MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - On again Off again Fuel/Temp Gauges

Hello All,
The last two time I took my 1977 US spec. MGB out for a drive the fuel and temp gauges would work then not work then work then not work. In other words gas tank is full but the needle will sometimes drop down to about 1/4 tank and at the same time the temp gauge drops down to close to the cold location. Then for no apparent reason they move back up to their proper indications. I cleaned the fuses and fuse holder thinking this might help it has not. Does this sound like it could be a faulty "Voltage Stabilizer"? If so how hard is it to get to. I've heard it is on the firewall under the dash. Is it located more towards the driver or passenger side? At my age and weight it is quite a chore getting up under the dash. I purchased this car 2 months ago and the PO has installed those pvc shields under the dash. I would rather just have to remove the proper side if possible.
Any thoughts would be much appreciated.
John Fraioli
79 Inca yellow mgb roadster
72 Teal Blue MGB-GT
and now a 77 Flamenco Red MGB roadster
John Fraioli

John,

This may be of some assistance, page down for a photo location of the voltage stabilizer. Start by verifying a good ground on the facia. Also plenty on this subject in the archives. Let us know what you find.

http://www.mgexp.com/phorum/read.php?1,1126677,1127480

Regards,

Larry C.
Larry C '74 B/GT

The fuse would affect the tach as well, so the instrument voltage stabiliser is the prime suspect, or the wiring to/from it.

The gauges don't need an earth for anything except the illumination, as the earth that operates them comes from the relevant sender. The stabiliser does need to be earthed, although if that goes AWOL the gauges read high.

Unless you can catch the fault while you are around with a test-lamp or meter it could take a while to find out just what is wrong. In the past I've driven around with a meter or test bulb wired up to various points in the circuit so I could see whether voltage is at a particular point all the time, and hence whether it is still there or not when the fault is present.

On a US 77 the fused ignition (green) fed daisy-chains off several other components, but the heater fan is after the stabiliser and the last thing in the chain, so if that still works when the gauges stop then 12v must be reaching the wiring connectors at the stabiliser at least.
Paul Hunt

Thanks Paul and Larry,

That is helpful information. The first thing I will do is take the B for a drive and when the gauges drop off I will try the heater fan. If the fan still functions should I then change out the Voltage Stabilizer? OR can I just bypass the Stabilizer? Or would that just give me fluctuating needles due to fluctuating voltages?

One more thing. Maybe I wasn't clear. The needles on the gauges don't drop all the way to the left giving NO readings, they go pretty far to the left but still give some slight readings. Stabilizer?
John Fraioli

The voltage stabiliser puts out about 10v for the gauges, so simply bypassing it is not a good idea! You have other MGBs - could you rob one of them temporarily of its stabiliser?

(Ironically the design of a VS is that it provides fluctuating voltages in that it constantly switches on and off so that the average is the required 10V.)
Michael Beswick

If the fan also stops working that indicates voltage has been lost from the stabiliser, so it is a wiring/connector (of which there are several) problem back towards the fusebox.

If the fan still works then it could be the stabiliser, or the wiring/connections to the fuel gauge, which is nearest in the run, being teed from there to the temp gauge.

Bypassing the stabiliser would give both high and fluctuating readings.

Ironically although replacement stabilisers all seem to be electronic now, amusingly many are still labelled with 'TOP'. The original was electro-thermal-mechanical, so the orientation probably did have some small effect on the readings. But given the non-linearity of the fuel gauge (and the crude and coarse markings on especially the later electric temp gauges) even the concept of orientating the original stabiliser is rather amusing.
Paul Hunt

I made electronic stabilizers for both my B's. One has worked perfectly for 10+ years. The components are cheap and manufacture simple.
http://www.britishv8.org/articles/mgb-voltage-stabilizer.htm
Have a look at this, I didn't bother with the LED.
Allan Reeling

