I made a number of electrical upgrades to my car this winter and while those are working well I'm having very odd problems with my gauges.

Strangest is the fuel gauge. When I press the brakes the fuel indication goes up! It does not peg--it goes up a bit.

My voltmeter reads high and the oil pressure reads low. I've verified that (engine running or not) is proper. I don't have a mechanical oil pressure gauge but fully believe it's a false indication.

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I did add a 3rd brake light. It's an LED array from a GMC truck. Thinking I made a mistake by getting its ground from the black wire in the rear lamp harness that serves the fuel gauge I gave the light a dedicated ground. The fuel gauge still goes up but not as much and not as quickly. +12V for the light comes from a new wire (added to the rear harness connector) that's spliced into the lead coming from the stop light switch. It stays on solid with brakes applied and a turn signal operating and flashes with emergency flashers.

I had a long conversation with DUB (THANKS!) and he gave me some suggestions but he too was pretty much guessing. I'm pretty much at a loss here and appreciate anything--no matter how wild it seems

LED daytime running lights are one of the things I added--their total power consumption is less than 5W. They work perfectly. Their power comes from the GAUGES fuse. DUB suggested that I unplug the relay to see if the gauge problems continue.

The rear view mirror with compass, garage opener, thermometer, etc. also gets is ignition power from the GAUGES fuse (tapped into the lead to the courtesy light delay module). I've disconnected it to find no change in gauge operation.

Since I completely disassembled the gauge pack I thought there was a possibility that I switched the resistors on the volt and oil gauges. I switched them to now find the voltmeter reading even higher and the oil gauge even lower

All of the grounds in the engine compartment harnesses and the cabin harness have been thoroughly wire brushed (terminal, mounting bolt and mounting point). While I can't be certain I would nearly swear that all of the front lights are now brighter. All of the factory splices (including grounds) in these harnesses now have solid copper soldered across them. I have not touched the rear lamp harness nor have I cleaned its grounding points.

I've checked, double-checked and triple-checked the revised schematics and find utterly nothing besides using the GAUGES fuse for very small amounts of power that relates to the gauges. I did add the HEADLIGHT warning to the gauge pack but it uses two dedicated terminals/traces. The indicator works just as it should and it is a new printed circuit with no loose "ears" or traces.

 

Dirty grounds or weak ground wire in the rear is my guess.

 

SOLUTION FOUND! Grounds were the problem. Which grounds? The body grounds not yet cleaned. Now ALL of the body ground points have been cleaned.

Cleaned the rear connection by the antenna. Also cleaned the connection near the starter.

Gauges all working properly.

Blower is delivering much more air. Rear lights are brighter than ever.

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MORAL: When removing some original grounds to find them very much in need of cleaning, continue and clean ALL of the harness ground pointṣ. Clean blower resistor and connector. Clean ALL connectors under the hood while you're at it

 

We suggest to a lot of posters to do just that. Doubtful that many actually do it. But, you are proof that taking the time to clean them all is worth it.

 

Definitely worth cleaning all grounds.
Battery to body ground.
Body to engine ground.
Body to frame ground.
Take a multimeter and set it to Ohms. Check the terminal wire and where it's supposed to ground to. Anything above .02 Ohms should cleaned and rechecked. Solves electrical problems and solves them before they start.

 

Heard loud and clear! I was one who thought, "Why bother disassembling and cleaning an untouched factory grounding point?" WRONG! I also though regarding connectors, "Why bother with deep cleaning when things work?" WRONG AGAIN!!!

In addition to my ongoing suggestion to replace vacuum lines to address many starting/running/idling/performing problems I will repeatedly remind that the most vexing electrical problems are often the result of dirty ground and other connections. More importantly I will stress that correcting ground/connection problems in one area is very likely to create new problems in areas yet to be cleaned.
I'll never forget a 200-mile or so test run after the major vacuum hose replacement found one of the few hoses not replaced--the little hose at the transmission vacuum modulator valve--failed (fortunately less than a mile from home).