Electrical problem

[Wangler]

I was coming back from a short drive this afternoon in my S3 when the engine just cut out. I suffered the ignominy of having to push the car home, fortunately just for a few hundred yards.

The number 1 fuse had blown (the first on in the left) and a new one blew as soon as the ignition was turned on.

Can anyone please suggest where to start?

[lancialulu]

Dig out a wiring diagram and trace back the wires from fuse n1. One side is 12v supply and the other side is where the problem lies. One circuit is going to ground. Could be ignition switch wiring coming out of the steering column….

[Spider2]

I think power from No1 fuse goes to the front junction box that mainly powers the lights including flasher control, stop lights and the switch etc. so i am guessing a short in one of those circuits. Think about the exact second before the engine quit. Had you put any services on? Lights, brakes, indicators, flashers, even reverse gear (back up light). Careful thought may give you a clue. Best of luck

[Wangler]

Thanks guys that’s very helpful. I had just been testing my brakes by using them very hard at relatively low speed. All of a sudden the engine cut out. When I got back home I found the fuse had blown.

I’ll have to spend some of tomorrow trying to read the wiring diagram. I’ve taken a photo of it so I can zoom in so it’s legible to my ageing eyes!

[Wangler]

Hmm - one step back and one and a half steps forward!

The fuse that has gone is the first one on the left, which most people would allocate the number 1 to. In the wiring diagram however, they seem to be numbered the other way around with the one numbered 1 being the one on the far right!  I spent ages carefully tracing the wiring diagram (or at least my blown up photo of it on my Mac screen) highlighting the wires in colour from their fuse number 1 and wondered why they weren't present when I went back to the car to locate them. Now I know.

Perhaps one day someone with a week of spare time will produce a full colour diagram for the S2/3 wiring!

[nthomas1]

I have a full colour high definition wiring diagram for Series 2 which I would be happy to share.  I'm not sure who it was developed by originally.  I keep a laminated A3-size print of it on my garage wall for easy reference.  The S3 diagram, only available in black and white I believe, has different  numbering from the S2, but I think the coloured S2 chart would still be useful to S3 owners to help trace cable runs.

[Wangler]

That's an answer to a prayer if ever I heard one!

Thank you - shall I send you an e-mail?

[nthomas1]

Yes, that would make sense.  And we can decide the best way to send it.

[davidwheeler]

Could you not post it on here in the Technical Information thread please?

[Beckerman67]

 Posted as requested

[dhla40]

To avoid the engine cutting out in a dangerous situation I have moved the coil feed to the incoming side of the fuse.  The ignition switch supply is unfused anyway so I just make sure the harness containing the coil wire is well insulated where it passes through the bulkhead.

Sean

[Wangler]

Ha! Thank you. Just what I was thinking before I read your message today. I have a feeling that I won’t find the fault and it would always be lurking there ready to strand me.

I hadn’t got as far as working out the best way to make the coil independent, but I’ll probably put an inline fuse in an appropriate place.

[Richard Fridd]

Perhaps a suitably fused override switch from 12v supply to the coil.
  I have the opposite. This takes the form two 'kill' switches, one dash mounted and one engine bay mounted.  These are used in the event of the engine not switching off with the key. Also a security measure.
 
Richard

[Wangler]

I found a loose wire that was meant to be attached to the handbrake warning flasher unit which might have been the culprit. All seems ok now, but I think it’s wise to take the live feed for fuse 9 and attach a separate fused lead to the coil via a switch.

The way I see it is that so long as you’ve at least got ignition power you can always bump or jump start the car.

[Jaydub]

Hi Richard, I suggest you move the coil feed wire to the input side of the Fuse Box (on the same terminal as the Blue/Black wire from the Ignition switch). That way you won`t have a possible dangerous situation of losing ignition if the Fuse No.9 blew. You do NOT want a fused feed to an ignition coil. I had a Race car with a fused coil supply and the fuse blew right on a fast corner causing a potentially disastrous accident!
But it`s your car your choice.
Good Luck.