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MG Midget and Sprite Technical - Temo gauge rises when cranking

This is a bit of a mystery to me. I have noticed lately that when cranking the engine to start, the temperature gauge rises in steps. When the engine fires, the gauge drops back to cold and then gradually rises again, operating normally as the engine warms up.

It is one of the dual needle electric gauges but clearly it isn't responding to the actual engine temperature when this happens, but it is some sort of side-effect of the current being drawn by the starter. (remember I still have that struggling Halfords battery).

Problem is now that when it happened today the needle then stuck at the top end of the scale and has stayed there. Any suggestions what is going on and how I might fix it? I am hoping that I haven't damaged the gauge irrepairably as they are quite expensive to replace.

Guy
Guy Weller

voltage regulator. £9.99 buys you a new solid state version. 5 minute fix
N Sayle

..or revert to standard capillary gauge... 'it was good enough then...etc'
;-)
David Smith

Im wondering if its a bad ground...as they say path of least resistance

>>>>dual needle electric gauges<<<<<.... whats the other gauge?

maybe some kind of cross over feed going on...


Prop
Prop

I'd be looking at bad engine earth connection. Put a voltmeter between the block and the battery earth post and see what it says when cranking.
Could also be related to your battery trouble, since charging rates get all wonky with bad connections between Alt & Batt.

FRM
Fletcher R Millmore

Umm, sorry about that - of course it isn't an electric gauge! - it is a standard dual oil pressure / capillary controlled temperature gauge. But the gauge is going up as it is cranked.

Thanks Prop and Fletcher. I think you may be right. If the engine isn't earthing properly and the capillary is acting as an earth, then the current flowing through it could be heating / boiling the ethanol and making the needle rise. I can check the earthing easily enough in the morning. But the problem is - the gauge having now stuck at max temp it won't go down.

Guy
Guy Weller

I am going to go and do a visual check first for the engine earth strap condition. I replaced it about 2 years ago so it should be OK unless it has come loose. The previous one lasted 37 years.
I know I could use a meter to check but putting it between block and negative post whilst cranking, isn't it just going to act as an alternative engine earthing strap and take far too much current for my meter?

Guy
Guy Weller

"take far too much current for my meter?"

Nope, I'm assuming that Fletcher meant you to try to measure a voltage between block and chassis. If your earth strap is poor, the 300 or so amps drawn by the starter will produce a measurable voltage across the strap. A voltmeter is a high impedance device and will draw hardly any current.
Allan ('76 Midget 1500)

Thanks Allan. I will try that now then.

The engine earth strap is still firmly attached both ends. A few loose strands but otherwise intact.

Any ideas on getting the temperature gauge needle back down from the high end on the oil pressure scale?!
Guy Weller

Well you are right. I get a voltage between negative and the block when cranking.
It is showing 1.4 v but is this enough to be significant? And is this likely to be related to my temperature gauge going off the scale when cranking?
Guy Weller

Oh heck Guy

seems likely that the capillary has become blocked during the cranking session maybe a highly heated spot of solder inna pipe and stopping the gauge returning to zero

Check for signs of "hotness-ness" where the capillary goes into the cabin, I had visible arcing damage when a similar thing happened to Lara, the protective spring had a burn across it and had opened up due to the lecktrickery
bill sdgpm

Normally my car starts extremely well (thanks to electronic MG Metro ignition I think) Once the fuel is there the starter only needs to turn over one compression stroke and it fires up. So normally even if it was earthing through the capillary it wouldn't be "live" long enough to cause excessive heating. On this occasion though, earlier this week I had been doing wet and dry compression tests, 10 rotations each X 4 X 2 and I am guessing that this is when it happened.
Guy Weller

Sorry Guy have I missed something while reading these posts? on the one hand you suggest you have an electrical gauge now you are suggesting you have a capiliary gauge? so just for my benefit which is it?
Robert (Bob) Midget Turbo

Keep up at the back there Bob!

I corrected myself yesterday:
"Umm, sorry about that - of course it isn't an electric gauge! - it is a standard dual oil pressure / capillary controlled temperature gauge. But the gauge is going up as it is cranked."

As the gauge seemed to be responding to the starter drawing current I sort of assumed it was electric - like my long-gone 1500 midget one. It isn't of course!

