Load test the wire end to end with a headlight bulb. I’ll bet it won’t light. You likely have high resistance or only a strand or 2 of wire that can’t carry any current.

 

If I unplug pcm harness, poke a wire (connected to a bulb) into the pin 71 socket, with key on, should it light up? I'm trying to understand if that's the case. Thanks for your help!

 

Pin 71 gets it's power from Fuse 9 through the PCM Power Relay. Fuse 22 provides the power to the PCM relay's coil. When the key is turned to RUN/START, is there power at the relay's output? See the attached diagram. Cheers!

 

Patrick Feeley,
How can I test for power at relays output? With the relay plugged in, or removed? Maybe I'm unclear on how that relay functions.
Thank you for your help!

 

When power is applied to the relay's coil (Terminal 89 on the diagram) it will close the switch portion of the relay, providing power to all the circuits connected to Terminal 87 on the diagram (including PCM pin 71). Cheers!

 

So I have a functioning pcm relay (tested with horn) and when I have key on, the relay is sending 12v. My problem must lie somewhere between pcm relay (terminal 87) and pin 71. I'm still not getting power to pin 71.
Do you know how/where power from the pcm relay travels before it hooks to pcm pin 71? I've traced pin 71 and it goes across the engine toward the glow plug relay. I assume it eventually connects again to a wire from pcm relay?
Thank you for your input, I'm pretty confused by all of this.

 

Do you get power to pin PCM pin 97? Or how about IDM relay terminal 85 in the underghood fusebox? Notice in the diagram that the power would go from PCM Power Relay terminal 87 in the underhood fusebox to a splice (S157) with 5 other wires that go to various things:

Circuit 361 for transmission controls (C1048 pin 1 from the manual)
Circuit 361 for IDM Relay coil
Circuit 361 for PCM Power at pin 97
Circuit 361 for PCM Power at pin71
Circuit 361 to Connector 138 (C138, 42-pin connector near the UH fusebox), then changes to circuit 552 for IPR power.

Location for S157 states it is in the Engine control sensor harness, near T/O (take out) to PCM. I would see if there is power to the other items as well. If there is, the open is somewhere after the splice in the wire to pin 71. If none of the other items have power as well as pin 71, then the open is from the fusebox to the splice. Cheers!

 

I traced the wires and found the splice, turns out I had mistakenly identified pin 71 and pin 97 from the rear(pcm side) of the harness, and had the wrong wires. So now I checked the correct wires and I do have 12v with key on.
Thank you Patrick for saving me hours more of wild goose chasing!
So I do have power to the pcm. I'll test the pcm diode one more time, and then I guess I'll look for a pcm. I know it's hard to find a DPC-311, is there a reputable rebuilder that you know of to send it to?
Thank you again!

 

Yeah, good luck in a search for that PCM. I've read of a couple of places that do repairs, but I don't know how good they are or anything. I'll have to look and see if I can find their names again. Before you do that, do you still have no WTS light? If so, and fuse 22 is verified good, check for the 5V reference voltage (this is produced by the PCM with key in RUN) at any of the sensors that use that reference voltage (CPS, ICP, EBP, TPS, MAP, BARO). The voltage is on the Brown/White stripe wire on any of those sensors. If no reference voltage, I've got more info. Cheers!

 

I still don't have a Wait-to-Start light.
Fuse 22 is good, ICP, EBP, TPS MAP all have 5v at the brown/white wire with key in RUN position.

 

Try Diesel Technology of Chattanooga for the repair.

 

Try Diesel Technology of Chattanooga for the repair.

Thanks, I'll give them a call. Have you used them personally?

 

I haven't... I have talked to them via email (I had questions not related PCM repair). As I understand it - the fellow that started the company was Swamps head electronics dude. There are several posts around PSN that have given favorable reviews. I would be willing to try them out. Before you send your PCM out... Do you have FORScan and an adapter? Have you tested the Wait To Start light to see if it is good?

Cj

 

I have a cheap Bluetooth reader, and a free app to go with it; it just says it can't connect to the truck's pcm.
I have tested the WTS light bulb, if that's what you mean, and it does light up.

 

What app are you trying to use to connect? Along with the adapter?

Are there any lights on the adapter when you plug it in? If you do have lights try downloading ForScan Lite to use.

 

You may have a dead PCM. Do you have 12 volts at the OBD port? I can't remember which pins off hand (there are threads that talk about it)... I think the power is shared with the power port/cig lighter.

 

I have power at the OBD port (Bluetooth OBD scanner lights up). And 12v at cigarette lighter. I just downloaded FORscan and it reads the BT scanner, but just says "unable to connect to vehicle".

 

The 97s had the OBD port enabled from factory... I'd send the PCM in for an evaluation.

 

The place that CJ posted is the one I was thinking of. Cheers!

 

Update:
I sent my PCM into "Diesel Technology of Chattanooga". They tested it and repaired it. Plugged it back in, and the truck fired right up and has been running reliably for 2 weeks. The pcm just died from old age apparently.
Thanks for the advice!