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Posted by: Bones May 20 2012, 05:36 PM

Hello out there. I just noticed that my VDO "graduated" temperature gauge (the aftermarket one with the temps on it (120 to 300F)) gives a reading that "jumps" up 20 degrees when I turn on either the parking lights or the head lights. I have tried wiggling wires and tightening the terminal on the sender. I know the sender's resistance goes down as it warms up, since grounding the wire out pegs the needle, so I am thinking that it must be some "partial short" involving the harness (tail lights?). Is there a place in the harness that is particularly vulnerable to this? Is there some other "weirdness" that occurs with these cars?

Car is a '73 2.0 four cylinder, pretty much stock.

Thanks for any guidance.

-Jim

Posted by: kg6dxn May 20 2012, 05:43 PM

Common problem for an old electrical system. Voltage drop in the system is the cause. You may be able to improve the conductivity at all the grounding studs around the chassis. Clean with small wire wheel. Adding an additional ground from the battery to the engine will help too.

Posted by: windforfun May 20 2012, 07:12 PM

Your local ground is floating. You shouldn't have to add an additional wire.

Posted by: Bones May 20 2012, 07:46 PM

Thank you both. I'll try to get back in there tomorrow and polish and tighten every ground I can find, starting with the trans.

Posted by: 76-914 May 21 2012, 11:23 AM

IIRC, the instructions say to use a dedicated 18ga ground. Also, where did you pick up the 12v source?

Posted by: Bones May 21 2012, 12:29 PM

I did not install the gauge. It was put in 3 or 4 POs ago and I never noticed this behavior until yesterday. Anyway, I cleaned the ground strap and hardware at the back of the transmission, and the problem seems to be gone. When the problem was discovered, I was testing the single wire to the sending unit, and taking that terminal off the sender and grounding it sends the gauge to maximum, so I think the ground is through the sender. I haven't examined the back of the gauge, but would guess there is only the one wire to it (plus one for the bulb to illuminate it).

Jim