Guy
Guy Weller

10x4x2 sounds like enough constant activity to allow the cobbleup to happen if it was that

heat generated lecktrickally causing the ethery stuff to expand inna toob

might cause it but why would it stay high unless the problem is at the needle sticking against a part of the gauge back plate

(I have that on my Airline tyre pump gauge, slight distortion of the stuck on backplate is my problem, I doubt if it is yours)
bill sdgpm

A 1.4Volt drop across a piece of wire is significant. It suggests that the termination or the wire itself is not good enough. How big is the earth cable and how is it terminated?

I think there is a strong possibility that the capiliary which is in parallel electrically to the earth strap has taken a bit of current. With this being small a bit of current would have caused a lot of heat and possibly strained the gauge at the uppermost end.
I think it could be toast. :(

Has anyone had a gauge apart? is the capiliary gauge a simple quadrant drive?
Robert (Bob) Midget Turbo

I managed to sell my spare one that I toasted the capillary on, but somebody must have one some place...

or google pictures? I'll have a look
bill sdgpm

Yes, I think that is what has happened. The 10 X 4 X 2 was over a period of perhaps 5 minutes as I tested each cylinder in turn. Nothing new about that though. I do such a test every 3 or 4 months. But the engine earth strap is not so good. It is the proper braided cable but the "few stray wires" that I mentioned earlier in fact turned out to be about 1/2 of the total when I took it off to clean the end terminals. Pretty poor quality for something that was replaced with new only a couple of years ago. Looks like that bit of crappy cable has cost me a new gauge!

Guy

PS and sorry about the title (Temo instead of Temp) Seems one cannot edit mistakes in the title!
Guy Weller

Guy -
Yes that V drop could be enough to heat the capillary, ans also cause poor starting, though your otherwise well tuned engine an electrosparky tends to hide that. Also, bad connections tend to be unreliably variable, so it could have had a worse episode.

If you have a cable from Batt to body, and another from body to engine, they can both be bad, and it is not unknown for the Batt one to be corroded internally so you can't see it. This can do evil things to alt/reg, killing lots of stuff including batteries. I run main earth from Batt to engine, with a strap from engine to body.

The gauge has a Bourdon tube in it, which straightens out as the temp, hence pressure (of the contained ether) increases. The motion of the tube straightening is transmitted by little gears (Bob's quadrant drive) to the needle. If driven to the end, the gears tend to jamb up, sticking the needle even after the tube returns to normal. You can usually free it up if you take the gauge out of the case. If, after the gears are free, it's still of kilter, you can carefully bend the Bourdon tube (tighten the "C" shape)until it's back to normal. If you have to bend things, watch you don't destroy the needle against the stop, as you will have to overbend it past zero.

Of course, if you burnt a hole in the capillary yr screwt!. But even those can be fixed by an enterprising young feller.

FRM
Fletcher R Millmore

Guy

You need to sort out your throttle cable; it's supposed to provide a back-up earth when the main engine earth is not up to the job. Over the years I've seen a couple of fried cables that have done a sterling job ;)

Some 25 years ago, a Scimitar Coupe I had just bought had an engine that worked intermittantly as it was driven slowly. The throttle cable earth wasn't working properly, and it was using the propshaft to earth through the speed cable. How inventive is that?!

A
Anthony Cutler

Yep, I wasn't sure if it was a bourdon tube or not FRM but if it is then I would expect that once released from a locked position at full scale then it could recover itself without problem. I think the only way to into the gauge is to remove the glass, then remove the pointer (pull it off) and then remove the face. If you do this Guy then it is a feel thing so go carefully. Initially you need to determine if the capiliary tube is intact.
Robert (Bob) Midget Turbo

Thanks friends, you give me hope! I guess the next thing is to remove the bezel and glass and see what a little gentle fettling might achieve. I shall enjoy that exploration.

Externally the capiliary tube looks fine, but who knows what lies within - if anything!

I have checked both earth straps. Battery to body was fine. Body to engine is the 2 year-old one nearing the end of its useful life. Build up of corrosion at the crimped-on terminal and about 50% loss of diameter of the braided wires. So that is the culprit I think. Need to get me a new one.

