hwyengr

Or it was an oversight and/or an error. Engineers are human, too.

sprie

I did various tests taking out fuses after removing the key. I was looking for a fuse that on removal would stop the engine immediately. I could not find such a fuse.
I then did a similar test with the relays, taking them out one by one. The only one that killed the engine that i tried was the LH, which i think is to be expected. I didn't bother testing the fuel pump and ECU relays, as I think they would also kill the engine.
I then noticed i had left the sunroof open when i had parked the car - at the time it seemed a good idea to help air inside of the the car, which is still musty from its 12 years off-the-road. I had not left this open this ever previously. So, after reinserting all the fuses and relays, i started the car again, closed the sunroof, turned off the ignition, and the engine stopped (which it has not done the last 20 - 30 times i have tested it).

I have since started and stopped the car about 8 times, and each time the engine stops. I have tried leaving the sunroof open, leaving it closed etc, but it doesn't change the behaviour of the car. The good and the bad news is that car is currently stopping correctly.

I guess it is possible that one of the relays wasn't properly seated and by removing and reinserting the relays, it has sorted an issue. But there was nothing apparent when i took the relays out.
I am wondering if a dodgy or dirty sunroof switch could possibly cause this behaviour, and by closing the sunroof i have avoid the issue?

Unless someone can suggest an explanation, i am guessing i might hit this issue again in the future - but at least i know which fuse to pull (fuel pump) and i just wait a bit for the fuel in the pipes to run down.

Mrmerlin

Ian please answer these questions.
Did you pull the defroster relay position I and II,
bottom row left on the CE panel its a big 2 position silver relay.

Did you verify the defroster switch in in the off position turned it CCW,
and that the defroster switch isnt sticking in the pod when you push it in and release it.

If you suspect the sunroof being open try removing the interior delay relay XIX #19 next time after you have pulled the defroster relay I and II

sprie

Hi

I did pull the big, rear defroster relay first, and the engine continued.
I had checked the defroster switch was off and turned to the off position.

I note what you say about the interior delay relay #19.

I need to wait if/when the problem happens again. And then make sure the switch is off, remove defroster relay and remove interior delay relay and see what happens.

many thanks for the help

Mrmerlin

Thanks I am trying to learn what you see to provide more concise info next time this happens.

Alan

The alternator will usually remain functional - its the old ECU's ($$$) that are at most risk of damage from wildly fluctuating spikes way above normal running voltage.

The battery acts as a ballast for the alternator - it cannot regulate well without the battery connected (even when severely depleted it will still help).
A good battery also directly protects against spikes from load step/dump changes from the many other major (esp. inductive) loads turning on/off.

So don't ever disconnect the battery when the car is running - it will almost never stop the engine - unless the alternator doesn't work or disconnection itself directly damages something

Alan

Alan

Fred this is not correct:
- The Ignition circuit (15) is there to provide power for the car to start & run normally. It is active during both starting and running, When you turn it off with the ignition key, the fuel pump and ECU's dropping out stop the engine

- The Accessory Circuit (X) [aka X-Bus] drops out during starting to remove non-critical/high loads to provide more power for starting (Headlights, Cooling Fans, Blower Motor, AC Clutch, Brake Lights, Wipers, Power Outlet, Rear Defroster, Cruise Control/Solenoid, PSD)

- The Starter Circuit (50) is active only during starting

Alan

FredR

Alan,

You are inferring something I have not written as did another of our friends - I am talking about what happens during the starting sequence whilst the engine is cranking on the starter motor- this defines the bus 15 design load period and is minimised for obvious reasons given the load demand from the starter motor. What happens once the motor is running is somewhat irrelevant as it does not really matter whether the item is powered by bus 15 or bus X at that point in time given they are both energised once the key is released and springs back to normal run mode.

However once we come to turning off the motor the entire point of the discussion is that for the motor to run on bus 15 must be energised thus fault analysis dictates that the focus of the discussion has to be centered on why the bus remains energised and there are only two possibilities- either the ignition switch has not disengaged or some other false logic has kicked in taking bus 30 voltage directly across to bus 15 somehow- given the OP has a new ignition switch and they invariably fail due to ageing, the logical source of his problem is most likely a false logic condition. Stan has identified the defogger switch and the defogger relay as the possible cultprits. I cannot see bus 15 connected to the relay but for some reason I know not the defogger switch has connections to both bus 30 and bus 15- ergo it would seem that somehow therein lies the fault condition. Why on earth bus 15 is connected to that switch at all is beyond me and I suspect it is a design flaw but maybe someone like yourself can understand this and explain why such exists- a defogger system on the rear screen cannot possibly be needed during starting can it!

Your thoughts?

Alan

Sorry Fred,

I was responding to how what you wrote typically reads - I can see how it was not what you meant. (perhaps: Bus 15 exists to power only the items needed for starting & running the engine...)
To the embedded question in that original first line that I didn't answer before - (15) is used to enable the defogger - it is not operative except with the engine running - on high it would deplete the battery too fast.

To your second paragraph, Bus (15) does actually connect to the relay logic (on pin 15). As I noted in the last reply the rear defogger is also one of the (high load) things deactivated when starting (this is actually accomplished in a negative way with a connection to the starter circuit (50) rather than via (X) but it amounts to the same thing).

There have been many previous posts indicating faults where a (15) <-> (30) connection is maintained through the defogger relay causing running-on. These were resolved with the relay removed or replaced. It is not obvious how this occurs since (30) is only a relay switch contact connection while (15) only connects to the logic circuit, so it does not seem to be a sticking relay issue. So while I don't know for sure - it seems most likely its some kind of mechanical breakdown allowing the logic circuit to connect with the (30) relay terminal or relay pin inside the relay.

Alan

FredR

Alan,

That logic makes sense- as per a point I made earlier should/could they not have put a diode in the bus 15 feed to this sub system to prevent a back flow?

If that relay is the same price as the headlight equivalent I think I would seriously consider disengaging the defogger coil given I have no need for it and neither probably do you! If I was in a miserable UK climate where this kind of feature is highly desirable I think I would seriously consider simplyfying the entire issue with a simple old school on/off switch.

Alan

Usually Porsche (and suppliers) do a good job of functional safety (fail safe considerations) - but the defogger issue while inconvenient isn't really enough of a safety hazard for them to focus too much time on. They must have eventually become aware through dealer reports that this happened occasionally, but I'd expect they'd just file it under "do a better job next time" - rather than consider making a running change. It requires a relatively rare fault to occur for this to manifest, and while certainly not benign - it's not catastrophic or even particularly expensive/difficult to remedy.

It is vaguely possible that the (15) circuit does have a diode into the logic circuit but that the fault somehow bypasses it (consistent with the nature of such faults). I do agree the defogger solution here is a overly complex - the 2 elements of the defroster can be run in series for low power or in parallel for high power - this adds quite a bit of extra complexity to the switch gear. The (15) connection to the relay is also really almost redundant* because the switches for high or low power modes are only active when (15) is active.

*It cannot just be removed but it serves no special functional purpose beyond powering the logic part of the relay - functionally (X) would be equivalent and perhaps better - no need for (50), and in case of this fault - the engine would still stop, (30) would also work - except for the extra power draw of the always powered logic part.

Alan