MGB Register Forum

Just found this site having returned to MGs after 10years away. Recently bought a nice 67 GT and started fettling it. Two oddities I’ve not come across before and a temp gauge that always reads too hot, in fact it’s off the scale and despite the engine temp being around 75-90degrees C

Second is a converted rev counter that only reads between 1200and 2500 rpm. ( converted from pos to negative earth. )

Both are original to the car so I don’t want to replace unless I have to but not sure either can be diy repaired.

Cheers. Ps recognise a few names from my MGOC days.

Hi Gary,
Have you fitted an electronic ignition system or have anything other than points and condenser in the distributor? If so the type of tachometer fitted to MGBs until about 1973 will not work correctly.
The temperature gauge will be one that uses a capillary tube from a sensor in the cylinder head. This type you can't repair easily (although Speedy cables might be able to do so), but you can test it.
Firstly, though I would change the thermostat as these do fail and are more likely to fail than the gauge or sensor. You will lose some coolant doing this so while it is low, take the sensor out and put it in some boiling water. If the water is boiling the gauge should read 100 deg C, a bit less if it takes a few seconds to get the sensor into the water. Be very careful undoing the sensor, as it is all too easy to damage the capillary tube, which will mean the gauge will have to be replaced, which isn't cheap.

The sensor is on the cylinder head under the thermostat housing and on the same side as the plugs. The Capillary tube is a thin tube with what looks like wire wrapped around it, and be very careful with it!
If you think the tacho or the temp/oil pressure gauges need repairing, contact details for Speedy Cables is https://speedycables.com/

hi Pete, thanks for the reply.

I'm quite familiar with MGBs having owned 8, restored two and converted two to V8s but Im grateful for the help. I'll get to the bottom of them soon enough but they are both problems I've not encountered before.

car is on points, temp is physically correct as tested with IR thermometer.

Think I recall you from MGOC forum years ago??

Hi Gary,
Given the temp is correct, then try replacing the thermostat. I think this is most likely this is the cause.

However another question - does it lose coolant on a regular basis?

Hi Pete

Give the temp is correct I'm struggling to see why the gauge over reading will be related to the thermostat. It reads just over half even when the engine is switched off, I think the hair spring has weakened and the gauge is duff. No it doesn't lose coolant.