Guy

Guy Weller

I had a problem with my engine ground cable and i just replaced it with a short battery cable. the throttle cable was the secondary ground for mine, but it wasn't quite good enough.
Is there any problem with using the battery cable? I do not plan to EVER drive the car in the rain (seeing as it is a rush-monster), if that helps at all.
Seth Brecklin

no Seth there should be no problem using a short battery cable for the job

I do
bill sdgpm

Ok, glad that i have done something right on that car.

im learning slowly.
Seth Brecklin

I've seen MGB melt choke, throttle, and heater control cables, and weld the heater control solid - spectacular in a dim garage! And my '47 Ford once fried the choke and throttle linkage ball ends nicely, that was my intro on the subject.

FRM
Fletcher R Millmore

I had a choke cable burn out in my wife's HA Viva!! (Them were days!!)
Robert (Bob) Midget Turbo

I have been investigating the gauge.
Everything looks OK, but it isn't working and I am not sure why.

The needle was hard up against the top stop so I took it off and set it too a random mid point. Then ran the engine for 10 minutes to see if the needle moved as it warmed up, but it didn't. :-(

I don't really see how the mechanism works. Essentially the oil and temp mechanisms look much the same. I presume somehow the change in pressure (oil) is mimicked by the temperature induced change in the pressure of the ether in the capillary tube. What I don't see is how that translates to straighten the curved metal (bi-metal?) strip. I can see this would straighten if heated, but how does this happen?

See photo. Moving the lever with a small screwdriver causes the needle spindle to rotate counter-clockwise as it should. But the actuating pin on the end of the curved metal strip doesn't seem to move as the car heats up.

I think the technical term is "terminally knackered", unless someone knows otherwise?

Guy
Also interesting to see how arbitrary is the positioning of the needle on the spindle. Next time someone complains about high or low oil pressure - the solution is very simple!



Guy Weller

the "curved metal strip" is a flattened bourdon tube & as it fills with the increased pressure it tends to uncurve itself. This pulls the bit the needle is stuck to and moves the needle round the dial

theoretically

if the needle doesnt move from inside there has to be a tube problem

both gauges look the same don't they but one upside down agin t'other

both use the same basic principal unbend the bourdon tube to drive the needle. Upper oil pressure attaches via the threaded end the lower has direct pressure feed off the temp bulb in the head

comparing "actuating pin on end of curved matal strip" with the same device on the oil pressure gauge it looks to me as if the actuating pin has slipped past the point where it can influence the needle and give a reading, by pulling the peg. Your temp "peg" has gone over centre it seems

try swapping it to t'other side of the lever and see what happens
bill sdgpm

Guy -
I haven't had one of these apart for a long time, but agree 100% w/sdgpm.

When you gently lift the peg over to the other side of the pin on the Bourdon, the Bourdon ought to close up to a smaller radius. Otherwise it is "permanently sprung" and needs to be gently squashed back to a smaller diameter. I would put the bulb end in a pan of water and heat it while watching the works and needle, before bending anything.

The only thing you really care about is that it say 100C/212F when the water boils, so it's not really arbitrary. That's when/where you push the needle back on the shaft. Same procedure for the OP: hook it to an airline at known midscale pressure and fit the needle to match.

FRM
Fletcher R Millmore

Bill,
Fletcher,

I thought the curved metal strip was a bi-metal that would straighten with temperature. But I understand the Bourbon principle works in much the same way in response to pressure.

I thought that the pin was on the wrong side too, but the lever is spring loaded and presses against the pin, rather than being pulled by it.

This may still be associated with the problem. If I gently lift the actuating pin to the other side of the lever, the lever itself is then free to move upwards (clockwise) and this in turn moves the needle counter-clockwise, without any assistance from the Bourbon tube thing. I had come to the conclusion that the temperature gauge "wants" to show a high temperature, but the Bourbon tube limits its travel, only permitting it to increase as the temperature rises and it uncurls itself.

The oil pressure on the other hand "wants" to show a low pressure and the tube thing actively pulls on the lever as it uncurls.

At least this seems to be how it should work. Only it doesn't!

Guy
Guy Weller

the spring thing is there to take it back to "start" I think

if you look carefully the oil pressure gauge is the exact same mechanism. I think the other peg on the bourdon is the actuator and the one that has "jumped" is prolly a return finger fingie

The actuator for the temp is almost hidden beneath the oil bourdon and the temp drive "finger" is almost invisible under the oil gauge finger

replicate the setup of the oil gauge pegs on the temp one and I wouldnt be at all surprised if it starts working again
bill sdgpm

Bill,
Irrespective of the Bourbon device, the temperature actuating lever and the needle spindle both move in the directions of the blue arrows on my drawing as the temperature increases. i.e. clockwise for the lever makes the needle go counter-clockwise.

