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Old 15-12-2018
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Question Do I need a new PCM

Hey guys, so I started having an electrical issue with my Sept 08-build JK V6 petrol 2-door manual (161K km's) out of the blue this week that I assume to be a PCM issue, however I thought I'd consult with the brains trust here just in case I'm missing something obvious before I bite the bullet and go & fork out big bucks for a new computer.

Symptoms are: After standing for a while with the engine off & the key removed from the ignition, doors will lock/unlock with the key fob & headlights switch on as per usual upon unlocking. However, when you open the door & sit behind the wheel the electrical system appears dead (ie: it hasn't woken up from it's "sleep" when everything shuts down after a period when you turn it off and remove the key). No interior light or odometer readout upon opening the door, and when you insert the key into the ignition and turn it through the clicks nothing happens either (no lights, chimes or electrical whirrs under the bonnet from fuel pumps, throttle body etc) until you advance all the way through to the engine turnover.

Engine will then start up and run normally, however dash display/instrument cluster is still dead with no lights/gauges and no functioning switches (indicators, ventilation fan, headlights etc); the windscreen wipers do however then turn on all by themselves and run at the constant slow setting. Disconnecting the negative lead on the battery (either momentarily or for a long time to discharge the capacitors) will rectify the issue immediately and the Jeep will run & function flawlessly until the next sustained shutdown.

The only red herring in all of this is to do with the radio/nav head unit (it's the 10yo factory option one with the touch screen and built in nav & hard drive). While as I mentioned earlier that everything else electrical is dead in the cabin upon opening the door, the unit's backlighting around the menu switches is on. Another gremlin to do with the unit (that has functioned flawlessly for 10 years too) is that it wouldn't eject a CD the other day and appeared to freeze-up momentarily after the screen pivoted down to expose the CD slot, before then closing the screen back up again and resuming normal function; CD ejection functioned as normal on my next attempt.

The only reason I mentioned the head unit issued is that it does connect to the main power BUS & communicate with the PCM; I'm wondering if an issue with the radio head unit could be causing the broader PCM electrical issues with the vehicle? Any input is appreciated guys!

PS: I did check for any fault codes thrown via the dash & nothing showed up.


Last edited by jklad; 15-12-2018 at 12:44 PM.
Old 16-12-2018
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Before you throw big bucks at the PCM, eliminate the easy stuff first. Try installing a known good battery for a while to see what happens. My son has a 2015 GC which was displaying similar behaviour but mostly while driving. All was good then the dash would either go ape shit or just go out for a while. Once or twice it even killed the engine. No codes of any description were thrown. It turned out to be an intermittent battery fault. My 2012 JKU also behaved in a similar manner when the original battery was beginning to fail.

Putting a load tester on the battery might give some idea as to its condition. Do this even if the battery is near new. Even new batteries sometimes turn out to duds.

Next I would disconnect that head unit for a while to eliminate that as a possible cause.

Get hold of a proper code reader and use it as soon as the problem shows up. A lot of DTCs that are not critical will get cleared by the PCM after a few complete driving cycles if they have not occurred again.

Hope I have been of some help.

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Old 17-12-2018
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I'd be looking at the under dash connections before replacing the PCM. I've had most of those symptoms in my 2013 and initially it was a faulty connection under the dash that was opened, blown out and reseated. About this time last year I got some engine codes throwing up with a cylinder misfire that ended up being the PCM after eliminating the other potential causes.. Still get the intermittent power issues inside, but I see a lot of dust, so most likely going to blow out the connections again over the xmas break.
Old 18-12-2018
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There are 2 grounding points on the side of the fender next to the battery and ECU and 1 on the other side of the engine bay (either on the firewall or a bolt on the fender, I can't remember) - have you checked that they are clean and their nuts are done up tight? I would also check that all relays and fuses are seated properly.
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