I've been looking at a friends 96 explorer ( 4.0 motor)because he has an electrical issue going on and I can't figure it out.
Symptoms:
There are multiple things going on at once. The headlights, signals, and blower motor (hvac) won't all work at the same time but will work individually.
The speed of the signal flash can be controlled by using the interior light dimmer switch. Lowering the dimmer switch speeds up the signal flash until the flasher relay will chatter.
The clock on the radio flashes with the signals. When the signal flashes the clock light goes out.
When starting the truck and turning the lights on both turn indicators on the cluster light up very dimly.
I could get everything to work if I went out and hit the top of the relay box for the hvac motor, it would then kick on. Replaced relay but no change. I figure it's a voltage issue.
With the key on if you turn the hvac motor on the clock light goes out.

When it first happened:
My buddy said the first time it happened they were driving down the road and his son plugged his dvd player into the power port. All the lights in the truck shut down and he said the truck jerked violently as if you put it in park for just second. He drove it home and a few minutes later the lights had came back and everything seemed to work for a while but have progressively gotten worse to the point it's at now.

What I've tried:
I checked the power port since he related it to the dvd player incident. Checked for voltage and had it, checked for a short and there was none.
It looked to me to be a ground issue. I've chased every ground I can find on the truck with no luck.
Pulled individual fuses to different components hoping to isolate the problematic circuit but no luck.
Pulled the flasher relay just to rule it out with no luck.

I'm just out of ideas. I started searching and reading but can't find anyone with these same symptoms. The GEM module is the only 1 component I can think of that might deal with all of these circuits although I'm not sure why it would effect voltage to the hvac motor. I'd appreciate and welcome any ideas or input you guys could give me.

 

I think it's a ground problem too. If you want to experiment, take a scrap piece of wire and bolt it to something on the firewall sheetmetal, and then run the other end over to the battery negative and see if that cures it. If it does, you can try to research more how the sheetmetal gets grounded from the factory, you could just leave the new wire in place.

 

Thanks for the reply, I appreciate it. I ran my own ground but I ran it from the negative battery cable down to the starter just like the stock one ran. It didn't help. I did have thoughts of pulling the starter to see if maybe it's corroded between the starter and the block but what's really the chance of that happening? Not to mention that starter looks like a PIA to pull off on a guess. Not sure why they put the connectors on the toughest part to reach, guess the engineers probably pay a mechanic to fix theirs.

 

It definitely seems like a ground issue to me but I've went over all the grounds that I could easily get to without tearing out interior pieces. The only ground that looks even remotely corroded was the ground on the starter body itself that grounds the starter and I kinda ruled that one out since the starter works fine although I did crawl under there and hit it with some sandpaper where the ground makes contact.

 

You have to remember the engine and the tranny are mounted in rubber mounts. The factory usually runs a ground wire from the engine block to the sheetmetal. The sheetmetal ground is the one that is giving you the problem. Go from the engine block to the firewall sheetmetal, or go from the sheetmetal to the battery negative.