You are right about the OP mechanism being the same, only it is flipped over , not just rotated by 180 degrees. But the spring acting on the two levers is different. The spring on the temp lever pushes the needle up to a high reading, whilst on the OP the spring pushes the needle down towards zero.

I think that the second pin at the end of each Bourbon tube is a stop pin, which would come into contact with the long "horn" of the cover plate.

I might try putting the bulb in some ice to see if it will condense and draw the ether back down the capillary tube. That might help.

Guy
Guy Weller

got it

no wonder it isnt working


you have a bourBon

the rest of us just have the bourdon

still yours will be far better "dunked"

:-)

I hope you dont have a holed pipe that has hidden from you

can you change the shape of the bourdon tube at all

maybe it set at a hot setting due to elecktrickery, might need annealing
bill sdgpm

LOL ! I thought the name was familiar !!

Not sure how else I can tell if the ether has leaked away into the ether.

Guy Weller

Just keep fiddling with it until you thoroughly understand how it works. The Bourdon tube is quite robust - pushing it one way or the other isn't going to hurt it. I have bent back ones that have been seriously overpressure, and it ain't easy! By all means cool and heat the bulb; too hot is the only BAD thing - use boiling water for hot, and ice in salt water for the cold test.
I can't give you exact instructions, since I don't have one apart in front of me now.

FRM
Fletcher R Millmore

Thanks Fletcher,
I presume that the Bourdon tube is effectively a direct soldered-on extension of the capillary - and should therefore be filled with ether as well. Unless there is some form of diaphragm arrangement inside the solid mounting block at its root.

Currently I am driving around with the gauges connected, but the glass and dial missing off the front so I can see what is going on when I take the car out. Yesterday in traffic the temp needle began to move upwards, travelling through maybe 20 degrees (angular, not temperature). So I thought it was working again. But when the car cooled down again the needle (and Bourdon tube) didn't return down again so it isn't working right.

But at least there is some life in it yet so there must be some ether still in there.

Guy
Guy Weller

Guy -
Yes the tube is directly connected to the capillary. The Bourdon responds to the pressure of the expanding ether, which is why getting it really hot by direct resistance heating might have permanently deformed it.
A few years ago we had a discussion about recharging the thing with ether. If you can't find it in some archive or other. I might be able to find a copy. at least of the part I wrote. Think maybe Gryf K. came up with the start of it.

FRM
Fletcher R Millmore

I think I have a copy of that saved in my "technical ideas" folder. Hoping that it doesn't come to that. I have an idea that necessary ether is not easy to obtain over-the-counter in the UK. Regulations!
Guy Weller

ISTR easystart is nearly 100% ether Guy
bill sdgpm

But if it still moves at all, it has ether in, and if there were a leak it wouldn't, so you are still at sort the mechanism and adjust accordingly.

FRM
Fletcher R Millmore

It is moving up as the car warms up, and moving further than it did. But then not going back down again when the engine cools. I think it is just sticking somewhere.

I have also remembered that I have another gauge in my box of bits for the Frogeye, so I do have another to compare with although I have no idea if that one works. As far as I know it has not been in use since at least 1984. But it will still be useful to take a look at!

Guy
Guy Weller

Ah lucky Guy

if the other one is "whole" use it instead (simples) it doesn't matter if it has the right legend on its face after all

Cold Normal Hot is just as accurate as a set of numbers now you see how precise they are inside

I'm glad you dont have to "g"ugger about with filling the capillary with ether, can't be a healthy career choice IMO
bill sdgpm

Bill,
I don't want to use the second one, I want to save that for the Frogeye. But I have taken the face off it to compare. The components are slightly different but the principle is the same. The Bourdon tube for temperature is definitely of a tighter radius. Interestingly, there is a Wiki on Bourdon tubes which gives specific radius limits. So I can check those, and maybe bend it back to the required curvature.

Guy Weller

This thread was discussed between 27/03/2010 and 04/04/2010

MG Midget and Sprite Technical index

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