes into the hot range. I don't know what the temperature will read when the needle reaches the Hot range and it is possible that you won't be able to drive it up to that point if it is above 212°. The simplest way to test the sensor is Ronald's method - just replace the bloody thing, but the above test will let you know what the actual temperature is that the gauge is reading (if you care).

I would agree with Ronald that the stabalizer is probably good since you didn't see any bad readings on the fuel gauge. I would not rule it out because it is new, as there seems to be a number of complaints about new stabailzers going bad in a very short time, but the fact that the fule gauge is behaving itself would indicate that the stabalizer is good since both fuel and temperature gauges get their power through it.

I don't know what would be considered a safe upper limit on temperature for the B series engines used in out cars. The gauges in the early MGBs have 230° as their upper reading and I don't think that I would worry until I saw the needle on my gauge started to go above that (into the oil pressure portion of the gauge) or until the engine started to miss, whichever came first. As long as the cooling system doesn't loose the coolant, the engine souldn't be damaged, but on the other hand a rapid rise in the temperature above 230° could certainly result in loose of coolant if it continued upward unchecked. While I am somewhat unconcerned about highe readings of the temperature gauge, I do keep a watchfull eye on it in situations where the temperature will normally rise, such as long periods of idleing in hot weather, long hard pulls up steep grades and such, but I have seen our MGB go as high as almost 220°, but always slowley and it always comes back to 195° - 200° when things get back to steady driving.

To see the rapid excursion that you did while running at a steady 65 MPH on a level road, I would suspect that instead of an actual rise in temperature, you had some kind of electrical failure, such as the sensor or gauge giving up or even a partial short in the wire to the gauge, or a failure of the water pump impeller as Ronald suggests. Good luck - Dave
David DuBois

A rise over one to two minutes does sound more like engine than electrical, but it doesn't rule out electrical. Before removing the sender I'd take resistance measurements at cold, N, and wherever it gets up to and ask others for what their readings are. Can't help you with those myself as the UK kept mechanical gauges until 76 or so. Also the aforementioned header tank temp measurement.

As for 'only a year old' I must remind you of Hunts Fifth Law: "Many break-downs occur soon after a car has been worked on; 'new' parts can be faulty when you receive them; 'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals." :o)

Paul Hunt

Ok, I took everyone's suggestion and just replaced the sending unit. It has made no difference. I have also reduced the anti-freeze/water mixture, to a higher percentage of water. I have also added a bottle of Water Wetter, made by Red Line, which many people suggested. No difference!
Now, I know that my car is not technically over heating, and many people have told me that as long as the temperature needle on the gage is still in the operating range, everything is fine. However, what I want to know is, why is my gage reading 25% higher then it did last week? It makes no sense. I also took my fan belt off and checked the water pump and it seems fine. It turns smooth and I can feel the water resistance. In addition, if I leave the radiator cap off and look into the radiator, you can see the fluid movement through the radiator.
I am having some problems with my alternator at the moment, so I have not driven the car long enough to get everything hot enough to take any temperature readings. Through my work, I own three, top end, infrared temperature guns to measure the engine. Hopefully, my electrical problems will be fixed tomorrow and I can take temp reading. I will let you know. Any additional suggestions and comments? Please!
Thank you.
Robert Browning

