Evening everybody…

I have a very strange issue with my 2004 TD6 that has me completely baffled.

Each time the ignition switch is turned to position 2 (just prior to cranking) fuse 39 goes pop. The car subsequently will then not crank.

I’ve had a poke about today and found that if you turn the key to position 2 without a 5amp fuse in slot 39, then put in the fuse it stays intact and the car starts and runs normally.
Suspecting the ignition switch I’ve had the center console out and the ignition switch out to check and clean the plug and look for any possible damaged wiring, I’ve not tried dismantling the switch as there is a big looking spring in there to return the switch to pos 2 after cranking and with my luck this would disappear somewhere never to be seen again.

What’s confusing me even more now is the fact that the fuse only blows if it’s in place when the key is turned, surely if there was a short somewhere it would go as soon as it’s inserted, all I’m doing by putting the fuse back in when it’s already in pos 2 is moving the point of contact from the switch to the fuse aren’t I ….?

It did have a replacement switch a couple of years ago, symptoms were very similar fuse 39 blowing but it only happened intermittently and would be fine for a while with a new fuse.

A new switch isn’t cheap and I’d rather have a little bit more certainty that this could be the issue before I take the plunge and order one up.

Anyone have any suggestions or opinions, anyone had a similar issue?
I can find similar threads but nothing relating to fuse 39 blowing

Cheers,
John

Fuse 39 protects the steering lock ecu.Why it should blow in the manner you describe l have no idea but it’s a start.

If you click on Wiki at the top right of the page and go down to INFO you will see electrical circuit diagrams.
Go to pages 30 and 32 of the diagrams and you will find what fuse 39 does.

If I’ve been reading the circuit diagrams and the system description and operation manuals correctly fuse 39 seems to cover the auxiliary feed to both the steering lock and immobiliser ecu’s.

When the key is put into the switch the steering lock can be heard to click and disengage, when the key is turned all the dash lights come on, the steering wheel adjusts to the driving position, radio comes on, blower starts etc….in short everything seems to be absolutely fine until the second position, when the fuse blows.

All of the above are not effected, dash, stays lit, blower still runs, radio stays on but the car won’t crank with the fuse blown.

Just for an experiment I stuck in a 10 amp fuse instead of a 5 amp and this still blows, didn’t fancy going higher than a 10 amp.

It’s got me well and truly stumped. Even if there was a high resistance in the ignition switch this should reduce the current wouldn’t it?..

A high resistance would reduce the current as you say, it does sound to be a really odd issue. BMW i3 Electric Car
2012 Full Fat RR 4.4 TDV8 (now gone)
2006 VW Touareg 3.0 TDi V6

The only thing the second position on the switch operates is the starter circuit. Given that everything is ok until you go to the the second position it does suggest that the earth fault is associated with the starter circuit. This is more so as when you start the car without the fuse and then put the fuse back in everything is ok ie the starter circuit is out of circuit.
I’m guessing that there is an earth fault somewhere on the starter circuit. Could it be simply a case of one of the wires chaffing?

Made progress but I honestly don't thing it the end of the saga..

I studied the wiring diagrams, looking in particular at the power distribution section on pages 11-15, Fuse 39 should only become live when the ignition is turned to AUX (position 1), it does and the fuse does not blow. When the ignition is switched to IGN (position 2) Fuse 39 blows so I went looking for somewhere that had feed from both.

On page 13 I found the seat relay had a possible link through something called relay 6, no idea what this is couldn't find it, don't even know if it exists and I'm sure its drawn wrong on the diagram but I thought with nothing to lose I'd have a look for the seat relay. Found it, buried behind the glovebox, took it out, looked at it, cleaned the terminals, put it back in, turned the key to AUX, crossed my fingers, turned the key to IGN and would you believe the fuse doesn't blow. Cranked the engine, car starts...

Tried it a good few times just to be sure and it seems to be OK

Now I have no idea why this may have caused the issue I was having and like I say I very much doubt that this will be the end of it. But for the time being at least the car starts and there are no flashes from the fusebox.........

Hope its still fixed in the morning.

Does anyone else find the Wiring diagrams so difficult to follow...?
Back in the day everything was so much easier....!!

Cheers for the help