Why is the voltage stabiliser necessary at all? On my 1971 Lotus (very similar electrics to an MGB) I have converted the generator to an alternator so ensuring I have a better charging system, and just to try, I didn't bother with the VS, wiring up the fuel gauge to battery voltage. I adjusted the gauge to match the tank unit, as Paul describes on his web site, and it is working perfectly well. The temp gauge is a capillary one so that isn't in the equation.
Mike Howlett

The electrical gauges run on 10 volts DC current. Without a VS, the gauges would change their readings every time the engine speed or load changed. RAY
rjm RAY

But it doesn't Ray, because the gauge mechanism is damped. In any case the original type of VS doesn't give out a steady 10 volts. It fluctuates all the time with 10 volts being the average output. It operates with a make-and-break mechanism that switches on and off a certain number of times a minute so that the approximate mean voltage at the output is 10 volts. Sometimes it will be 12 volts, sometimes zero. You don't get much bigger fluctuations than that. My Lotus is specified to have a 10 volt VS just like the MGB but the gauge works perfectly well on battery voltage.

My theory is that with a DC generator the battery voltage really did drop off as the revs fell, especially if the headlamps and wipers were on. You used to see this with the directional indicators blinking more slowly while waiting to turn. It made sense to make the gauge operate on a cut-down voltage that could be maintained, more or less.

Now with a decent alternator the voltage through the system is much more constant. Even at idle revs, your lights and wipers keep going at normal brightness and speed. The fuel gauge is adjustable and can be easily set up to run on the 12 or 13 volts supplied. I know, mine does.
Mike Howlett

"But it doesn't Ray, because the gauge mechanism is damped. In any case the original type of VS doesn't give out a steady 10 volts. It fluctuates all the time with 10 volts being the average output. "

Yes it does. It's the on-off that give the regulation, as the average output over time is 10v, and the damped gauge barely responds to each on and off. The higher the system voltage the shorter the on periods, but the off period is relatively constant. A damped gauge will still fluctuate with voltage, which could be anywhere from 12v to 15v as Ray says varying with engine speed and load. That's why a stabilised system was necessary, the manufacturers didn't put it in for fun.

Alternator or dynamo makes no difference, but the type of gauge and sender does. Early MGBs didn't have stabiliser because they used a different gauge and sender which are inherently self-stabilising. It uses a principle similar to a Wheatstone Bridge which is a method of determining the value of an unknown resistor (the tank sender) using three known resistors when the voltage is from batteries whose voltage falls over time. That had two drawbacks - one was the needle flips about all the time you are driving, the other is one of cost. However some say the damped gauge has its own disadvantage as it is so slow to return to a 'true' (a relative term) reading after you have gone round a bend or curve.

I'd be very surprised if your 71 Lotus had damped gauges and no stabiliser from the factory.
Paul Hunt

My 1971 Lotus does have a damped fuel gauge (rises slowly when first switched on) and should have a stabiliser, but since I didn't have a working one when I was rebuilding the car, I decided to try it without, and it works fine.


Mike Howlett

OK, I crawled up under the dash today. Took a picture of the stabilizer. It is so high up in there that I am not sure I can get to the screw to remove it. I am old and stiff. Could probably get to it better if I took out the seat and took off the steering wheel. But I digress. So instead I unhooked the green and light green/green wires and they drop down far enough for me to work with them. There is still a black ground wire on the stabilizer that I left in place.
I connected the two wires together. By the way one has a female connector the other a female. I guess that is so you can't hook them up wrong to the stabilizer. Anyway I could go over to my other garage and rob the stabilizer out of my '79 B which currently is not running anyway as the engine is out of the car. In the mean time I started the car and the gauges work the car was filled with gas yesterday. The fuel gauge needle does go all the way to full now and the temp gauge went to mid way after 4 minutes of the car running. I'll have to take the car out for a long drive and see if the gauges act properly. I plan on purchasing a new stabilizer from Moss. They sell electronic ones so I don't think I will have to orientate it in a level position. I will let everyone know how things work out. I was able to take a picture of the stabilizer before I pulled the green wires off he it is.