The point of all the talk about the voltage stabilizer is that the temp and fuel gauges are operated by heating a bi-metal strip. As the rate and degree of heating are voltage dependent, any voltage change ANYWHERE in the system of the gauges will affect readings. The stabilizer creates a pulsating voltage that is on average 10v or whatever AT THE STABILIZER. If the stabilizer changes, both instruments should change in the same direction. Possibility 1 is that in concern over the temp, you didn't notice that you "had" more gas than before.
Beyond the stabilizer, the two gauges are in different environments. The fuel gauge/sender grounds through the tank/body, but the temp grounds through the engine/body. Any changes in engine grounding devices could easily alter temp readings, and they could also result in flakey alt/reg operation.
The goofy alt/reg system can feed strange voltage transients through many components, and it will respond to bad connections, hot or ground, in odd ways. If the alt/reg is actually bad and there are bad connections, you could have any fraction of 18 or 20 volts feeding randomly into various components, especially if they share common defective grounds. The same bad connections could be causing the alt/reg to act badly or to be damaged. Same for the stabilizer. Bad connections of any sort react strangely to the heat caused by the resistance that they themselves create. The BIG point here is that the grounds are as important as the power side.
I recently had an XJS with myriad strange and varying electrical problems, including gauge readings that varied by day, temperature, time it had been sitting. I checked voltages between bat grd, body in various places, subframes, engine, the alternator case, and various component housings, in every possible combination. All should have been zero, but they varied from 0 to about 3.5V, and they varied in groups, each according to its own set of variables. Drove me batso.Methodical one by one cleanup of grounds caused the groups to improve, some all at once, some in stages. Once I got the grounds straight, I could go after hot side voltage drops, one by one. The same situation applied, but these were easier to isolate by measuring volt drop acrcss each connection in turn.
When doing these voltage drop checks, it is important to load the system. With the engine running, turn on every electrical device you have. This will maximise the voltage drop. IF THERE IS NO CURRENT FLOWING THROUGH A RESISTANCE, THERE WILL BE NO VOLTAGE DROP! Start by checking alt case to bat grd, and alt power to bat hot.Both should be zero, and alt hot to alt grd should be the same as bat hot to bat grd. On the Jag I found hot circuit drops as high as 3V, some were single drops, some were several lesser ones adding up. I found a 1.5V drop between the alt case and the alt mount bracket, leading to the realization that what appeared to be a defective regulator was simply a bad ground. This would be disturbed in removing the alt, which would mean that putting a new alt on would fix it solely because the ground got fixed accidently!
Have fun!
FRM
http://www.usachoice.net/gofanu
FR Millmore

I think that FR and I saw the same thing in your last post - that is "problem with my alternator at the moment". That statement implies an electrical problem of some sort and since the temperature gauge is an electrical instrument, this could be where your problem is coming from. The ground and voltage drop problems that FR speaks of are very subtle problems and take a bit of electrical circuit understanding and patience to troubleshoot. The electrical problem you are having with the alternator could be a problem with the alternator itself or it could be due to bad grounds or voltage drops on the hot side of the circuit. One check that should be made is the output voltage of the voltage stabalizer, since they are really prone to failing while still new. The output should average out at 10 volts. I am not sure how well that can be read with a digital voltmeter or digital multimeter, a analog meter would be a much better instrument to read it with, but regardless, the voltage on the output side of the stabalizer should be a difinite lower voltage than on the input side and should average 10 volts. Anything higher, will cause your temperature gauge to read corespondingly higher. Just out of curiosity, what is your battery voltage reading when the car is running? It should vary between 12 and 14 volts depending on what the alternator is putting out. If it is reading higher than 14 volts, you either have a bad regulator in the alternator,or one of the bad grounds/voltage drops that FR speaks of.
Good luck - Dave
David DuBois

Once started (which takes a couple of seconds) the 'stabiliser' switches 12v (or system voltage) on and off about once per second , so you will not see a lower voltage on the output of the stabiliser with either a digital or an analogue instrument, only one that switches between 12v and 0v. The stabiliser is designed to give relatively constant output voltage across a range of input voltages, including a higher than normal input voltage.

