I recently restored a 69 Camaro, 4 speed, with added factory console gauges. I replaced the console harness with a new one from Classic Industries. The temperature gauge works fine and registers correctly - except - when I turn on the headlights, the needle pegs the upper range stop and when I turn the lights off, it goes back to normal temp. I checked how the wires were hooked up and it was fine. The old harness that was on there was an automatic harness but had the neutral safety wires tied together and the console ****er lights tied off. Everything else works fine.
I do have a ground cable between the battery cable and the fender, but I an not sure that the headlight switch is grounded. I think that I got the ground washer and wire back on it. I will need to check. The car is 90 miles from my 'other house' so I will check that when I go there on Friday. Any other thoughts.
ground strap from front frame rail near the starter to the firewall? Also from back of the block off a valve cover bolt to the firewall near the distributor. That takes the ground into the body for guages and lights. The rubber frame bushings isolates the interior grounds, so without the straps, weird things happen with elec. system.
Click..is Jim..former owner of a 69RS-LM1 350/255 - 700r
"There will come a time when you believe everything is finished,
That will be the beginning." Louis L'Amour
I don't remember how the console gauges are illuminated, but the problem sounds like the gauge illumination is influencing the temp gauge. I've seen this many times in vintage GMs. If you can make the temp gauge read normal, by dimming the dash lights to 'zero', check the bulb sockets and their grounds, and the ground for the gauges, in the console gauges.