Aftermarket Temp Gauge False Readings..

I have a B&M black out temp gauge installed and from what I can tell it reads accurate except when I turn on something electrical (lights, heat, etc..). I know a bad ground can cause false readings but would a bad ground cause the reading to differ only when turning something electrical on?

For example, when I'm going down the Interstate my temp usually reads around 210, if I turn my heat or lights on it will immediately jump to 230 or so. I can turn that device back off and it will instantly drop to the lower temp again.

If the sending unit is set up (designed) to increase resistance as temp increases, a voltage drop will show up as a temp increase. I would take a voltmeter and test directly at the battery with the lights on and off at idle. Your alternator may be borderline. You should get at least 13.5 volts. Of course check your battery connections and make sure they are shiny clean inside.

How do you have it wired up? Does it have it's own voltage source or is it wired up in series with something else? Depending on where you have ground, I'd relocate it elsewhere. Closer to the battery the better.

I'll check the volts from the battery today. I know the ground connection for the battery is in rough shape I just haven't got around to change it.

As for the hook ups I have the power connection tapped in with the radio. The ground is also under the dash.

I'll vote ground on this.

And now I get to tell a story: Brother's 2nd Ex had a Mustang II with 302 V8. She had to commute about 12 miles a day, with one stretch with a mountain that was essentially a 4 mile climb. She complained the engine was overheating. Checked the system, everything fine. Suggested a fixed flex-fan and she said Ok--still overheating. Suggested a larger core radiator and she said Ok--still overheating. Now, she has paid for the parts, my labor free, but we are both getting frustrated with this. So I said fine, let me drive it over the mountain. Up and over, all the way to her office--ZERO problems. Drove it back to my shop--ZERO problems. Guess what she tells me--IT ONLY OVERHEATS WITH THE HEADLIGHTS ON!

Installed new ground strap, "overheating" problem solved.

I have the B&M gauge and it works perfect regardless of what I turn on or off. No changes in the reading.

My guess is a ground problem, as they will create some very weird problems.