sprie

I was investigating where my exhaust was blowing (turned out to be from where the exhaust meets the manifold, gasket not quite doing its job), and having finished, i pulled the ignition key - and nothing happened, engine carried on running.
I now know that fuse 38 for the fuel pump is the best way to stop the engine in such cases, but i tried disconnecting the battery which didn't work, and then resorted to pulling ignition leads, until the engine had had enough and stopped. I now know better.

However, this hasn't happened before. The latest work i have done has been for the HVAC: i have pulled the centre console, and moved cables around a bit whilst i sorted out the vacuum diaphragms. A few months ago I did replace the ignition switch, as part of the many steps in sorting out my headlight issues.
I am wondering if something i have done in this latest work could have caused this issue - is there anything vulnerable in or around the centre console that could cause this ignition issue?

gazfish

Look at this thread for suggestions: https://rennlist.com/forums/928-foru...l-running.html

hwyengr

Fingers crossed for the health of your alternator, having disconnected the battery while the engine was running.

Mrmerlin

next time pull the defroster relay.
Make sure your defroster is also turned off it has three positions.

Off CCW
Timed on, by pushing in the button. the lamp should illuminate, the relay will turn it off
On , by turning the switch CW the internal lamp should illuminate.

NOTE dont disconnect the battery ground strap or positive lead while the engine is running,
damage to the light or engine computers and or the regulator and or alternator can occur.

NOTE if the engine turns off after pulling the defrost relay,
you could take it apart and clean the contacts, then put it back in to test it.

Petza914

I know another 928 that does this and turning on the headlights that creates a load.once the key is removed causes it to turn off.

FredR

Ian,

If the engine is "running on" chances are this probably means that for whatever reason the ignition switch is not dropping the EZK relay in the central electrics.

The most likely reason for this [I suspect] would be if the EZK relay has welded itself stuck in the running position. Easy to test for this by pulling the relay and checking for continuity across terminals 87 and 30. Even that may not fully explain what is going on- I need to look a bit deeper into my system analysis diagram I prepared a few years ago.

Update:- my system diagram was conveniently to hand and the EZK relay sticking on is not a viable reason for the run on condition as the bus 15 supply dropping out would also cuase the fuel pump to stop running.

Thus the only thing that can cause the condition you experienced would be if the ignition switch fails to isolate bus 15 from bus 30 and I fai to se ehow you could have inadvertently caused this.

sprie

I had 30 minutes free today when I could do some investigations and the main result is I have just confused myself, and I need to be more methodical.

The first thing I did once the engine was running and the key removed was pull the defogger fuse (no 25). The engine continued running. There is a buzzing noise which i think is coming from a relay, but i haven;t worked out which one.



I then started pulling fuses from the left hand side, and it was only when I got to fuses around 36 – 40 that the engine died.

I did some more tests but with no conclusion, and I think I am confusing two different actions

1. If I start the engine and remove the ignition key, if I then pull the fuel pump fuse (no 38), am I right in thinking that the engine will stop but only after a passage of time whilst it uses up the pressurised fuel in the pipes/rails? If so, about how long does this typically take – as it does seem to be a long time.
2. On reflection, what I need to do (but happy to hear if this is not the right action) is leave the fuel pump fuse in place, start the engine and remove the ignition key, and pull other fuses (probably from 36 upwards) in turn. I think I am looking for a fuse where it stops the engine immediately – and if/when I find this, this would give me a good idea of where a short might be.

Does this make sense?

davek9

Yes the Engine will run for a bit after the fuel pump fused is pulled.
Also the advice given you was to pull the named RELAYS, not to pull fuses as there is a difference.
Pulling fuses is quicker and easier however may not have the same results, if you do not have a relay puller that gets behind the relay so as not to just pull off the cover, get one they are not expensive.

Dave K

Mrmerlin

Ian pull the defroster relay,
its a big one,
this should stop the engine otherwise pull fuse 42 the fuel pump fuse.
Please re read my earlier post,
as it describes what your seeing.
And how to deal with it so you dont damage other systems in the car.

FredR

Stan,

What is the logic here? The only explanation I can think of would be if there is some way this item can cross connect bus 30 to bus 15 unintentionally.

Bus 15 exists to power up the items needed for starting and dump everything else during cranking [high battery load] as I recall so why would there be anything related to bus 15 be needed for a defogger circuit?

Is this a design flaw or some kind of overkill feature that causes issues?

JET951

We have seen some where the ignition barrel is so worn that the key is able to be pulled out without turning the the key to the left. So basically the ignition switch is still in the on position.

Mrmerlin

Fred the engine running after the key has been removed is a common issue in the 928 due to a stuck defroster relay,
or the switch being turned on.
NOTE the defrost relay is seldom used and if the car sees moisture it can cause the relays to get degraded ,
and sometimes the defrost relay will stick on,
thats why to make sure its turned off. CW.
removal of the relay removes power to the ignition coils.
This issue has been documented more than a few times

76FJ55

The statement above isn't completely correct.
Bus 15 is ignition on voltage. it will have power in both the run and cranking position.
Bus X is the one that has power in the accessory and running position but cuts out during cranking.
and for reference Bus 50 is the power that is only active during cranking.
and Bus 30 is battery hot at all times.

FredR

Simon,

What you have written is a good overall summary of what happens but I did not write a summary as such is not needed - what I wrote is spot on correct.

Bus 15 exists primarily for the starting condition which is higly stressed electrically speaking due to the starter motor load thus critical that all non essential loads are dumped and that is precisely what bus X does whilst the motor is cranking.

Once the motor is up and running it is somewhat academic which bus the power comes from and the battery adds what the alternator cannot add as and when the engine revs are too low[ like in the idle condition]- thus why it is important that the battery is never disconnected when the engine is running as it can screw up the alternator if the demand is higher than the output as will be the case at rpms below about 2500 or so. Very easy to tell when this is the case as the voltage indicator will show 12 volts rather than the 13 volts or whatever the system is regulated to.

FredR

Stan,

As I see it there are two possibilities here viz:
1. The ignition switch is not doing its job.
2. The ignition switch is defeated because of a system leakage from bus 30 into bus 15.

I tend to discount the ignition switch because Ian advises the switch is new- not a guarantee of course but ignition switches invariably fail due to mechanical wear and tear over time.

You state that the defrost relay is seldom used- rather true in the Middle East! I do not even know which is the defogger switch - probably one of those items next to the sunroof switch above the transmission tunnel?

Out of interest I took a look at the wiring diagrams and I cannot see any indication to suggest that the power supply to the coils runs through the defogger relay but maybe I missed something. What I did see was a bus 15 supply to the defogger switch. Why that should exist I have no clue but given that is the potential flow route causing this issue one might think they would have installed a diode to prevent such from happening. This is the point of discusson that needs to be opened up some [I would think]. Why on earth would such a feature be required- Porsche must have had something in mind but what?