I am at a loss.

My 1990 XJ is reading below charge when running. no hesitation starting and drives fine but there is a delay in my turn signals, my charge and oil pressure reading drops heavily when headlights are turned on, but my engine temp gauge goes higher????

I have tested the batt and alt. both are fine. I don't know where to begin to even diagnos this problem let alone fix it. Any help?

 

Might be a short on the light switch or loose contact on the dash. Check fuse box and replace busted fuse.

 

replaced all the fuses. still nothing.

I was wondering if one of the relays under the hood passenger side might be the culprit? or maybe the power distribution box under the hood?

 

Also the voltage drops w/o headlights on occur while driving, but return to normal (which is still below charging) at idle. the turn signals will flash when the brake is released but freeze while pressed.

 

My 89 did that alot but sporatically. I had it tested when the gauges read low and everything tested fine. I ended up cleaning all the contacts on the plug and the ribbon on the back of the gauges that seemed to fix it

 

how about a busted ground strap. broke about 3 inches from the engine mount point and about one inch from the body mount point... hopefully that is the only thing.

 

XJ's are known for bad grounds.

Fix it with a good replacement and let us know if it cures the problem.

If it were a bad ALT..the PCM/ECU will give you a fault (CEL)

ALSO: Do not always trust those "free" checks at places like A/Z.
I had a bad ALT that tested good at A/Z, Pep Boys and a National.

 

it was the ground strap. replaced it with a battery to starter cable (4g, better contacts and protected by casing). also ran a ground (same cable type) from the battery to frame. best of all, it was cheaper to use the batt/starter cable then it was to buy a ground strap....