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MG MGB Technical - Temp/Fuel/Rev conter Gauges playing up

Hi I am a new MGB GT owner and this is my first post here, hopefully all you fine people can point me in the right direction.
My 71 GT has got a problem with most of its gauges.
The rev counter won’t go past 3k most of the time, the fuel gauge only shows half full when the tank is full and the temp gauge doesn’t get past quarter way to normal. The car has an electric fan with its own thermostat that I have checked and is coming on about 80 degrees. The good news is the speedo and oil pressure gauges work fine :-)
Has anyone had anything similar? Is it a power source issue to all 3 gauges? Or are all 3 playing up independently?
Thanks
M R Silcock

MR - All three get power from the same fuse, but the symptoms are different from a common power problem. the tach probably just needs the calibration pot tweeked or has some other internal problem. the fuel and temperature gauges both get power through the voltage stabilizer, so you may have a problem there (check the mounting of the voltage stabilizer for a good ground). Cheers - Dave
David DuBois

Is the 71 B fitted with a remote voltage regulator/stabiliser behind the dash which supplies these guages??
Ian Jackson

All MGBs with Smiths gauges have the remote voltage stabiliser behind the dash.

If this isn't earthed properly it causes *high* readings, not low.

The stabiliser only feeds the *fuel* gauge on a UK MGBs up to 1977, not the tach or temp gauge which should be the dual oil and temp. If could be a bad connection in the green circuit feeding the tach and stabiliser, both those two do come off the same point in a 4-way bullet connector behind the dash, the other outlets feed the indicators and the HRW. If the bad connection were on the fuse side of this 4-way then the indicators would be the first thing to not work properly, if they are OK then decent voltage must be getting as far as that conenctor where they split off. The easiest thing to do is check the voltage on the green wire of the stabiliser, which should be screwed to the bottom edge of the firewall behind the dash on the right-hand side. The other wire should be light-green/green, don't confuse it with the indicator flasher which just pushes into a spring clip and had green and light-green/brown wires.

If you have a minimum of 12v on the green then you have two separate problems afecting tach and fuel gauge. The light-green/green should show 12v switching on and off about once per second (after being on for about 2 or 3 secs after first turning on the ignition) and with it being on longer than it is off. If there is less than 12v coming out of the stabiliser, or its off period seems longer than its on period, then the stabiliser is faulty. There could also be a bad connection through the gauge and tank sender, to check these with the ignition on briefly conenct the two wires that should be on the sender together and the gauge should rise smartly to and above F. Don't leave these wires connected together any longer than you have to.

The temp gauge reading low is quite possibly that the after-market fan set is set too low, it shouldn't be coming on till about 90 degrees F (you surely don't mean 80C?) and MGBs don't need after-market electric cooling fans, especially in the UK. Or there is no thermostat in circuit or it is stuck open, or the gas is leaking out of the capilliary pipe to the gauge which is a known fault and does cause them to read low before stopping working altogether. With a working stat starting up from cold you should feel the head warming up gradually but the radiator header tank stays cool. When the temp gauge needle approaches N the radiator should suddenly get very hot. If they both warm up together the stat is missing or faulty, which isn't doing the engine or fuel consumption any favours.
Paul Hunt

Hi
Thanks for the responses. I haven’t been able to check anything yet but plan to next week when I have some time.
To confirm some areas, my indicators are working fine although someone told me my brake lights flicker some times. Not sure if that is related.

Regarding the engine temperatures I can confirm there is a thermostat fitted, whether it is stuck I don’t know. The temp I recorded was with an infrared thermometer after driving home from work so everything was up to temperature, the 80ish degrees C recording was from the top of the rad tank, around the top hose housing on the head and near the temp sensor location. On cooler days the temp gauge doesn’t move at all hotter days it might reach quarter of the way.

I will check the electrical readings as you suggested and report back.
Thanks again
M R Silcock

Brake lights on a UK 71 are hydraulically operated rather than by pedal movement so only come on when the brakes have started to be applied and can flicker with light braking if there are any pulses coming back from the calipers or slave cylinders. They can also have bad connections of course, common to other circuits off the same fuse or their own.