Robert tells me he has about 12v on the brown/yellow when it should be 14v, and this is quite likely to be the cause of his glowing light. It also goes out when revved to about 3k which is a documented symptom of diode failure in the alt. One point I have asked Robert to re-check is that he says the brown and white show "between 14 and 16 volts - the best I could tell". Obviously 16 is too high, and would indicate an alternator voltage regulator problem, but even so I feel this should be within the compensation range of the stabiliser.
Paul Hunt

Furthering Paul's explanation of the stabiliser function, the only way to check the output accurately is with a bimetallic voltmeter which will average the reading. That is precisely what the temp and fuel gauges do. If you really want to know, get a selection of flashlight bateries and measure the voltages they have. They are nominally 1.5V, but will vary. Connect varying stacks of these to the temp or fuel green input and mark the gauge for each measured input voltage. You can easily calibrate it in 1.5 V steps or closer if some of the batteries are low. Remember that if you fiddle about too long, the bat voltage will drop as you work, since you are loading them, and that this calibration is dependent on the resistance in the circuit including the sender. It is a calibration of "here and now"
On the regulator issue, Paul is 100% right, but under some conditions like a loose/dirty connection in the alt, the regulator will "se" less voltage than actually exists and try to compensate.
FRM
http://usachoice.net/gofanu
FR Millmore

Here is the latest of my problems: ( car problems! :))
Paul was 100% correct in saying that the alternator was bad. I bought a NEW Lucas alternator this morning and it immediately solved the alternator light problem and I now have the correct voltage with the brown/yellow wires. So the electrical problem seems solved. Thank you very much.
Now back to the original problem of my temperature gage reading hot. This is what has happen so far:
1) After replacing the alternator, I also replaced the voltage stabilizer just to be on the safe side - but there is no difference.
2) As I have said previously, I replaced the temperature sending unit yesterday - which had no effect - good or bad.
3) I changed the water/anti-freeze ratio to 70/30 - 70 water
4) I added a bottle of Water Wetter
5) I replaced/installed a new 165F thermostat
6) I disconnected the wire at the temperature sending unit, then turned on the key, and the temperature gage did nothing, thus no ground fault in the wire
7) I disconnected the wire at the temperature sending unit, and hooked it to a direct ground; then turned on the key and the temperature gage increased from cold to hot - in a slow normal increase, as expected
8) Now here is the real catch,. I replaced the temperature gage with a new gage and it is does the exact same thing as the original gage. When I start the car cold, the temperature increases as normal, but then keeps going till the needle is in the white under the “H”. Thus, it is not the gage.
9) When I replaced the alternator, I checked the water pump and it seems fine. When you take the radiator cap off, you can see fluid movement
10) I have three infrared temperature guns and an infrared camera. At normal operating temperature, after driving the car thirty minutes, the maximum temperature recorded on the engine, the head or the radiator, was 180F.

So the problem I am now having is knowing what else do I check. I have checked everything on the cooling system. I know the engine is not running hot, and some people will say, just leave it alone, but if the needle is always in the hot range, I have no way of knowing when/if the car ever does run hot.

I am very frustrated and do not know what else to check. Please help. Surely someone else has had this problem. As I said, up until last Saturday, the temperature gage has always been in the middle, under the “N”. It just suddenly increased over a period of about two minutes and now stays in the hot range, even though the engine is not hot.
Robert Browning

Let me add a few additional things.
This evening, I checked the archives again, and in the electrical section it listed the Resistance Ohms at different temperatures for the temperature sending unit. When the engine is cold, the resistance is very high, thus a very low connection (ground) is going to the temperature gage, thus creating a cold or low reading. As the egine warms up, the resistance drops and a better or stronger connection is made, allowing a stronger ground going to the gage, causing the temp gage to increase. For my car, when the temperature is 180F (my normal operating temperature and I have double checked it), the resistance ohms should be approximately 82 ohms. On my car, it measured about 85 ohms, well within the range and as the archives indicated, this should be in the "Normal" range for the gage.
The second thing I did tonight was run a direct line from the sensor to the gage, just in case there was a ground fault in the wire or a connection (green/light blue wire). This had no effect.
The third thing, is that I found out that the voltage stabilizer is suppose to reduce the power to the fuel gage and the temp gage to 10 volts. When I measured the input power at the temp gage, the light green/green wire, it measured exactly 10 volts.
So, I have the correct power going to the gage; the correct Ohms coming from the temp sending unit; two different gages; but they both read the temp as hot. Why? What do I do?
Robert Browning

