Temp gauge rises with headlights on

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plymouth73

plymouth73

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modesto ca
1973 Plymouth scamp 318

I haven't heard of this one but apparently it's common. My temperature gauge jumps up immediately after I turn on my headlights and that's with a non runninge engine. I'm thinking it's engine to chassy ground. Do the headlights have a ground wire or ground to chassy? I will look at repair manual later when I get home.

Any thoughts?
 
You said jumps up. Rapid needle movement all the way to max ?
Can you vary how fast/far that needle moves with the dimmer rheostat ?
Does the fuel gauge respond to headlight power also ?
 
Temp gauge moves up to 120 and stays there with headlights on. As soon as I pull the switch. Also I should have mentioned the gauges are after market and electronic. The fuel Gauge does not change with headlights on. 6 gauges daisy chained for power .
 
Aftermarket gauges. You didn't say that in the start. I'm out. Good luck
 
I have all aftermarket gauges and wiring. I have a ground to the body from the headlight pigtail at both headlight shells. Hope that helps.
 
plymouth73 said:
I'm thinking it's engine to chassy ground.
Click to expand...

So? CHECK IT LOL Hook a meter to the engine block and to the battery NEG post on low DC volts. Turn the key on to activate the temp gauge. Now turn on the headlights. Any reading? Should be zero

I doubt this is the area of the problem. An engine ground that bad would probably give you other problems.

More likely this is a problem with gauge / panel grounding. Some of these gauges are what is called a 'bridge' circuit. The gauge itself (or ground terminal on the gauge) must be grounded.

Another thing you can try is with the key on, and headlights on, run the instrument lamp dimmer up and down and see if that changes the gauge.
 
I did the engine block to negative battery post test with temp gauge activated and the meter read below 1 volt.

with the negative cable off of the battery and nothing powered on it read 11.5 volts , that's hard to believe . don't that make a direct short?

also during testing I found that the ballast resistor is not getting warm with key in on position but the ballast ohms out correctly. I am at my wits end
 
I just dealt with the same problem. Make sure there is a good ground from the negative battery terminal to the engine. Preferably to the front of the head. Scrape all the paint off and use star washers. I also ran a ground from the battery negative side post terminal to the chassis in the engine bay, also with a star washer and my issue was resolved.

plymouth73 said:
I did the engine block to negative battery post test with temp gauge activated and the meter read below 1 volt.

with the negative cable off of the battery and nothing powered on it read 11.5 volts , that's hard to believe . don't that make a direct short?

also during testing I found that the ballast resistor is not getting warm with key in on position but the ballast ohms out correctly. I am at my wits end
Click to expand...
 
plymouth73 said:
I did the engine block to negative battery post test with temp gauge activated and the meter read below 1 volt. with the negative cable off of the battery and nothing powered on it read 11.5 volts , that's hard to believe . don't that make a direct short? also during testing I found that the ballast resistor is not getting warm with key in on position but the ballast ohms out correctly. I am at my wits end
Click to expand...


The test with the cable unhooked doesn't tell you anything

Detail this test you made which showed 1V

"Exactly." Where exactly were the probes, and what in the car was turned on / off?

And.........where was the meter set? (Post the model or a photo of your meter)
 
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