What started out on a '95 3500 as no cruise control has escalated to no charge, tach, speedo and TC lockup. I would say bad engine speed sensor which is correct but no, the problem is no 8 volt supply from the PCM to the to the ESS.
Bad PCM right?
No, the PCM and another junkyard PCM work just fine in another truck just not on the problem truck. All the 12 volt inputs, outputs and grounds check out fine right at the PCM. The connector pins are straight. Only thing that doesn't work is the 8 and 5 volt PCM outputs measured right at the PCM.
I'm at a loss, dealer is at a loss.
Any ideas?

 

Believe it or not, it is your ignition key switch. You are grounding the ASD sense relay of the PCM in a failed ignition switch. Everything will start and run, just all the control voltages used by the PCM will be dead. This is a very common problem and is caused by high impedance on that key control circuit.

 

New ignition switch in, no luck.

 

Check the wiring harness from the firewall to the engine. Really look over the pins where it connects to the steering column harness, and also down on the floor above the foot feed.

If that does not do it, disconnect your battery grounds and lift out your battery and check the battery temperature sensor for corrosion. If it opens the common ground to A4 it will kill the signal from the PCM that is common to pins C15 (battery to sensor signal), A4 (sensor ground), A10 (engine speed sensor), B10 (alternator field driver), and B20 (wait to start lamp driver).

Also check your starter relay, if the terminals get corroded internally they can create enough feedback through A6 (neutral switch sensepoint) to shut down the low voltage along the entire 5 and 8 volt buss. This will usually show up as a hanging starter or difficulty in shutting down before it gets this far, but once in a while it just kills the low voltage side.

 

thanks RCW i'm needing this type of info also

Mel

 

Did the new ignition switch come from the factory with dielectric grease on the connections,if so clean it off with some elect contact cleaner as they will ground over in the connnections. I am Rcw on the ignition switch first then other grounds. You know Bill these elect gremlins is what is pushing me to find me a old 68 Ford camper special 3/4 ton and put a plain jane stripped of all elect 5.9 in it, manual shut off,big ole push button starter button, one wire alt etc..... Goodluck,,,Rick