John Fraioli

John, The other wire to the blue canister is a radio suppression capacitor.
Allan Reeling

As Paul Hunt stated above.... that bypassing the stabilizer would make the gauges run "High". The fuel gauge did tend to look that way.

So I did take the stabilizer out of my idle '79 B. I decided to mount it on the lower edge of the fire wall so it would be easy to replace if I ever need to replace it again in the future. Hooked up the 3 wires and took the car for a long ride and all seems to function properly. I will still purchase one of those electronic voltage stabilizers so I can have it on hand.

See the attached picture and you can see where I now have the stabilizer mounted.

Allan, I assume that other wire (black) that goes to the suppression capacitor also functions as the stabilizer ground?


John Fraioli

Here is a picture of the gauges operating with the car running.

If that wire to the blue canister that Allan mentioned above is not the ground could it be that the stabilizer is grounded by being screwed to the firewall?

Also while I was under the dash I went ahead and bypassed the Dash Lights "Dimmer" Switch. Could not image why any one would want to make the dash lights dimmer. They are dim enough even when turned up all the way.


John Fraioli

John,
The stabiliser is not fitted with a "ground". The electronic units are not actually stabilizers, but voltage converters, i.e., virtually whatever Dc you put in comes out at 10v.
Allan Reeling

Allan..Paul's words from his first response to this thread.."The stabiliser does need to be "EARTHED", although if that goes AWOL the gauges read high."
John Fraioli

John, There are only two terminals. Because the stabilizer is a make and break circuit I suppose it could have lead to clicks on old AM radios hence the supressor. As for Paul's comment I read it as, "if the unit goes AWOL" not a ground that doesn't exist.
Allan Reeling

The original stabiliser was earthed thru the canister. A electronic voltage regulator is either earthed thru the metal tag that bolts it to the body or the centre pin.
Michael Beswick

"The stabiliser does need to be "EARTHED", although if that goes AWOL the gauges read high."

The above statement is correct. The earth to the stabiliser is required to pass current through a heating coil that is wound round the bimetallic strip. That causes the strip to bend, which opens a contact, which disconnects 12v from the heater and from the gauges. The strip cools, closes the ontact, and reconnects power to the heater and the gauges. If the heater is not functioning for any reason, a missing earth being one of them, the strip will never bend, never open the contact, and the gauges will get 12v (or system voltage which could be up to 15.5v) all the time which will make them read high.

The attached shows the effect of a faulty stabiliser - engine running on the left, engine stopped but ignition on on the right - a very significant difference.

All the electronic stabilisers I have seen are earthed in the same way i.e. through the mounting tag, just as on the electro-mechnical oneas. On the insulated surface there are the same two terminals each with two spade-type connections. However the disposition of these varies from all males, two males on the B terminal and two females on the I, or one of each on each connection. The original stabilisers can be either of the first two, the 'all male' being earlier. The third one is an after-market 'universal' type which is suitable for either type of harness.



Paul Hunt

My point, Paul, was that John was confusing the suppressor black wire for a ground. Not good if he added an actual ground from the "through" terminals! The old make and break stabilisers are indeed earthed via the canister mounting screw. The solid state voltage regulators have two forms of earth, the mounting tag itself and also a third contact leg. The mounting tag is quite a beefy thing and I always assumed it also acts as a heat sink.
Allan Reeling

Took me a while to realise that the references to 'centre pin' and heat sink were referring to a voltage regulator obtained from an electronics supplier as opposed to an instrument voltage stabiliser obtained from an MG parts supplier, current stock of which all seem to contain electronics.

There seem to be several ways of building your own using an electronic VR, including cutting the centre leg off and just using the input and output with the mounting tab screwed to the can to pick up an earth, or using all three legs plus the mounting tab as the output, which therefore can't be attached to the can, and with various numbers of resistors and capacitors. Moss show theirs with just the legs soldered to a circuit board and the mounting tab not connected to anything, so heat sinking isn't a requirement.
Paul Hunt

This thread was discussed between 31/08/2015 and 16/09/2015

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.