Pretty sure my PCM is dead

[JMo]

Hey Guys. new to jeeps and enjoying my very first major problem. 06 LJ 4.0 manual 116K. I've run several tests and they all point me towards a dead PCM. volt meter and fuel gauge do not register (cluster passes self test though). my OBD2 scanner will not connect, and the SKIM light is on. I have verified none of the engine sensors are shorted and they all are being powered and grounded properly. I have verified that power and ground are being delivered to the PCM when the ignition is on, and when you turn the key to the start position there is + power delivered to the start relay coil. I've found however that the PCM is not providing the ground to the ASD and Fuel pump relay coils as it should when you turn on the ignition. also, at one point with the ignition in the on position the ASD and fuel pump relays were chattering in unison (they share the same coil ground from the PCM). sounded like a telegraph. so I think it's dead, but I'm looking for any guidance out there so I know for certain. Does anyone know of a specific test(s) I can run to verify it is in fact dead? Note I did run the ASD relay circuit test procedure in the TJ Drivetrain manual (best I could without the DRBIII), and the conclusion there was to replace the PCM. any advice? would like to know for certain before I buy something... thanks!

John

 

[Leinhit]

https://wranglertjforum.com/threads/05-06-pcm-issue-is-solved.21699/
Maybe this will help...

 

[Jerry Bransford]

Try unplugging and then reseating the PCM's connectors a couple times to see if there's just a bad connection inside one of the multi-pin connectors.

 

[Imapepper]

If the relays chatter they're getting ground, maybe not proper ground...

 

[JMo]

Thanks guys. Yeah I read the Wranglerfix thread. looks promising. definitely played with the connectors quite a bit. they've been on and off several times now. we also tried manipulating the connectors and harness to see if anything changes. I haven't been able to "cause" that chatter to occur. it just happens randomly when it wants to. the chatter is what has me thinking the power switching inside the PCM may be bad. I have good continuity on the ground wire from the PDC to the PCM even when you abuse the harness, so that chatter must be happening upstream of the PCM connector, so either inside the PCM or upstream of that.

 

[Imapepper]

Use a jumper wire to ground suspect relays, take the computer out of the equation.

 

[Painfultruth]

Thanks guys. Yeah I read the Wranglerfix thread. looks promising. definitely played with the connectors quite a bit. they've been on and off several times now. we also tried manipulating the connectors and harness to see if anything changes. I haven't been able to "cause" that chatter to occur. it just happens randomly when it wants to. the chatter is what has me thinking the power switching inside the PCM may be bad. I have good continuity on the ground wire from the PDC to the PCM even when you abuse the harness, so that chatter must be happening upstream of the PCM connector, so either inside the PCM or upstream of that.

Did you ever get this resolved? I’m having the same issue.

 

[JMo]

not yet 1RAD. I've done a lot more testing though, and I'm fairly convinced it's the PCM. most notable is that the PCM has all the power and grounds on connector C1, but when you turn the ignition to the on/run position there is no signal out of the PCM to the ASD relay, which I verified at the PCM connector. per the FSM there should be a command signal (ground) for a couple seconds when the PCM is powered on. and I've tested the crap out of the ignition switch which appears to be fine.

I'm planning to call Mark at Wranglerfix and get his take. I'll post when I have more info...

 

[Painfultruth]

Ok, my overall symptoms are a bit different, but my ASD and Fuel pump relays seem to behave as yours are. I have spent a lot of time troubleshooting as well with no definitive answer.

I have already spoken with Mark, he seems like really good guy, but he is not able to help my situation due to his product not supporting tuners.

 

[JMo]

ok here's an update. Just to satisfy my curiosity I thought I'd put in a new ignition switch and see if that changes anything, to my amazement I put the key in and it fired right up! holy crap! but it did throw the dreaded P0031 P0037 P0051 P0057 code set for the O2 sensor heaters. ok so I figured just to be certain I had isolated the problem, I put the old switch in and guess what... it fired right up! sooooo long story short I still have no idea. I'll be researching the O2 sensor codes, which may again point to the PCM. out of time for the day but I plan to measure the voltages at the sensors and really scrutinize the grounds. I'll update as I make progress...

 

[JMo]

I did all the tests in the FSM which seemed relevant, and all pointed to the PCM. I bought the PCM from Mark Cantrell at WranglerFix. jeep runs great. one minor issue which I'll post about here: https://wranglertjforum.com/threads/05-06-pcm-issue-is-solved.21699/page-14