PCM not switching on in 955 Cayenne S! :(

[cayenne11]

Hi All

Having recently suffered the now all to familiar coolant leak issue, and paid for the repair myself - as Porsche wouldn't pay a penny - I came back from a weeks holiday to another problem with my 2006 Cayenne S which has only done approx 40,000 miles;

After about 15 minutes in the car I noticed the screen on the PCM would flicker for literally a split second probably every minute, when doing this it was literally a blink and you'll miss it type of thing where the screen looked exactly the same before and after the flickering but after it had done this a few times I turned the screen off and then back on again to see if this would fix it but it didn't. I then turned the PCM off for a minute and turned it back on again and it was fine and had no further issues with it that day

The next day when driving back from work I turned the PCM on and the Porsche logo with the white background (start up screen) appeared for a lot longer than normal (probably about 20 seconds), after this it went on to function normally where I could control the radio, CD etc.

However, the next day I couldn't switch the PCM on at all, and have been having this problem now for 2 days. Yesterday the PCM came on by itself without me pressing any of the related buttons but since then I haven't been able to switch it on at all.

From reading other posts about people having problems with their PCM's I've seen some have had issues with the screen and when they've pressed it this seems to have solved the issue, but I can't even get it to switch on. I've tested the battery and it's fine so it can't be that.

Can anyone PLEASE help with this!! It seems to be one issue after another with this car.

Thanks in advance

[bigbuzuki]

Mine had similar start up problems that you describe especially when the unit was cold.

Unfortunately the PCM has been known to have these issues and mine was a bad solder connection to one of the ICs on the circuit board.

It could be a solder issue, requiring a new / exchange PCM, or you can try this

Edited August 19, 2011 by bigbuzuki

[cayenne11]

Mine had similar start up problems that you describe especially when the unit was cold.

Unfortunately the PCM has been known to have these issues and mine was a bad solder connection to one of the ICs on the circuit board.

It could be a solder issue, requiring a new / exchange PCM, or you can try this

Thanks for this, by process of elimination, and exhaustive searching on the net, I think it may be the battery and this afternoon a friend and I went through the rather over the top procedure of getting access to the battery and am having this replaced tomorrow. I think this should solve it as the PCM came on earlier today when I'd only be in the car a couple of minutes, so it wasn't even as if the battery had a long time to charge. I'll let you know if this solves the problem and if anyone else has any other ideas in case this doesn't solve the problem I'd love to hear them.

[hahnmgh63]

Also try to do a PCM reset (doesn't cause it to loose any data). With the car started or ignition on and the PCM off push and hold the numbers keys 1, 4, & 8. The screen should come on with the Porsche Logo for a few seconds then shut down. Then just press the volume knob as usual to restart it again.

[cayenne11]

Also try to do a PCM reset (doesn't cause it to loose any data). With the car started or ignition on and the PCM off push and hold the numbers keys 1, 4, & 8. The screen should come on with the Porsche Logo for a few seconds then shut down. Then just press the volume knob as usual to restart it again.

Thanks for the tip, will try this.

I tried resetting it a couple of days ago by disconnecting the battery then reconnecting but it didn't do anything. My local garage have ordered a new battery as my previous research had led me to believe it could be this - I'm not so sure now - but as they've ordered it in I'm going to replace it anyway as it's about 4 1/2 years old and could be this. Otherwise, does anyone else have any ideas, some people are talking about solder joints, does anyone have any ideas about how to do this??

[Andrei Marton]

How did you solved the problem ? I now have the same identical issues with the PCM. My car is 2008, but the PCM's i think are identical. And the simptoms are the same. Won't start at al, and the, out of the blue, turns on. I noticed another thing. When it starts , looses all memory, and car clock, remains behind, 2-3 hours. Minutes are correct, hours are behind. Is the battery the issue ? Is this some kind of protection for the battery ? I run allways with my headlights on, music on, a/c on.

[mudman2]

The PCM in 955 is very different from the PCM in 957, although they look very similar. The rest of your description is very typical of a battery going bad tho, I would change that before you do anything else

[Andrei Marton]

So, today i changed the battery. I replaced the OEM batt with Bosch Silver S5. Same specs, but different brand. The Original Porsche battery, was made by MOLL, producer i did not heard until now.

After the replacement, the PCM is still dead. Like the usual, it started in the morning, and after i stopped the car for the first time, it would not start anymore.

So , new battery, no difference (except starting much much better).

I keep on investigating.

[airbil]

Any update on this? I think it's a cold solder joint, but would love to know where.. Perhaps it's a common fail in the same location?