Rear Fog Light stopped working Mk7 R Estate

[bainbad]

Don: that "virtual mk7" of yours looks extremely impressive! Thank you very much for the detailed analysis that you have done with it looking into my Rear Fog issue.

Yesterday I did try the "total vehicle reset" by removing both battery terminal connectors for 30 minutes. The anticipated bunch of errors re-set, most from turning the wheel lock to lock. The rest went away with the OBD eleven. However doing this made absolutely no difference to the problem with the Rear Fog.

I have just back-pinned terminal 5 of the Headlight switch with a T-pin (didn't pierce the wire) and done the DC Voltage checks that you advised. Results look essentially the same as yours.

• No fog-lights (centre turret in OFF position) = Rhythmic pulse fluctuating 2.3 - 2.8 V
• No Front fog-lights fitted to car so n/a (switch only has Rear Fog)
• Rear fog-light (centre turret out) = Rhythmic pulse fluctuating 0.1 - 0.2

I assume the rhythmic voltage fluctuation is due to the nature of the signal which you initially explained to me (don't have an oscilloscope).

I repeated the tests with the original Headlight switch and obtained the same results which confirms my suspicion that there was nothing wrong with it.

I have checked carefully in the passenger footwell (RHD) for any sign of water ingress and all looks good.

I agree that it seems unlikely that the problem is at the BCM end of the wire from pin 5 of the Headlight Switch given the nature of the 3 x multi-pin sockets and how firmly seated they are into the BCM housing with a low probability of moving. As you say the A29 wire is directly connected with no intervening terminals/plugs on-route to the rotary light switch. Looking at the wiring loom at the back of the Headlight Switch the wires are all wrapped together in protective black tape so it is difficult to believe that the pin 5 wire would have been singled out in isolation for abuse.

However I might need to check the signal is getting to the pin A29 end before the car goes to VW to minimise cost!

I wonder if VW have a way of reprogramming the BCM unit rather than just swapping it out for a new one (which sounds expensive).

 

[DV52]

@brainbad: hmm...... yes, the fluctuating voltages in your readings are indeed the result of a DC volt-meter trying to measure a waveform with a pulsating frequency!

Also, in respect of your statement "I might need to check the signal is getting to the pin A29 end before the car goes to VW" - don't be concerned about this aspect of your readings. I don't know how the internal circuits inside the BCM operate for the signal from the fog-light switch, but it seems from my testing that the A29 pin provides a quiescent voltage for the yellow/green wire.

If I remove this wire from pin #A29 on my test-bench mk7 and I measure into pin #5 of the rotary light switch, I get no DC volts!! So - your test confirms that there is good electrical contact between the rotary light switch and the BCM.

Whilst this is good news - I'm totally baffled about the cause of this fault!!

To recap:

1. You have proved that the BCM successfully powers the fog-light with the output test
2. The Leuchte-set coding has been checked and it's the same as the factory settings
3. You have reset the CAN system on the car with the battery lead removal procedure
4. You have confirmed that the BCM is receiving the correct switch-ON signal from the rotary light switch via the DC Multi-meter test

Very stubborn problem indeed. My only niggling doubt remains your statement that the fog-light indicator lamp doesn't illuminate on the dashboard when the output test is initiated. As I said, on my test-bench, the indicator did turn-on.

I'm clutching at straws - but are there any other error messages on the SCAN report for this car - maybe on other modules?

As for your closing thought "I wonder if VW have a way of reprogramming the BCM unit rather than just swapping it out for a new one" - the answer is YES!

Every mk7/mk7.5 vehicle that rolls-off the VW production line has a copy of its entire software coding from ALL installed modules stored on VW's central server which is called FAZIT (FahrzeugAuskunft und Zentrales Identifikations Tool = vehicle information and central identification tool).

All registered VW agents (i.e. dealers) have remote access to FAZIT via VW's own diagnostic device - which is called ODIS (this is the device that dealers plug into the car's OBD port)

Don

 

[bainbad]

Don: thank you!

I don't think there is a problem with the fog-light indicator lamp on the dashboard. It does illuninate when the output test is initiated. I had posted:

• Using the Output test for Rear Fog light both the Rear Fog Light and the amber Dashboard Warning light do come on.

Confusion is probably because I later replied to ZERO815:

"Realised that the small indicator light for the Rear Fog on the Headlight Switch doesn't ever light up even when doing a successful Output Test for the Rear Fog."

I should have clarified that the anber Dashboard Warning light was still coming on with the Output test, but I was concerned that the little fog light icon in the headlight switch was not illuninating. ZERO815 then clarified that the little fog light icon on the headlight switch itself is not supposed to illuminate. Sorry - my bainbad.

Looks like I now have enough information to give the VW dealer service department another call and enquire about the FAZIT BCM reprogramming. However I will also run another Obdeleven SCAN report for fault codes on other modules.

Thank you again for all your invaluable input.
Tony

 

[bainbad]

Epilogue...
Sorry for delay in updating what happened about the fog light and the BCM.

In April I took the car to the nearest VW main dealer 56 miles away.
They had it for three days.
They confirmed all the information that I had given them (as discussed in the previous posts) - labour = £199.99 inc VAT
They replaced the BCM and recoded it - labour = £117.60 inc VAT
The BCM itself (J519) was = £576.00 inc VAT
That gave a grand total of £893.59 inc VAT just to fix a non-functioning rear fog light!

They tried to get another £147.36 out of me because the under bonnet sound deadening material was distorted and hanging down "slightly" (image attached).
I declined that offer.

They had not warned me to give them both sets of keys when they installed the BCM. I subsequently found that the BCM was not recognising the spare key (which previously worked fine).
Initially they wanted another £75.00 to program the spare key.
Eventually I was put through to an actual technician who confirmed with his superior that the spare key should have been adapted to the new BCM at the time it was programmed.
Yesterday I took it in and ten minutes later my spare key was working again (£00.00).

Thanks again for the invaluable support which I have received on this forum.