My instrument lights go out when the headlights are turned on in my ’89 GT. They turn on when the ignition is turned on and remain on when the marker lights are switched on but turn off when the headlights are switched on. The “potentiometer” functions normally whenever the instrument lights are on and the digital portion of the display remains lit when the rest of the instruments go dark. Turning the fog lights on and off makes no difference to the symptoms.

I suspect the light switch itself after staring at the schematics for awhile, but I thought that I would ask if there is some seemingly unlikely cause for this.(Finding that a bad ignition switch caused my headlights to extinguish when I stepped on the brake pedal makes me a little uneasy with a simple diagnosis…)

Thanks,

Bob

 

I hate it when this happens, but the symptoms are gone: the problem cleared while testing. I wanted to confirm the symptoms in a more thorough manner (I realized that among other things, I had not confirmed that the marker lights remained on when the headlights were lit) before dismantling things, but everything worked normally.... for now, anyway...

Nonetheless, I would still appreciate knowing if this could be due to something other than the light switch.

 

Classic advice number one: Clean your grounds. Ground problems probably fix 75% of electrical gremlins. Of course, 83% of statistics are made up on the spot

 

Absolutely. Search for Wally's yearly electrical maintenance.

Since I started doing that (well, I do it every couple of years anyway) I have pretty much stopped all the electrical system voodoo.

 

Good advice. BTDT.

I cleaned up all of the grounds (once again) and positive connections (to include alternator and re-termination of the cables on the starter), as well as the removal and general reconditioning of the central electric panel, within the last six months in the process of finding that the near-perfect looking ground strap from the battery to the chassis was bad. (Actually, I found the bad strap first, but did everything else that I could think of for good measure....)

The only other electrical problem (that I know of other than the digital dash displaying only in German, but that is not strictly an electrical problem...) was low voltage output when the alternator got heat soaked on a hot day, which was cured with the installation of a new regulator/brush assembly from Roger.

I am still inclined to think that the switch was/is at fault. I seldom drive the GT at night, but usually have the marker lights on during the day, so the light switch had not been turned to the headlight position for a number of months as I don't usually bring the headlights up when the car is washed either.