After receiving the factory service manuals today, I was able to diagnose with about 90% certainty, a failed PCM. Here’s what I found. I won’t detail tests already described in previous posts.

First I suspected (hoped) that the Park/Neutral Position Switch was defective. It supplies a ground or low signal to PCM pin 76 (Park/Neutral Sense). In Reverse and Drive it supplies an open or high signal and disables starting. This is a physical switch that when engaged grounds the center pin of the 3-pin switch connector to the transmission case. It’s a black switch on the far driver side front of the transmission. The white switch next to it toward the center is the Transmission Range Switch.

The first thing the manual calls for in diagnosing a no-crank no-start condition is the use of a DRB (Chrysler diagnostic and scan tool) to show the registered position of the P/N switch. In my opinion, the very first test should actually be to swap the starter relay with another relay in the Power Distribution Center. It’s the most obvious fault possibility and the easiest thing to test.

I don’t have a DRB so the alternate P/N test procedure is to remove the appropriate connector from the PCM and check resistance from pin 76 (P/N Sense – BK/WH) to ground. In Park and Neutral, a reading above 5 ohms is considered a failure of the switch. Mine was showing about 50 ohms but did go open in Reverse and Drive. The switch-to-PCM wire resistance alone was about 5 ohms. The switch contacts are likely failing. However, grounding the P/N center conductor to simulate a Park or Neutral switch position at the PCM, did not solve the problem.

The next steps were to test for voltages and control signals at the starter relay, and to jumper the relay to test the starter. I had already tried these.

With everything else checking out correctly, the last step was to pull the other PCM connector, ground pin 8 (Starter Relay Control – TN) and “briefly” turn the ignition to start. If the starter runs, then the PCM is likely defective. The starter worked, so there you go, PCM failure. To be honest, grounding pin 8 and cranking the starter would be another quick way to immediately isolate your no-crank problem to either the PCM side or starter and starter relay side.

An alarm condition was another early consideration. However, the manual states that an active security inhibit condition would still allow the vehicle to start, but would shut it down after a few seconds. I've never had that occur so I can't verify the claim.

As part of testing, I also checked resistance from the negative battery terminal to the PCM connector grounds, engine and body. They were less than 1 ohm. So those grounds appear to be good.

A rebuilt PCM from Auto Computer Exchange has been ordered. For $100 it seems very reasonable plus it has a lifetime, non-transferrable warranty and they pay for return shipping of your old PCM. I’ll post again when the new PCM is installed.

 

 

Let me just say the method you used to summarize your post (test methodology) is an extremely valuable learning tool for future readers.

Thank you for spending the time and following up with the final solution.

 

There have been a lot of times when I've followed a thread hoping it would lead to a possible solution only to have the thread abruptly end with no follow up. It's disappointing and I believe discourteous to those who spend the time offering their experience and advice, not to post the results. So I appreciate your comments.

 

Agreed. The internet forums hav been a place where either a) one is too lazy to search for solutions, so just posts the question hoping(expecting) a free answer to their problem (which was likely asked hundreds of times before them), b) those who take it upon themselves to find the solution first, before taking up valuable time from the volunteers.

I'm in the second group, and like you, many times have to piece together tidbits of info from various posts b/c there is no follow-up solution. Then here or there you find that gold nugget of good information -- like your post -- that kick-starts your personal knowledge base again. /soapbox off.