Robert I measured the resistance of a hot sensor yesterday, but since you have changed the sensor and gauge the figures may not be helpful. After a 50 mile run with gauge pointer at very near the center of the N, I measured 68 ohms on my 74. With a cold engine the sensor on my 68 read 731 ohms. The 68 gauge works ok but I guess to be fair I should measure the cold resistance of my 74 sensor. Wonder what voltage you have on the sensor with the engine at normal temp? If you think checking that may help I'll run one of my cars tomorrow and make the same check. Good Luck Clifton
Clifton Gordon

Clifton, at this point I will take any and all information everyone can provide. This is really driving me crazy. It is totally illogical. Any information and data will be helpful. Thank you.
Robert Browning

Robert - Which sensor did you get for your car? There are two different sensors for the later MGBs. The correct one for the 73 is Moss P/N 760-180. I don't know how much of a difference this makes, but we are looking for the needle in the perverbial hay stack here.
Good luck - Dave
David DuBois

Robert,
So, now you have found that the car is not overheating, which several people already told you.
You have shown that the gauge(s) and sender(s) are not bad.
You have shown that the wire between them is not bad.
You have spent a lot of time looking elsewhere for and through info that we already gave you.
You have spent a lot of time, money and effort for nothing.
You may have bought an alternator you didn't need.
Whether you needed it or not, you may be buying another sooner than necessary.
The bottom line is that "it" is not driving you crazy -"You" are.

Many of us know how the system works, and we have had similar to identical problems.
We are willing to help you, even more, but only up to a point.
Reread my post above -the long one first, then the rest. Do it again until you thoroughly comprehend it.
Anything you don't understand, or if I have gone over your head despite trying to keep the descriptions at a level most enthusiasts can comprehend IF they are willing to make the effort, call or e-mail. The offer is still open and I already sent you my phone # last night.

In addition to the extra info I sent last night:
On the XJS mentioned after fixing the main bat grd, I had 14.5V between alt power and alt case. I had app. 17-18V fluctuating between alt power and body grd. That looked like a bad regulator. I had 1.5V between the alt case and engine. The alt mount bracket was loose, tightening got the alt-eng drop to 1.0V. Removal of alt and cleaning contact surfaces (which appeared to BE clean) got rid of that 1.0 drop. I now had app 2.0 fluctuating between eng and body grd. Fixing the eng to subframe to body grds eliminated the 2.0V drop there. Now I had about 14.2 V between ANY power and ANY grd. This is right where it should be; the drop from 14.5 is because the regulator was no longer maxed out trying to charge what it thought was a low battery. IN THIS CASE, the alternator was fine but other things could get very unhappy indeed.
The point is that these sensor devices can only measure voltages between their connection points. The stabiliser measures from its power to its grd. If the dash is "floating" 2 V above true grd, then the output of the thing will be 12V relative to true grd, which may be what the temp sensor is reading at the engine. Alternatively, you may have had the engine floating 1 or 2 V above grd and it "fixed" itself, giving higher temp readings.
Another point re these loose/bad connections is that transient voltages up to the 700V range have been measured when fully loaded alternators come disconnected as a RESULT OF BAD CONNECTIONS. They have a very short (no pun intended) lifetime under these conditions.
I have also posted B specific basic check info on the thread "Charging System Help" /Matt Kaser
Sorry if I sound like I'm yelling at you, but my intent is to help you fix your car and understand it. That way you can fix other things and help other folk. My offer is still good.
FRM
http://www.usachoice.net/gofanu
FR Millmore

This post makes some excellent points. You are relying on the engine grounding strap for this gauge reading . A high resistance here will cause volt drop with other loads on the alternator , and this will show up on the temp gauge . It would be worth either checking and cleaning the strap and its connections , or measuring if there is a voltage between the engine and chassis .
S Best

180F/82C is spot-on so it isn't getting hotter than it should. Which leaves the gauge system. I'm interested you say the new voltage stabiliser outputs exactly 10v. This implies it is an electronic unit, but the Parts Catalogue quotes the same mechanical unit for late model cars as earlier. Nevertheless, it is only of interest, as it is giving the 'correct' voltage.

