JKlad

Hey guys, so I started having an electrical issue with my Sept 08-build JK 3.8L V6 2-door 6-speed (111K miles) 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 hood 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 windshield 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 for a time after the screen pivoted down to expose the CD slot, before then closing the screen back up again by itself without ejecting the CD and resumed 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.

Cheers!

TheDirtman

It could be the fues panel as well. I just had to replace mine after some electrial gremlins. There is a company in flordia that rebuilds them but mine was too far gone they said and I had to get a new one. They are plug and play though no dealer flashing required. Have you had any strange battery draws? That is one symptom.

cybersonarbat

HI JKlad

You may not need a new PCM. You spell out some symptoms of a failed instrument cluster. It needs to communicate with the PCM and you're saying that it's sometimes dead, but you can reboot it when you take off the ground strap.

The comms is sophisticated and would probably require the services of the stealership. One of the members may have just bought one and put it in. They might be able to tell us.

When my instrument cluster failed, I got one fitted by the stealership.

Mr.T

Doesn't sound like the PCM to me either. It takes a high-end scanner to check out the various devices that communicate with the PCM like the instrument panel. One time I was unsuccessful trying to figure out a gremlin on a friends JK that turned out to be the compass module, which was messing up comms of other devices on the same bus. It takes the right tools...

jdinny

Yea, like Dirtman says, it's probably the TIPM

JKlad

Hey again guys!

Thought I'd post an update just in case anyone else has a similar electrical issue & is looking for answers (and cybersonarbat you were right after all!)

Turned out to be a faulty instrument cluster that was causing the problems. The dealership mechanic's report detailed the troubleshooting that they went through; after initially suspecting the TIPCM they eventually ruled this out after receiving wake-up signals and good voltages on the appropriate PCM output wires, but were getting nothing back from the cluster until a hard reset (battery disconnect & reconnect) so narrowed it down to this.

Next logical step to me would have been to isolate & replace the bad $10 relay switch on the cluster that was responsible however there are evidentially "no user-serviceable parts" in the cluster unit so replacement was the only option. 800-odd bucks later for parts & labour and I'm back in business (the Aussie stealerships over here really know how to ream us good & proper), plus they did the airbag & clockspring recalls while they were at it. I won't go into the many months left waiting for the backordered parts to come in from the 'States, just glad to have my Jeep back on the road.

Thanks again for all your input!