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Temp Gauge Problem Update And Solution

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I had a Topic a while back about my temp gauge which would work for 10 min then go full scale, even with a new temp sender and a different instrument cluster.

After staring resentfully at the disconnected temp gauge for a week or two, I had another go at it..

I ran a couple of wires under the bonnet to the temp sender and cable, and had a switch on the passenger seat that let me connect the sender to the gauge.

When the gauge went right up, I stopped and did some measuring with a multimeter. Voltage from the gauge was 12 volt. resistance of the temp sender measured against the earth on the cig. lighter frame was- ahh. funny- 70 ohms one way, with the probes reversed, minus 120 ohms. ???

I don't think so, a minus resistance is getting into quantum mechanics,.......

Clearly a voltage was getting between the temp sender and my ground connection.

So, an earthing problem, not uncommon, but usually a bad earth is a resistance to earth, bit of a puzzle where the voltage rise was coming from.

Anyway, I got out my old temp sender and soldered a wire onto the bottom half of the flats, so I could still get a socket onto it, and fitted that, earthed directly to the Battery neg terminal.

Problem solved.:hehe:

I guess my mistake was to assume the engine block is one conected metal lump at the same earthed potential. I suspect the temp sender is screwed into the water pump or the thermostat housing? with the gaskets, its only the bolts earthing to the block, throw in a bit of steel./ aluminiun corrosion, that might do it.

Anyway, anyone else with a temp gauge problem, try earthing the sender unit

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funny you should mention , when fitting a new radio was having issues with interferance.

my earth (the large bar in the dash) and the radios original earth would vary between sometimes up to 5v.

traced it to the fuse box under the bonnet , a fair amount of corrosion ~__~.

some fuses and stuff refused to come out (didnt want to break anything), so i have just added extra earth straps from neg terminal to body , wiring block and engine. all sorted.

I guess even good cars can have bad earth at old age

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