Quite honestly the only thing I can see left is to put it side by side with a similar era car and measure the *voltage* at the sender terminals and the same terminal (green/blue) at the gauges. Clifton's lower sender measurement at correct engine temperature (which Robert's engine also is despite the high gauge reading) would give an even higher gauge reading on Robert's car.

I have to say I don't find FRMs last post particularly helpful. The alternator *was* faulty, isn't now as it has cured other problems, but has made no difference to the temp reading. The sender *hasn't* been proved as OK yet, the new one could be the wrong one (although with a higher 'normal' resistance than Clifton's probably not). And if there were a high-resistance engine/transmission ground-strap two things would be apparent - one is slow or non-existant cranking, the other is that the temp gauge would read lower, not higher. But even then the engine to body resistance would have to be tens of ohms to make any difference and the 'alternative' engine grounding paths through accelerator, choke (?), and heater control cables are usually much lower than this.

However Robert should measure the potential difference between the body and the engine, with the engine idling at 'normal' temperature but a high gauge reading, to eliminate it.
Paul Hunt

Robert,

Does the guage read the same with the radiator cap on as it does with the cap off? When you change the thermostat to a 165 do you also change the cap to a lower pressure one?

Fiat's (at least the 124/2000 models) have a similar problem to the one you describe and their problem is due to air in the cooling system. Because some parts of the Fiat cooling system are above the water-fill point it is difficult to purge the air. There are many articles about "burping" the Fiats after any change to the cooling system. I think the only portion of the MG cooling system that might cause such an air pocket would be the heater. Since summer is here, you might try bypassing the heater to see if the temperature readings improve.

Doug
D. Cook

Some more thoughts:

Get the engine up to normal temperature i.e. temp gauge reading high, leave it to idle for a few moments to let things stabilise, then stall it but leave the ignition on. What happens to the gauge then? Ordinarily I would expect it to rise slightly over a couple of minutes because of the effects of heat-soak, then slowly drop as everything cools down. Unless yours drops to N in a few seconds then it isn't a voltage problem. Is you stabiliser output still reading 10v?

Secondly you haven't mentioned the cooling fan in all this. What is it doing? Never comes on? Always on? Or cycling on and off particularly when idling. When looking at the cooling system on a car with electric fans I have them running constantly so as to avoid the normal rise and fall in temp that occurs as the fans cycle on and off.

Disable the cooling fans so that the temperature *does* rise above normal, say, 195 to 200F as measured with your temp guns. What is the gauge reading now? If it is well into the H or past it, then for the moment I would treat the reading at 180 as 'normal' so at least I could drive the thing while pondering just what the cause is. I might even put a few tens of ohms in series with the blue/green to drop the gauge reading to N, but ensure it does go up to H if you disable fhe fans and let the temp rise.

The radiator cap was the same regardless of the stat fitted. The cap pressure is such that it should only open if the pressure gets higher than it should in normal use. If it *does* open, and this is due to temperature and not a leaking head gasket, then any stat would be wide open anyway.
Paul Hunt

Robert: I just ran my engine about 30 min. and did some voltage and resistance checks. With a cold engine I read approximately 10 volts at the sender. Alternator output was 13.95 v. Measurements were made with a good quality digital meter. Of course voltage readings vary at the sensor due to the voltage stabiliser so I tried to catch the higher readings. With pointer just below normal the voltage was 6.8 volts, several minutes later the voltage ran between 6.6-6.8 v. Resistance was around 82 ohms at first but kept coming down as the engine heated. After about 30 minutes voltage was 6.4 v. and resistance 72 ohms with the gauge pointer near the center of N. I switched the engine off and monitored the resistance while the engine heat soaked. Resistance went down to 65 ohms. I turned the ignition on and the pointer registered less than 1/16" to the right of the right leg of N. I didn't measure voltage for the last check but it should have come down a little. All resistance and voltage measurements were made with the common lead on chassis ground.

