Strange electrical gremlins

[scottyboost]

98 Limited.
Crank car, runs fine.
Turn on parking lights, all is fine.
Turn on headlights, all goes to hell:
dash light 99% gone/dim
AC turns on to MAX
left headlight very dim as well as left marker light
left fog light is very dim, even tho fog lights are off
left rear turn signal light has a low glow

Turn off headlights, all is fine except everything has been erased from memory(computer display and radio).

Help!

TIA
Scott

 

[jgilbs]

sounds like there is a short somewhere between your headlight switch and the headlights/parking lights. check for any frayed/crushed/punctured wires. i know I installed some wigwags and accidentally ran the wire for the headlights close to the battery, and when i turned them on, everything went dim.

 

[RiverRat]

Check the Ground by the LH rear corner of engine compartment, at fender apron. It's near the starter relay(solenoid) on the fender.

 

[Opera House]

I assume the starter cranks ok. That would indicate a bad engine to frame ground wire or the tap off of the 12V going to and including the 40A fuse. You have resistance in the line that drops the voltage at higher currents. The fact that you loose radio memory indicates the problem is very close to the battery before it branches out. A voltemeter between bat neg and frame is one test with the lights on. Other is bat pos to 40A fuse. One of these will show a lot of voltage. Enough that test lamp would glow. Since the engine runs fine with lights on I wouls suspect a bad net to frame.

 

[scottyboost]

Did I mention that it isn't consistant?
I coudn't duplicate it today if my life depended on it. I even drove it around a while with the lights on, but I didn't have much time to diagnose. Thanks for your suggestions and I will put some of them to use tomorrow, time permitting. I did remove the battery and have it tested. It tested good and I cleaned the terminals, but no time to continue.

BTW, the radio and computer did not lose memory. The only change was that the "info center" changed to metric.

 

[scottyboost]

RiverRat is the winnar!!!!
I had already checked the short ground (about 4" long) that goes from the battery to the core support, as Opera House suggested. After reading your post, I searched for more ground points. There are 2 more "eyes" right behind the battery on the inner fender (just a few inches away!), and they were flopping around. I can't believe I didn't notice them yesterday. They didn't even appear to have been tightened at the factory because the "eyes" weren't bent like they do when you tighten down the bolt.

Thanks for the replies!