Carrera 3.2 engine won't stop after switching off

alniki · 05-22-2010, 04:19 PM

The engine starts without problem, runs fine, but will not stop after the ignition switch was turned off. I disconnected the wire connector from the electrical part of the ignition switch but the engine continued running. The problem was intermittent and in the last instance the engine suddenly stopped when I happened to turn off the fan switch of A/C in the central console.

Any advice? Thanks!

rick-l · 05-22-2010, 07:29 PM

I think the DME relay could hold the car on if the contacts stick shut. Can you turn the ignition off and beat on it and see if the car shuts off?

Quicksilver · 05-23-2010, 01:34 AM

The DME relay only turns on the high power devices that are controlled by the DME: fuel pump, injectors and coil. This doesn't keep the brains on and functioning.

The whole DME/ignition system would have to be powered up. Fuse #7 is the "run" feed out of the ignition switch. When it is continuing to run I would pull fuse 7 and see if it dies. If so the leading culprit would be some sort of odd failure in the ignition switch. You can also just unplug the ignition switch under the dash. That will tell you if the switch has control of the issue. (The Red/Blk wire at the switch is the "run" circuit)

Is there an add on alarm system on the car? Just trying to think of things that would monkey with the power distribution.

Quicksilver · 05-23-2010, 01:37 AM

BTW - If the run circuit is staying hot all the time I would be worried about the battery being drained and what it might do to the DME by being energized all the time, etc...

alniki · 05-23-2010, 02:36 AM

Thanks a lot!
I did unplug the ignition switch under the dash but to no avail. The battery was not drained after sitting for two weeks so the ignition switch is likely fine.

The DME brain was repaired recently by replacing a part, I'll ask the shop if the problem is related to the repair or the replaced used part.

Any way, will try to knock on the DME relay or replace it next time the problem shows up again.

rick-l · 05-23-2010, 08:00 AM

Quote:

Originally Posted by Quicksilver

The DME relay only turns on the high power devices that are controlled by the DME: fuel pump, injectors and coil. This doesn't keep the brains on and functioning.

Look at a wiring diagram and tell me what would happen if the contacts from pin 87 to 30 of the DME relay were welded shut due to weak spring, age etc.

Quicksilver · 05-23-2010, 08:06 AM

The wiring diagram for these things are almost as much of a disaster as the basic methodology of the wiring...

The power distribution diagram shows the circuit that is hot during Start and Run as going "to fuse 7". Looking at that page of the diagram it actually shows that it connects at fuse 7 but is actually feeding: fuse 7, fuse 8, the air conditioner control relay, the ignition coil, blower control unit, anti-theft control switch, fuse for convertible top (optional),... Any of these could backfeed and provide power for your unintentional running.

Geeze...
On another thread you mention that you switched the AC fan and it died. Power could be feeding back from the AC relay. Seeing that you removed the AC just try removing the AC relay. If that solves it the car is usable but actually finding the root cause will require a lot of work and a wiring diagram.

(The wiring on these cars makes me nuts. I have half a mind to develop a wiring harness that goes straight from the battery to the fuses and then to the control switches followed by the load. Then I remember how much trouble wiring a simple race car from scratch is...
But if I did it would be proof against electrical fires and the wiring diagram would be readable and comprehensible!)

Quicksilver · 05-23-2010, 08:14 AM

Quote:

Originally Posted by rick-l

Look at a wiring diagram and tell me what would happen if the contacts from pin 87 to 30 of the DME relay were welded shut due to weak spring, age etc.

It would power up the injector harness.

rick-l · 05-23-2010, 08:23 AM

Quote:

Originally Posted by Quicksilver

It would power up the injector harness.

OK.... and what would 12 volts on pins 18 and 35 of the DME do?

rick-l · 05-23-2010, 08:27 AM

Forget the DME relay theory. The coil is switched separately. I thought it was on the same relay.

Quicksilver · 05-23-2010, 09:00 AM

Quote:

Originally Posted by rick-l

OK.... and what would 12 volts on pins 18 and 35 of the DME do?

Geeze, I just didn't see that...
(This is exactly why the wiring diagrams drive me nuts. Why wouldn't they draft this so it distributed at the DME instead of at the injector harness? What a bass ackwards way to describe what I'm assuming is the main power feed for the DME.)

Fuse 2 is hot all the time and powers the AC relay which could be the source of power if it was shorted in a major way or the wiring had been badly messed with. It appears that this fuse is upstream of the relay. (Just don't trust Porsche with fuse location anymore...)

alniki - Try pulling the AC fuse (#2) and see if that makes it act normally no matter what position the fan is in.

rsscotty · 05-23-2010, 09:33 AM

I've seen this before and the diode in the charging light circuit has always fixed the problem. Use a 1a-100v diode installed like shown in the link below. It works if it is installed in the proper direction as shown in the picture. You can do this at the alt. light right at your gauge.

Porsche had this diode in series at the alt. light, but I don't recall what years this applied to.

MSD Ignition

alniki · 05-24-2010, 05:26 PM

Gentlemen, thank you very much for the info and suggestions. I'll try accordingly and report back what I find later.

Thanks again!

alniki · 06-08-2010, 06:36 AM

Gentlemen, finally got to drive my Carrera and did some test.

The engine would continue running after being switched off if AC had been turned on but would stop immediately when AC was turned off. So I think Quicksilver is right, the power was fed via the AC relay. I didn't have such problem before because only recently did I turn on the fan to get some cooling and the AC temperature knob seemed to be inadvertently turned on as well.

Thanks a lot!