I have a factory fitted water pump driven plastic fan
on my car, Robert's car should have the same type fan, and it was running during the test. I also have a 10" electric Hayden fan and it never came on until after the engine was turned off and heat soaked several minutes.

BTW, This morning I checked resisrance of both cars heat sensors and both showed infinity. I don't understand why I read 731 ohms on the 68 on Monday. I used the same meter on all checks. I seem to recall making a check on a new sensor last year and it read infinity when cold.

I hope you have found the problem while I have been typing, Clifton




Clifton Gordon

My latest thought is that there may in fact be 12(or more) V getting to the gauge, despite the stabiliser output of 10V. Cooked wires in the harness could do this. Check voltage at the gauge input LGG to see if it is 10V at that point. Checking at the stabiliser output may show this if checked with everything connected, but not if the suspect LGG lead is disconnected. If it is the old mechanical stabiliser then it will pulsate so you can't get a true reading, but if it is steady at 12+ you have a crossfeed. I have no experience with any steady 10V units, must be something new, but again it should be 10 not 12+. This problem should affect the fuel gauge as well though. Robert, are you positive that the fuel gauge isn't reading high as well?
Note that the grounding of the stabiliser is critical and so is its physical orientation - "B and E uppermost and not exceeding 20 degrees from vertical" From factory shop manual.

I finally got to check out Paul's website - very good and helpful and I recommend it highly. He actually covers most of my points re checking electricals; I am simply giving alternate statements and an additional layer of diagnosis.

Robert did mention the fan, in his second post. It is belt driven, no electric fan on 73.

AS to Paul's comments on my comments:
I am not denying that the alternator "was" faulty. One reason I am being adamant about all this is that Robert said that the alternator was new at rebuild only 2000 miles back. It may have been a "bad" alt, esp. if it was a rebuilt, or it may have been a "killed" alt, due to problems like I have pointed out.In that case, he may soon have another, unless the trouble is fixed first. While nothing is ever "proved", Robert's replacement of both sender and gauge with no change in readings, can reasonably be taken as ruling out faults there. While bad engine grds frequently give poor starting, sometimes it is not obvious. The XJS cranked on the very slightly low side of OK even with a less than fully charged bat, it is now very noticeably on the fast side. Paul is entirely correct that the resistances would have to be in the tens of ohms to cause the readings observed, but "floating" voltages would be much smaller -tenths to single volts.This can easily happen with defective multiple grds connecting powered devices.
Behind dash multigrounds are frequently not replaced correctly as they are so hard to see.
I am intrigued by Paul's info (his site) on recalibrating the fuel gauges, it's a new one on me and long desired. It should apply to the temp gauge as well (I don't have one at hand) -but only after the current trouble is sorted -NO MORE variables!
I have just had a long talk w/Robert. We sorted some stuff out, and he has done a lot re ground connections etc. Trouble is, he keeps shooting down possible causes with no improvement. Am posting this to keep others up to date. I look forward to a solution-good mystery!
FRM
http://www.usachoice.net/gofanu
FR Millmore

What an enormous thread! I have just breezed throught it, so forgive me if my comments are duplicated elsewhere.

Having seen all the troubleshooting which has occurred, I'd like to suggest an additional possibility: that the gauge was in fact mechanically sticking at the 'N' point, and for whatever reason became freed to operate correctly. Don't forget that the gauge is an elctro-mechanical device.

Instead of thinking it is the same as a new one, so it's not the cause, consider it may have had a fault which has corrected itself. Now, being an electronics technician, I'll admit it doesn't happen every day; but it is certainly possible.