Definitely sounds like the stat is stuck open, even with a low-reading gauge it should read much the same on cold days as hot, just taking slightly longer to reach it's 'normal' point on colder days. You can check the stat by feeling the head and top of the radiator as the engine warms up from cold. The head should warm gradually, the tank should stay cool as the temp gauge rises, then when it gets to about 2/3rds the way to normal the radiator should suddenly get very hot. At this point you may well see the temp gauge drops a bit, then continue rising to its normal position. My roadster exhibits two or 3 of these slight drops and rises, the drop is when the opening stat allows a rad-full of cold water into the engine, which causes the stat to close a bit, then that gets hot and it opens again and so until until the whole system reaches an equilibrium.
Paul Hunt

Small Update:

The thermostat seems to be working fine, the head warms up and the radiator stays cool till the head is just about too hot to hold your hand on then the radiator starts to get hot also. This is long before the aftermarket fan comes on and the temp gauge has barely moved. I would like to replace the temp sender but for the life of me can’t unscrew it from the head. There is a larger diameter boss in the head that will unscrew but the temp sender won’t unscrew from it. Can the larger boss be purchased?

I found the voltage stabiliser was loose on the bulkhead. The fuel gauge rose to full when the stabiliser was unscrewed to the bulkhead and has returned to about half full (about right level) since re-screwing it to the bulkhead nice and tight. Does it need a good earth? I won’t know if that was the problem till I fill the tank back up.

I thought I had found that the problem with the rev counter was a lose wire to the back of it but it is still playing up. It has a bullet style connector pushed into a plastic moulding towards the bottom of the gauge. It doesn’t feel like it is making contact with the metal female connection, is that correct?
Thanks in advance.
M R Silcock

The after-market fan stat is set way too low, if it comes on before the radiator is getting hot then it is just wearing itself out for no benefit. The factory electric fans come on about 1/3rd the way from N to H. Many people are paranoid about MGB temp gauges, being used to modern cars where they never seem to vary. They can go almost to the H before you should be investigating (assuming very hot conditions and high loadings), if it isn't steaming or losing coolant it isn't overheating.

The instrument voltage stabiliser *does* need a good earth for it to work properly.

Any electrical wire *should* be making a good electrical connection with whatever it is connected to, of course. Ordinarily a bad connection will either cause the rev counter to not read at all, or go up and down between zero and the correct level. Consistently reading low, or not going above a certain level, is more likely to be problems inside the tach. A 72 should have have a male and a female bullet for the white ignition and sensing wires, as well as spades for the green and black 12v and earth connections. If one of the white wires isn't making a good connection then you would be having problems with how the engine performs, as that is the 12v ignition supply to the coil.
Paul Hunt

i dont know if it makes a difference but the car has been converted to negative earth with a single 12v battery. the tach says its negative earth on it.

i have filled the tank up now and the fuel gauge goes a little bit further than it did so i will test the sender in the tank.

sorry i think you read my post wrong about the temp, the electric fan doesent come on till after the radiator gets hot to touch. as i said in my first post the fan is coming on when the radiator is arround 80ish degree C which relates to an thermostat around the same temp. maybe its coming on a little early but thats not what im trying to sort out its the temp guage that is not moving much or at all. at the point where the thermostat opens where would a normally operating temp guage be indicating?

thanks for all your help.
M R Silcock

Yes sorry I mis-read 'after' as 'before'. Fuel gauge system isn't polarity sensitive, unlike the tach. Even less so the temp gauge as it is purely mechanical - if you have the dual temp and oil gauge that is which UK cars had until the 77 model year, although US spec cars were different. The dual gauges have to be replaced with the sender as the connection between the two is a gas-filled tube, the normal failure mode of these is first low-reading then non-reading. I'm not sure if you are saying you can undo the hex boss from the head but the pipe and bulb is stuck in the head, or the two come out of the head but you are trying to part the boss from the bulb. If the two are coming out then you exchange gauge, pipe, boss and bulb for new, just disconnecting the oil pipe from the back of the gauge. If the bulb is remaining stuck in the head then it doesn't matter if you wreck the pipe or bulb getting the old one out, just hand over all the bits when buying the new gauge.
Paul Hunt

This thread was discussed between 11/08/2009 and 11/09/2009

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