As I'm working on my replacement engine to replace the engine that was killed by the KDP I started tracing down a short in the engine bay wiring harness. I've unwrapped most of the harness and will probably need to remove the entire harness to make the changes I need to make. The PO spliced and changed a lot and it's quite a mess. The PO bypassed the entire wiring for the injection pump solenoid and strung a wire from the ignition switch to the IP. When the truck is running the fuse box is always hot and the brake warning light and some of the other dash lights are always on.

I removed all the mechanical parts related to the cruise control, including the wiring harness that goes into the cab. Now my question as it relates to my 1991 W250, what is this box in the picture? It's located behind the battery inside the fender well.



If it's related to the cruise control I'm just going to delete it and all the wiring attached to it. If it's not related to the cruise control, what is it?

I also have a complete wiring harness from a 1991, non 4WD, that is close the the harness I have but is different because of the black box in the picture and some differences related to the sensors on the head that regulate the IP timing advance cold start mechanism.

 

Anyone know what this black box is?

I'm guessing it's the control for the injection pump timing advance and pre-heater. Just a guess.

 

It's your PCM i believe. It controls the cruise, whether your alternator charges, gauges, etc.

 

Thanks for the reply! So I guess I won't be deleting it!!!! Unless.....

I do have another 1991 harness that's somewhat different from the one in my truck and it does not have the bunch of wires going to that PCM. It has a smaller (about 4" by 5" by 1") black box that is the PCM for the truck it came out of. I'm wondering if I just use the new harness and that black box would I be able to delete the original PCM pictured.

I'm attempting to get rid of a short in a mess of wires in the main engine bay wiring harness. It's been cut and spliced and modified so much it's hard to tell what wires go where.

 

The PCM for the pre 92 truck is pretty basic, I believe that it does have some control on the cruise, but my understanding of it is that it actually controls very little on our trucks (91 and older, esp. manual trannys). Of course the problem is there are a whole lot of opinions on how much it controls. Even though, for example, it receives a signal from the VSS regarding vehicle speed, AFAIK, it doesn't do anything with it on manuals, and (once again) it may not do anything with autos (three speed vs four speed). If you can put your hands on a good wiring diagram (there are a couple floating around to be downloaded) you can see what signals it receives, but (there's that word again) whether or not you can easily bypass it is open to interpretation. Easy is in the eye of the beholder. There are a couple references on here to cleaning the plugs that go into it and getting some solid gains in the electrical system, and I know of one poster who swapped out the PCM from a same year gasser and it worked fine. I do know that the pins in it change since the harness changes in 92 (tach lead, CPS, etc, so if you have to swap one out, make sure you get the right year.
I think if you're swapping a harness, I'd prolly yank everything I could from one to the other, but make certain that my build dates are correct. Remember, just because the title says its a 91, that doesn't mean its a 91. Check your VINs, and compare the harness to a known schematic before pulling any wire...

Cree

 

Great info and advice, thanks. When I pulled the harness out of the donor truck I thought it would be a straight replacement; both 1991's, but they are quite a bit different.

I'm waiting on a wiring diagram from another member here. I've found a few on this site but nothing that shows the differences between the 1991 models I have. I've got vin numbers from both vehicles if they are ever needed.

I also found the sticky about correcting/removing the fusible (fuseable?) links and I know that's part of the problem. Didn't even know what those were but I'm pretty sure that's a main part of the problem.

I'll take some postable pictures this weekend as I do the work and post them as I have additional info or questions.

 

About to tear into a wiring harness swap on my 89 cummins, did the engine bay harness the other day. Smoothe sailing. Ripping into the dash now. Hopefully by swapping out my melted wires stops my harsh battery draw. The truck acts like the alternator isn't putting out and only get a couple days out of a freshly charged battery. Any tips/thoughts on this headache would sure help! Thanks

 

I'm not a 100% for your year but does it have an external voltage regulator on it had the same problem with my 91 for awhile and replaced it and problem fixed