The easiest solution is the most likely...
Dave Whitehead

Robert - One point that I was going to make on my last post after reading that you measured exactly 10 volts going to your gauge from the stabilizer and building on one of FRMs earlier posts is this - Remove the voltage stabilizer from the bulkhead and thouroughly clean the mounting point to insure a good ground at that point. See my article on ground point preparation at: http://www.omgtr.ca/technical/General%20Technical/grounding.htm
Not only is the stabilizer notorious for failures, it is also notorious for not operating properly if the ground point for it is not 100%. Good luck - Dave
David DuBois

First, I want to thank everyone for their suggestions since my last posting.
Second, I will try to address some of the points made:
1) Just to reconfirm, I had the old alternator tested and it was bad, so replacing the alternator as Paul suggested was the correct thing to do.
2) Doug ask what happens if I take the radiator cap off. Last Saturday, when I was replacing the thermostat, I noticed the seal on my old radiator cap was torn, so I bought a new one this past Monday. It is the 10 lb pressure cap, as what was on the car and what is listed in the manual as the correct pressure cap. However, removing the cap has no effect.
3) As for bypassing the heater, when the heater is turned off, it is basically cutting off all flow to the heater core, so I do not think this will be an issue. Also, when I turn on the heater, it has no effect on the temp readings on the gage.
4) Yes, Paul is correct in that the Voltage Stabilizers are all electronic, thus giving a steady 10 volt reading.
5) Paul asked about the cooling fans and what they were doing during all of this. My car is 73 MGB, so it does not have electric fans. I have the multiple blade, yellow plastic fan that is directly mounted to the engine/water pump, and is driven by the fan belt.
6) Someone else has suggested putting some ohm resistors in line on the green/blue wire going to the temp sending unit. If all else fails this is an option, but to me this would be a temporary fix and not solving the problem.
7) Paul wrote, “Get the engine up to normal temperature i.e. temp gauge reading high, leave it to idle for a few moments to let things stabilize, then stall it but leave the ignition on. What happens to the gauge then? Ordinarily I would expect it to rise slightly over a couple of minutes because of the effects of heat-soak, then slowly drop as everything cools down.” Paul, this is exactly what happens.
8) For David’s comment, I double checked and the temp sending I have is the Moss # 760-180.

NO READ ON!!!!

This afternoon I had the opportunity to talk with Fletcher Millmore for about two hours. We went over every aspect of everything I have done and everything that everyone has suggested. When talking with Fletcher, he told me about a part on his Ford Taurus, that would fail only at a certain times. He said that the part would indicate being correct before and after a certain point, putting out the correct ohms and volts, except for the one small point, which happen to be the area where this part was used most often in normal operation. This got me thinking.

I ran a new, direct wire, from the voltage stabilizer to the temp gage; and a new direct wire from the temp sending unit to the temp gage. This means that I had isolated everything out of the system. When I did this the gage still registered in the hot area.

I then measured the voltage from the voltage stabilizer again, and it was registering between 10 and 10.5 volts, sort of moving between the two numbers.

I then changed to the other gage I got yesterday. It also registered in the hot range.

Now, this lead me back to the temperature sending unit at the block. I started thinking about what Fletcher had told me and wondered if it was possible for the temp sending unit to do the same thing. I had stated previously that when I checked the ohms on the temp sending unit, when the engine was at normal operating temperature, it registered about 85 ohms, and the motor temperature was approximately 180F.

When I checked the temp sending unit today, when the engine was at 180F, it still registered approximately 85 ohms. So I decided to heat the engine up to a higher temperature (by blocking the air flow through the radiator) When the engine temperature was up to about 200F, the ohms had dropped to about 40 ohms, where they should have been about60 ohms. When I let the engine cool down to approximately 150F, the ohms at the sending unit were 95, when they should have been about 150 ohms.

