I have this same problem with
I have this same problem with the two gages in my instrument cluster, but haven’t gotten round to it yet.
Two tests of the theory I have been meaning to run
“Caution first – on the C4, it is negative ground and all the wiring that caries current from the positive side of the battery is insulated and fused. Second, verify the ground on the instrument with the first test before you make the direct connection to the battery in the second test”
– With the gauge pulled from the panel so you can get to the backside. Turn on the ignition and verify that the lights sill increase your gauge readings. Then, attach a voltmeter between the case/ground of the gauge in question and the negative side of the battery. With the lights off, the reading should be a very low voltage (milli-volts.) Then turn the lights on and observe the reading. If the ground is week, you should see an increase in the reading on the voltmeter.
The second half of the test - acquire a large diameter wire with insulation ( 14 – 10 gauge), strip ¼ inch off both ends and connect one end of it to the ground/negative side of your battery. Connect (touching is fine) the other end to the ground/case on the gauge. This wire is a “strong” ground. With the wire connected as described, turning the lights on and off should no longer have an effect if the problem is weak grounding.