Reverse switch - no power

[FelixBuchanan]

Hello all,

Done all the usual stuff I could find suggested online, new bulb, checked every fuse, new reverse switch, checked for power and broken wiring to reverse switch. Still nothing.

I have determined that there is no power going to the reverse switch. Only pin 1 (Blue/Black) has any form of detectable resistance to ground and it's around 1MΩ.
So, I assumed pin 2 (Black/Green) is for power and is simply broken and is an open circuit. I've stripped back the protection all the way back to where it splits into a Y-joint near the nearside headlight and appears to continue back through the firewall and then splits off into one of two directions, the ECU or behind the instrument cluster. Does anyone know which of these two this wire is supposed to terminate? (Hoping not the ECU because that looks like a pain to remove from a few YouTube videos I skipped through).
Because I was a little desperate I checked for continuity between each pin in the connector and poked a tiny hole with my multimeter probes through the insulation just before that first Y-joint to make sure there wasn't a hidden break within the insulation, but unfortunately it buzzed out on both wires. I find it hard to believe there could be a break any further up the harness as everything appeared to be so well strapped down.

Haynes manual reckons the reverse switch power comes directly from the passenger fusebox fuse (FC4) (Heating, Air conditioning and some other things). As I said earlier none of the fuses are bad and my heating works perfectly fine. Also, Haynes reckons the two wires should be Black/Blue and Black/Purple where as my wires are Black/Blue and Black/Green so not sure to trust it.

Absolutely any suggestions would be greatly appreciated. Got some pictures if that helps anyone visualise what I'm trying to describe.

[jonathanfenton]

Have you checked the fuses in the ecu box?

[Crasher]

The reversing light switch is fed by fuse SC4 so fuse box C fuse 4, the box in the drivers side of the dash. SC4 must have a feed as otherwise you would have lots of other problems. The power feed to the switch is black/purple and comes from a junction in the main wiring harness but if you had a problem there you would also have an oil level sensor warning. The feed from the switch is black/blue and this goes into the Body Control module where you can see the signal and test the output with suitable diagnostic equipment.

[FelixBuchanan]

Thanks for the reply, if you mean the fuse box under the bonnet then yes all fuses check out okay.

[FelixBuchanan]

The reversing light switch is fed by fuse SC4 so fuse box C fuse 4, the box in the drivers side of the dash. SC4 must have a feed as otherwise you would have lots of other problems. The power feed to the switch is black/purple and comes from a junction in the main wiring harness but if you had a problem there you would also have an oil level sensor warning. The feed from the switch is black/blue and this goes into the Body Control module where you can see the signal and test the output with suitable diagnostic equipment.

Okay that's good to know the power feed is probably okay as no other issues with other sensors.
Just so I understand how the system is supposed to work. Black/Green joins a junction in the main wiring harness and splices into the power feed Black/Purple? Black/Blue returns to the BCM with feed when reverse is engaged?

I'll try and take a look to see if the BCM is getting the signal from the reverse switch on the Black/Blue wire. Is the BCM located under the passenger dash?
The only thing unclear to me is why I can't see feed on the Black/Green wire going to the reverse switch.

[Crasher]

The black/purple (or violet as it is in the VAG diagram) from SC4 is a 0.5mm CSA wire which goes to a junction in the harness where it splits off the heated washers (when fitted), the heater panel (with manually controlled heating) and to the oil level/temp sender in the sump. Out of the switch it is a blue/black (I got it the wrong way around before) back to the BCM at a plug called T8h in socket 6.