Panel voltage stabiliser?

[Dave Wheatley]

I've been trying to get my head round how this works, but not getting anywhere.

Up till last weekend, both the fuel gauge and the temp gauge were working fine.

The fuel gauge appears to be working fine, showing the same reading with ignition on/engine running and ignition on/engine not running.

The temperature gauge however shows a very low reading at normal running going down to nil reading when the engine is getting very hot. Gauge used to show about half way at normal running temp.

Anybody come across this?

Could this be a faulty voltage stabiliser, or or a dud temp gauge?

Car is standard 1978 GT.

[Michael Barclay]

Hello Dave.
Both the Fuel gauge and temperature gauge are powered by the voltage stabilizer so if the Fuel gauge is working correctly I don’t think the issue is with the stabilizer. However if you has access to a multimeter, it is easy to check. The stabilizer has 2 wires going to it. (Green an Light green/green) Measure between earth and the Green wire you should read battery voltage. Then measure between earth and Light green/green wire. This should give you a stable 10 Volts as this is the feed to the instruments.
If this is Ok then I would check the temperature sensor. Disconnect the wire to the sensor and If you have a multimeter put it on to resistance range and connect the meter between earth and the sensor terminal. I do not have any actual data for the sensor but usually the resistance decreases as the engine heats up. No change between when the engine is cold and hot means the sensor is faulty.
May be a stupid question but are you sure the radiator thermostat hasn’t failed in the open position?
Hope this helps.
Best regards
Mike Barclay

[Vic Butler]

The way to test whether the gauge is faulty is to disconnect the wire from the temperature sensor and with the ignition on short it to earth. The gauge will read hot if it's working. If that's ok then it's either the sensor or the thermostat. Is there heat from the heater?

[Charles Farran]

Most unlikely to be the voltage stabiliser as the fuel guage works fine & the feeds to each guage are piggy backed. My money would be on the temp sensor or a dodgy thermostat. In addition to the test earlier if you hold the wire that connects to the temp sensor to earth the guage should read hot.
Silly question, but have you checked the coolant levels as you can get strange readings on the guage (it happened to me some years ago when i was losing coolant due to a leaking water pump)?
Cheers, Charles

[Dave Wheatley]

Many thanks for these posts.

When I get out to the car this morning, I will do all the checks mentioned, and report back.

[Dave Wheatley]

Did all the checks mentioned above.

Turned out to be low coolant level. Expansion tank nearly empty, and level slightly down in the thermostat housing.

Topped up less than half a pint in thermostat housing, and a bit more than that in the tank, and temp gauge back to normal.

I'm surprised that such a small decrease in coolant level could cause the gauge to malfunction to that extent, but it did.

Next job to sort - where did the coolant go to?

Thanks again to all.

[Charles Farran]

Check that you are not losing coolant from the small hole below the wter pump pulley as this is often the place on mgbs which are not used regularly. Other than all hose connections,check that the water valve on the side of the engine block is not weeping. ( Again a known regular fault area - i am on my 3rd or 4th in 39 years of ownership). The pressure cap on the expansion tank ,if not sealing also causes leaks.Hopefully your heater matrix is not weeping - a pig to change (one of the worst jobs in my opinion!
Good luck!
Cheers, Charles

[Charles Farran]

If there is a leak from the bottom of the water pump hole,then unless it stops after regulatr use then a new pump will be required!
Cheers, Charles

[Vic Butler]

If it is the water pump it's a very easy job to replace. I 'm on my 3rd. I replaced the heater valve 5 years ago as a matter of course when I fitted the second modified cylinder head (the first one cracked.)

[Dave Wheatley]

Ta for the replies. No obvious leaking ploints. Tank cap holding good pressure overnight.

A couple of weeks ago I spent a long time tuning the carbs, half an hour with the engine running and the electric fan coming on intermittently. It's possible that I may have lost some water then.

Anyway I will just run the car and monitor the coolant level for a while.

I'm still surprised that the gauge behaved like it did with such an insignificant coolant loss.

Anyway the car is now my sole means of motoring. During the winter I used the Standard 10 daily, and have layed it up for the summer pending comprehensive checkover.