Engine/Transmission (1994 - 1998) Dumfounded on PCM problem

[ABorchard]

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?

 

[mechanic]

I would check for a short to ground on the 5 volt reference voltage circuit. If it is grounding out the signal, then you would have a low or no volt reading. Check for the reference voltage when you first turn on the switch after a cool down time. If you have a reading for a few seconds then it drops to 0 or near 0, then the PCM may be cutting the power to that circuit to keep from burning out that supply.

The next test I would do would be to remove the 5 volt reference voltage wire from the PCM plug, then turn on the switch and see if you get a reading at the PCM terminal. If yes then you most likely have a heavy drain on that circuit. If all this fails, if you call me I will look at my ALLDATA and MITCHEL on DEMAND5 to see if they will help. A good source of help is IDENTIFIX. I don't have their number in hand now, but I will tomorrow in the shop. (They have a fee of $40. or so) My number is 405-399-5172, email is -- email address removed --

Contact me if I can help in some way.

David Lapham

 

[DWitcher]

What about checking the wire harness connectors at the firewall next to the brake power booster for loose conection or corrosion.

 

[ABorchard]

Good call Dave, will try a resistance reading between the 5 volt pin and ground with the connector unplugged, if it measures too low can isolate sensors one at time by disconnecting each at their connector, a direct short to ground could even be caused by a bad WIF sensor. Have already tried this method on the 8 volt supply, everything checked out fine.



Thanks DW, but we've been over the wiring including the driver's side connectors several times, everything checks out fine.