Therefore, I pulled the temp sending unit out of the engine, and ran cool water over it to cool it down. I also did this to my original temp sending unit. So both temp sending units are now about 75F. At this point, they should have been registering about 700 ohms. The old, original sending unit, well, the ohms were all over the place. The needle just bounced between 300 ohms and 1200 ohms. However, the second sending unit, which was registering 85 ohms at 180F, was registering 180 ohms at 75F, which corresponds to a water temperature of about 140F.

I had said that it seem to me that the temp gage was rising very rapidly when I started the car cold. It was a steady, but solid increase in temperature all the way up to hot. This was because it was indicating it was starting out at 140F, about 70F hotter then reality.

Therefore, this evening I went to NAPA Auto Parts and bought a new temp sending unit - a non-Moss part. When I measured the ohms before putting it into the car, when the actual temperature of the temp sending unit was about 80F, it registered right at 600 ohms - right where it was suppose to. When I installed the temp unit in my car, at 180F, the unit reads 84 ohms. The temperature gage however is reading right in the dead center, under the “N”, on both of my gages, with the voltage stabilizer hooked up normal. When I increased the temperature of the engine up to 200F, it was reading approximately 60 ohms - right where it is suppose to be, and the gage showed the increase temperature. Then when it cooled down to 150F, it was reading approximately 125 ohms - again, where it is suppose to be.

Now, maybe I am just stupid for not seeing this before, but when it was reading the correct ohms at 180F, I never thought it could have a “glitch” in it - at least not until I talked to Fletcher this afternoon.

As of this minute, everything is working just like it is suppose to. Why this happen and why the new sending unit was bad - or calibrated wrong, I have no idea. In addition, I did as Fletcher suggested and checked the voltage and grounds throughout my car and they all were just as they were suppose to be - no voltage leakage.

I want to thank everyone for their help and advice. I really appreciate this site and the advice everyone offers. I hope this thread will be of benefit to others in the future. It contains a lot of great and valuable information.

Thanks Paul, Fletcher, David, Clifton and everyone else. This was very frustrating and I am glad I did not have to go through this alone.

Cheers. Robert



Robert Browning

Robert - Nothing beats success!!! congratulations. One of the most suptle things to troubleshoot and will lead one down the garden path every time is the new part that is failed in the same way as the old part. It will drive you crazy everytime. Now that you have found and corrected the problem, take the MGB out for a drive and enjoy. Cheers - Dave
David DuBois

Robert, congrats!
It all illustrates my basic philosophy:
It ain't what you don't know that's the problem, but the things you do know that are wrong.
The car has no expectations, only its experienced reality.
If you can't figure it out, you're wrong, not the car.
When things get goofy, go back to the absolute beginning basics.
Thanks for the picture -pretty car.
Happy motoring!
FRM
FR Millmore

I have heard about bad sensors giving crazy readings before , but never 2 one after the other giving a story that was almost beleivable as an an engine cooling problem. Congratulations due on fixing this .
S Best

Congratulations Robert. Glad you found it.

FR Millmore, Thanks you for your recent contributions to the BBS. I have read most of your responses and they contain some very valuable information that will be archived in the BBS and be helpful for MG owners in the future. Thanks for taking time to share your experiences. BTW, Nice hit and miss engine model on your site. There will be dozens of hit and miss engines at the Southeast old Threshers' Reunion on June 30-Jul 4 at Denton, NC. See FR's model:
http://www.usachoice.net/gofanu/ModEng.htm#Model

Cheers, Clifton

Clifton Gordon

"but never 2 one after the other "

Hunt's Fifth Law: "Many break-downs occur soon after a car has been worked on; 'new' parts can be faulty when you receive them; 'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals."
Paul Hunt

As a new MG Owner('74 chrome bumper)concerned about the temperature guage drifting towards hot in traffic, I cannot tell you how much this thread has helped. You "experts" who offered valuable assistance are truly saints. Thank you.
G.W. Lee

This thread was discussed between 09/05/2004 and 18/05/2004

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