I have 2 problems at the moment, both happened after I accidentally tried to start the truck without the large ground wire hooked up. Of course the small ground to the core support was hooked up.
1- A/C clutch will not engage. I have power at both the blue and brown wires at the compressor, both at 13.5 volts when running. How/where does the clutch ground? Through the no longer available temp sensing cycle switch? Looking at the wiring diagram it does not appear to ground through the ECM.
2-Wait to start light stays on, replaced ECM with a spare and still the same. Speedo works which is grounded through the ECM so I would think the ECM is properly grounded. Thanks

 

I would look for a ground wire from the engine block to the firewall or frame either burned off or roasted.

Best guess is one of those tried to handle the load of the starter without the negative cable hooked up.

 

The battery to core support wire cooked so I replaced it. Got the A/C compressor to start buy bypassing the clutch cycle switch with a relay. I'll check for more ground issued. Thanks

 

I wonder if the power surge might have back-fed one of the temp sensing switches.

You may need to debug the PCM system and it's I/O from the git go. There's a Chrysler manual that has maintenance analysis procedures for all that. I have one.

 

I've got the 1991 FSM but not does not cover the PCM and related stuff. Looking back I should have got the 92 FSM. I may just ground the speed sensor for the speedometer and put the grids on a switch and do away with the PCM. Have not had a water in filter sensor in years so and I believe these are the only 3 things the PCM does in a 91.5.

 

Alternator, Tach drive, and OD control.

 

Thanks for the info.My alternator has external regulator, no factory tach and manual trans so it should be simple to eliminate, I hope.

 

92 is totally different, that FSM would just screw you up. For your 91, do you have a printed manual or the CD Both my 90 and 91 CD's have the diesel supplement, which brings in all the diesel specific stuff.

Not sure you can read this, but here is the pinout for the 91.5 PCM.

 

Check the crimp weld on the grounds for the starter relay, wif, fuel heater, etc. IIR, there are four or so black wires that all come together sorta behind the intake horn, there is a crimp weld, and then maybe two wires going to the ground terminal screwed to the intake . . . It is a good spot to burn out & give intermittent grounding for at least some of those systems.

 

I've got the printed manual but no diesel supplement. I did find the 91.5 electrical in the sticky and downloaded it which is helpful. So far I have found no bad grounds except for the one I replaced that goes from the battery to the core support. I did check the ones behind the intake horn and they look OK I will pull them and check closer. Thanks.