I just wanted to run this by you guys and see what you think, since I've never dealt with a problem like this before.

A week ago my '93 3.9L Dakota died on my way to school. It would still turn over, but I couldn't get the engine to start. Immediately I noticed that the fuel pump wasn't running when I turned the key. I checked the relays and fuses, all good, so last weekend I pulled the fuel pump. Turns out the fuel pump was fine. I could easily get the pump to run bypassing the relay.

So then I had to figure out why the relay wasn't getting the signal from the computer to come on. I read here that I should check for codes, so I went to do that next, but... no check engine light. That's when I got suspicious. Turns out that the check engine light also works when I bypass the powertrain control module (PCM), but it just flat out won't come on when connected to the PCM. Checked that PCM has power and ground at proper pins. All seems good.

Here's the summary of problems:

-engine dies out of nowhere one day while driving
-turns over but won't start
-fuel pump relay not getting signal from computer
-ASD relay not getting signal from computer
-check engine light not getting signal from computer
-Wiring for relays (splice) and check engine light all seems to be good
-computer (pcm) has good power and ground
-PCM seems to be emitting a high-pitched whining noise when power is on (I don't think that's normal)

I had planned to do some more diagnostics, but after I found that the pcm wasn't even able to properly turn on the check engine light, I figured that I must have located the source of the problem. So at this point I figure my PCM must be dead. Does anyone have any other thoughts?

I tried to open up and check the PCM itself but the whole circuit is encased in rubber, so I can't even check to see if it's something simple like a bad capacitor. Has anyone had any success in removing the rubber and performing a repair?

And if I have to replace my PCM, does it have to be a specific match to the one in my truck, or can I just grab any old '93 3.9L PCM off of ebay and expect it to work? The one in my truck is a 56027774. I see several 56028483 PCM/ECU units on ebay for like half the price of the 7774 (even a 7775 is a good bit cheaper). They look the same, and are from the same model year 3.9L, but will they substitute?

Thanks.

 

Sure sounds like the notorious power splices. It provides power to the fuel pump and ASD relay.

 

I was thinking that originally, but I checked continuity on the wires from the PCM to the relays with an ohmmeter, and all seems to be good. I can bypass the ASD and fuel pump relays with jumpers and the engine still won't start.

Also, the the check engine light doesn't work, so I can't even check for codes. The check engine light not coming on is why I figured the PCM must be dead, since the light itself works when I power it up through the PCM connector manually.

 

Just take a look at the splices, the FAQ has a good post on the locations of the splices.

 

I'll take another look.

 

I'm certainly no expert, but a couple of years ago I could not get my '95 Dakota 3.9L to go into overdrive. I had my shop look at it and they determined it was the ECM/PCM unit. A new ECM/PCM from the dealership cost over $600, so I looked online and found Auto Computer Exchange, called them and ordered a remanufactured OEM computer, shipped my defective computer to them and now the overdrive works perfectly. Oh, the remanufactured computer cost less than $275.

Hope this helps.

 

+1 on that. When I bought my truck, the PO was selling because he thought the fuel pump was bad. I got it towed home. Went to check for codes, no check engine light. Took the wire tape off the wires under the PDC, and sure enough, the wires had snapped due to corrosion, right at the splice. I stipped and re-spliced the wires, shrink wrapped them, and the truck started right up.

 

Do not take the black stuff off of the circuit board, that stuff protects the board. Is the black stuff cracked in any places. If yes moisture got through and is shorting a circuit. I would just resolder the splice first to see if that may fix it. My truck randomly died and when I checked the relay I had three with 12 volts and one with 7.1 millivolts which lead me to a computer.