P1604 PCM code

Thor

Thor

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Good day! New to the forum and I have seen a lot of great information posted here.

My wife has a 2006 LJ auto with the 42RLE with about 111,000 miles on it. The check engine light came on yesterday after it got started and had a strange idle. got the P1604 code from the self diagnosing we are able to do without a scanner. From all the research we have done, it seems like it’s the dreaded PCM.

Our question is will there be any benefit taking the Jeep to an auto mechanic to see if they can fix the problem or with the history of this code is it best to replace the whole PCM? What have you guys done to solve this problem? Is there an easier or quicker fix to this problem?

Any information is appreciated as we would like to gather as much knowledge on this matter so we are confident in our decision on how to repair our Jeep.

Thanks!
 
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I would first get a scanner (one that can read transmission codes) and do a full diagnosis before jumping right to a new PCM. Report back here with any additional codes....I’m sure you’ll get plenty of good guidance.

Can you more fully describe what you mean by “strange idle”? Any other symptoms?
 
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My wife said as soon as she started the engine, it was revving or idling higher than usual. She turned it off immediately and gave it a minute and restarted it and had no idle issue but that’s when the check engine light can on. The only other symptom we can come up with is a hard cold shift from 1st to 2nd only. Once the tranny is warm, no hard shifts.
 
You need to have it scanned first with an OBDII scanner to make sure you are getting all the codes.

You could have a bad PCM, a bad connection, a broken wire somewhere or even a bad ignition switch.

A bad PCM is challenging to prove in some cases. Even veteran techs are known to eliminate all the other possible causes and condemn the PCM last by proving all else is good. Internal electronics are near impossible test as they require special equipment and more importantly information to test them which is NOT provided by the manufacturer to the public.

Start with what you can test first and prove whats good.

Locate a pinout digram for connector 1 and inspect pin 12. Look for corrosion, discoloration or looseness in the connector. Clear the codes, start it and try wiggling the wire that goes to pin 12. See if you can induce a code. It takes more than one failure to set the code so it may come back as a pending code the first time you recreate the failure.

Try locating a wiring diagram for the ignition switch. Wiggle test the wire that is the output from the ignition switch and goes to the PCM circuit. Look for any melted connections, discoloration or loose connections. Same thing with pending code applies. May need to fail more than once to set a hard code and illuminate MIL (Malfunction Indicator Lamp).

Locate fuse that protects the switched ignition circuit to the PCM and check connections there.

You may need to check all the wiring and connections (plugs) in the entire circuit to eliminate those possibilities.

You have an advanced level diagnostic to perform and it will take a lot of hard work and patience to perform.
 
I agree with the Wrangler Fix. Mine has been in for months and works great. Try disconnecting the battery for a while. This will reboot the PCM. Drive it, if the trans problem goes away, probably the PCM. This is only a temporary cure. Problem will come back.
 
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