I think I have a bad PCM and wanted to see if anyone thought the same...

Yesterday I replaced my PCM (A9P) with a 'rebuilt' one, also A9P. (My intent is to have a known-good working backup). So I installed the PCM and verified the truck ran (just started it for ~10 seconds). Disconnected battery.

Today I started the truck and it ran for about 5-6 min before I turned it off. After that I wasn't able to start it again. It sounded like there wasn't spark so I checked and spark is coming out of the coil but it seems very weak. It's a new coil but JIC I tested with a used, known-working coil and no change, the truck still wouldn't start. Also, the same timing light that I used just yesterday (before I replaced the PCM) doesn't pick up the signal for the cyl. 1, also leads me to think spark is weak.

Last thing that made me think it was the PCM was I am not able to pull codes. Same code reader as before, used it a few days ago. Code reader turns on as it should but the KOEO test sequence doesn't start. I hear the fuel pump cycle for 2 seconds when I turn on the key and the lights in the cluster seem OK but can't get codes from the PCM.

I didn't want to replace this PCM with the original one yet, it's a chore, and I am hoping I am wrong with having a bad PCM.

Thanks.

 

Unplug the PCM and see if spark returns. You do not need the PCM for spark but the fuel injectors will not fire with it unplugged.

You can also plug the old PCM in without removing the PCM that installed in the truck. Just lay it somewhere out of the way and try running codes. I do this all the time.

 

Spark seems to be the same before and after I have the PCM plugged in. I can't tell a difference.

I did verify the fuel injectors are firing when PCM is plugged in, I have a light I can connect.

The truck was sputtering a bit, like it wanted to run but never did. Still makes me think of something to do with the spark, it's either weak or not receiving right timing.

Tried pulling codes again and no change.

 

May you have a bad Ignition Switch.
What is the voltage at the coil with the key on and in start.

 

Good point. It's also a good thing to remember if you ever have to do a quick road-side swap.

 

With KOEO I read 11.37 volts across the pos and neg terminals on the coil connector. I read the same between pos of connector and neg of batt. At the batt I read 11.71v.

The not being able to pull codes bothers me. I'll replace it later today, just need to go through a few extra steps because I can't run the truck without the Tweecer to write a tune.

 

OK, the rebuilt PCM was bad but I have a question about what might have caused it.

After I pulled out the bad PCM, I saw a rather large (12ga??) black (ground) wire coming out of the same wiring bundle as the connector for reading codes that was not connected. I reconnected that right away. Before I reconnected that ground, I think I got a code 15 from the good PCM and that refers to keep alive memory. Could I have damaged the rebuilt PCM by running the motor without that ground connected for a certain period of time?

Right now I can start the truck with the rebuilt (bad) PCM but it doesn't sound well. The CEL comes on but still can't pull codes. As soon as I start the motor with the original (good) PCM, the truck sounds like it should and is going through warmup. I can also pull codes from the original PCM but still can't from the rebuilt one.

 

The ground you are talking about is the case ground I think and goes to computer plug #20 (J1-20). I would not think that would have hurt the PCM but do not know for sure as I have never operated a PCM without pin #20 grounded. See diagram below.



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There are other ground points for the electronics so I don't think that would damage the computer.

Did you try rotating the Tweecer 180 degrees? Ya that aught ta work!

I got a newly refurbished EEC that didn't work on the first try. I have voided the warranty by pulling the sticker and opening the J3 port. I am going to try it again with a better cleaning of the J3 contacts and will ensure I have the Tweecer seated properly..................fingers crossed. I hope I didn't just dump a couple of bills on trash.

 

Ummmmm........

I'm thinkin your answer is in BOLD above.

Bad computer ground or corroded cable(s) will cause this.

My .02 cents.

Bob