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MG MGB Technical - fuel and temp gauge problem

I got obsessed with brighter gauge lights to help my 50+ year old eyes. After reading about people who painted the inside of the gauge housings white I decided to try that.

So I removed, disassembled, painted,reassembled and reinstalled them. I made a mistake with the temp gauge and let the post touch the housing when I put it back together, which caused a short and blew a fuse. I corrected the gauge and replaced the fuse, but now the fuel gauge reads less than half when full, and the temp gauge does not move. Both move across the range when connected to a DC voltage source varied between 3 and 12 volts.

So I am wondering if I either damaged them when working on them or if the short damaged something, including maybe the voltage stabilizer. Any ideas?
Btw, lighting is a bit brighter, but I'm not yet sure it was worth all this trouble.
Mike
mike

You may have damaged the voltage stabiliser. This, despite its name, actually switches the supply voltage off and back on again about once per second (needs a couple of seconds to 'warm up' when first turning on the ignition during which time it is 'on' all the time). It stabilises the voltage to the slow-acting thermal gauges by varying the relative amount of time it is on to the amount of time it is off, called the 'duty cycle'. With higher supply voltages it is on less and off longer, with lower supply voltages 'tother way round.

But that should affect both gauges equally, i.e. if a full tank only reads half then an engine at normal heat i.e. half-way up the gauge should read 1/4. So you may still have a problem with either or both gauges or their connections as well as possibly with the stabiliser. With both gauges you can *briefly* ground the sender wire just long enough to see of the gauge starts moving up to full scale deflection or not. Don't leave it on any longer than necessary as it drives more current through the gauges and stabiliser than normal. To test the temp gauge and its wiring ground the green/blue connector at the sender on the head. To test the fuel ground the green/black at the tank.
Paul Hunt

Always disconnect the battery when working on the dash. There is so much jammed into a limited space that it is very easy to short things out when trying to assemble anything into or behind the dash.
John H

Thanks for the help. After driving around more I find that both the temp and fuel gauges register, but read low. Fuel is just over 1/4 when full and temp is just over the cold mark when warm. since they are reading similarly I am thinking it is the voltage stabilizer.
Also when i ground the leads as suggested the temp gauge goes to full range, while fuel does not move. Odd since fuel indicates, just reads low.
I suppose I will replace the stabilizer and see what that does.
Mike
mike

If the temp gauge goes to full scale when you ground the temp sender but not when you ground the fuel gauge sender then that indicates to me that you could have more than one fault, one of which *may* be the stabiliser, but then maybe not. Personally I'd be investigating why the fuel gauge *doesn't* peg when its sender is grounded first, before spending any money. But then I'm just tight.
Paul Hunt

Mike, check to be sure that you replaced the gauges into their proper locations, i.e.: fuel gauge not plugged into the temp gauge hole (and wires) and vis-versa.

I know that you probably didn't do that, who would be dumb enough to mix them up like that? I had really similar problems with mine and let's just say I really felt dumb when I figured it out.
Gerald O'

This thread was discussed between 25/10/2008 and 10/11/2008

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