Reverse starts engine

Howdy, never ventured in here before, figured I'd try it.

So I have a 110 300tdi (R380 gearbox) at work that has had a fuse popping fault on the stop/indicator/reverse lamp circuit. While trying to fault find with the ignition on I placed it in reverse, at which point the vehicle started itself and I quickly found out that the handbrake doesn't hold an idling 300tdi in reverse.

I assume a short somewhere, but has anyone had a fault like this and have any ideas where to start? The problem stops when the fuse pops, so it has to be something on that circuit, but I couldn't see anywhere on a wiring diagram that'd logically lead to this issue.

Any help would be amazing.

Cheers gents.

jamesmartin said:

id try a new ignition switch, theres 2 small screw hold it into steering lock housing

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We have tried that and it still triggers reverse. Best I can tell, the wire from the output side of the reverse switch and the reverse lamp is shorting to the starter circuit, but I can't find any signs of it. It is most bizarre.

has it still got immobiliser box under passenger seat ,and look at starter relay

jamesmartin said:

has it still got immobiliser box under passenger seat ,and look at starter relay

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It hasn't got an immobiliser as it is a bare bones vehicle. I'll check the starter relay in the morning. Although I can't see how the starter relay could be connected in any way to selecting reverse. Without it being trigger by other events on the reverse lamp circuit, so brake lamps or indicators neither of which start the engine.

I suspect there is a problem behind the dash as this is the only area the two lines for starter exciter and cable for reverse switch would be close.
I have for some months been having intermittent starting problems with mine.
I have traced the problem to be in the line from the ignition switch to the center relay in the fuse panel. However finding the exact spot of the problem in the loom mass is another matter.
I have bypassed the ignition switch line from switch to relay for now, starting on a button from another live.Will investigate more when weather improves.

tottot said:

I suspect there is a problem behind the dash as this is the only area the two lines for starter exciter and cable for reverse switch would be close.
I have for some months been having intermittent starting problems with mine.
I have traced the problem to be in the line from the ignition switch to the center relay in the fuse panel. However finding the exact spot of the problem in the loom mass is another matter.
I have bypassed the ignition switch line from switch to relay for now, starting on a button from another live.Will investigate more when weather improves.

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I thought of that, but unfortunately the reverse switch takes power from a common fuse (indicators and brake lamp), and shares power up to the brake lamp switch, so it would stand to reason that using the indicators or depressing the brake pedal would also start the engine. That said I have made 2 bypass looms to try in the morning. One from the output of the reverse switch to the lamp, and one from the crank terminal of the ignition switch to the start solinoid on the starter motor. Hopefully one of them can help pin point the issue.

Wireing problems can be a barsteward. It still may have nothing to do with brake and indicator lines,you could disconnect them and see if the fuse still blows. Starter solonoid needs little current to trigger it,less than 5 amps.

Okay, just to follow up on this, I found the fault. I forgot to do a follow up as things got busy.

Two parts of the loom had worn through and touched, the white/red that goes from the relay in the cab to the start solenoid, and the green/yellow that goes from the brake switch to the reverse switch. I cut out the bad bits, put in some longer heavier gauge wire that had a stronger jacket (all we had at work) and rerouted them so they shouldn't rub anymore. That was about 6 weeks ago now, and the vehicle is being used daily over a lot of rough terrain and water crossings (it is used for driver training) and so far so good.

[Edit: Tell a lie, it was the green/purple (I think, or green/brown) from the reverse switch to the lamp, it wasn't the green/yellow from brake switch to reverse switch, that was stuck in my head because I kept having to tell people it wasn't that or the brake pedal would've started the vehicle. I found it was switch to lamp when I removed the bulb and it still started in reverse, which meant the fault existed before the lamp, but would stop if I unplugged the reverse switch, so I knew it was power out from the switch to the lamp. I was then able to chase the wires to where they had rubbed against the body and shorted.]

sorted . I made up a new direct line from switch to starter and mine has been good to.