I will try to be as concise as possible with out leaving any thing out.

I bought a second jeep (blue 92’ XJ 4x4 4.0L) that has some interesting parts on it that I wanted on my (black 96’ XJ 4x4 4.0L) jeep.

I swapped the intake manifolds (99+ with TB riser)

Blue jeep runs fine. Put 500 miles on it.

Black jeep crank no start. No spark, no fuel pump prime when key on, and no communication to pcm (computer)

5 volt supply to cps and all the TB sensors now reads 0.6 volts.

I crack open the computer, after checking everything I could think of, and a resistor (?) has gotten hot enough to melt its blue plastic wrapper and burn up some of the board.

So now I have a new computer from napa but don’t know what fried the last one and am scared to plug it in.

With the negative battery hooked up and a multi meter between the battery positive and the uuuh positive lead terminus connector… what ever that is called. I get a voltage. [11.83V] out of the potential [12.30V]

If I remove the Fuse 19 (20A small) in the PDC (fuse box under the hood) I get a drop in that voltage [10.94V]. If I unplug the TCM (silver box by passenger’s knees) I get another drop in voltage [9.86V]. Any other plug or fuse has no effect.

I have a short right? But where or what?

 

I am just going to treat this thread like a log. I feel like i am screaming in a vacuum but still feel free to join in.

And now for something completely different:

The aforementioned voltage dropping rigamarole is no longer repeatable (it was repeatable).

There is no change in voltage or 6/12V test light between battery terminal and positive lead when I remove the TCM or fuse 19 in PDC.

Today removing a new fuse, 3 in the PDC (40A circuit A4), drops the voltage about one volt and shuts the test light off.

Fuse 3 in PDC feeds two black and red wires one to the starter relay and one to the fuse block (fuse 18 25A) in the cab. after the fuse the wire changes to brown and goes to the Rear Window Defogger relay.

pulling fuse 18 in the block cuts the test light off when connected to the positive lead. However, the test light illuminates when I touch any grounded component and the multi meter reads full voltage.

am i using the equipment in correctly? am i supposed to get a voltage between the positive battery terminal and any grounded component? no, right? why do i have a completely different problem today?

 

I had my PCM commit suicide because of a bad ground at the battery. I went crazy trying to find the cause. Then I learned that the voltage regulator in my alternator is in the PCM. With a bad ground, best we could decide is that the alternator was sending high, low, high low . Could not regulate. A few times before this happened before it burned up, the electrical instrument panel went crazy. I saw this before PCM finally burned up but so quick then gone. No way to see what happened. Couldn't find it. Final time that white smoke poured out and PCM nearly caught fire. I called the fire department it was so bad. Luckily all my wiring was ok. Check your grounds. Keep your battery contact clean.
With new PCM, keep ground on battery clean and tight, no problem now for 4 years.

 

well, i just plugged the computer in and every thing seems fine now. i guess that test light was worth the $5. so some voltage may be present with out a short? i guess if its not enough to trip a test light your cool. i will tell you if it blows up again. i guess my computer just decided to die when i plugged everything back up. so strange. i guess i could not find a problem because there was none....

 

Good luck. I had everything good too.
My new pcm (4 years now) is fine. But I check and clean my battery ground often.