Weird start/non-start problem - help needed
 

I have a suspicion what this might be but don't know enough...…
Started the S8 this morning, no problem, even though it hasn't been used for a couple of weeks. Let it run while I checked the tyres - one has a very slow leak yet to be fixed.

Went out, stopped to post some Xmas cards, got back in the car and it wouldn't start. Lights on as normal but no action whatsoever from the starter.
Bit of a panic as I was on a mainish road and half-blocking the driveway next to the post box!
Somehow by moving the gearlever and operating the key it suddenly started so I went back home.

Turned off, removed key and tried to start - no starter action, not even a click.
Moved the gearstick to different positions and tried again, still didn't start.
Then I kept my foot on the brake AND the key turned to start and began moving the gear lever around and it fired up.
I think it was as I was moving from D to N that the starter kicked in. The engine actually started in a fraction of a second as it usually does.

So I'm guessing this is something to do with safety circuits (and switches?) ensuring you can only start the car in P or N.
But how to I check, confirm and rectify whatever is going on? Thanks in advance for any advice.

Sounds like... F125!

This is the same problem Neil had when the neutral lockout terminal fell off inside the switch - there is a relay in the starter solenoid circuit which is only activated in P or N, otherwise disconnects the solenoid.

It can be bypassed by a bit of chunky wire and some 6.3mm flat terminals if you're unable to get it to a garage to swap the F125 - basically the same configuration as a manual car. There might be a pic in my manual swap thread.

Don't throw the old switch away as I can usually refurbish them if its just a burned or bad contact (like most of them). Not so much if the contact has actually come adrift though like Neil's did.

Whoever designed that stupid switch has a lot to answer for :rolleyes:

Oh dear, it's not that long since I had the F125 replaced. :(

Where do I need to access to bypass it, is it inside the car, engine bay or underneath? and how easy is it?

I'll plug in VCDS sometime to see what that shows, if anything, but I've got other more important things on my plate at the moment (ask Neil!) so the S8 will probably be just parked for a while.

PS I've also still got the old F125 somewhere, I opened it up and took some photos then fitted it together again with some reusable screws/nuts.

Had a quick look but couldn't see anything obvious (to me).

Is it possible that your gearstick movements merely coincided with the intermittency of the fault? I've had starter motor faults that resulted in similar, seemingly random behaviour.

I don't think so, I had the key twisted into start position and held it there with no "click" of the relay.
As soon as I moved the stick it started.

If the F125 checks out, have you checked the main battery cable on the drivers side chassis? When that corrodes out you have power to everything but no starter action. If it’s getting crusty (damp and salt won’t be helping at the moment) it might start causing intermittent no start?

Also check if yours has had the recall done for a slipping park interlock cable - if there's no fault codes suggesting F125 it'd be worth checking....

Thanks for the input folks, I'll not rule any of them out. :cool:
But right now MJs F125 theory fits the symptoms way better.

And I investigated that recall Adrian, ages ago, and I'm quite sure it's not relevant to my car. Nor does it match the symptoms.

Right now I really would like to know more about bypassing the F125 starter solenoid lock...…..

I don't appear to have a photo of it unfortunately. Its J207 which Elsawin claims is top left in the relay panel in the footwell. I remember it being more in the middle of that panel tbh. The manual has a jumper between pins 2 and 8 per this diagram:

http://forum.a8parts.co.uk/attachmen...1&d=1575758921

I used some 2.5mm wire and some 6.3mm terminals to bridge 2 & 8. The factory one is a bent bit of copper, but I have no idea what the number is so just made my own.

If you jump power to pin 8 it will spin the starter which might be diagnostically useful.

Late at night and just been sampling 27 and 36 year old whiskies, but that looks good.
I'll investigate further tomorrow. +++

Do you remove the relay and put the jumper in or need both?

Also I can't recall the layout there, is that the main panel under the "board" under the passengers feet, or the section that I think is above it?