Audi A3 Cabriolet 2008 - Left side tail light problem

[Phil A3 8p7]

Hi, I'm hoping someone may have had a similar issue. My left tail lights (inner boot cluster and outer LED's) are not working despite both clusters working on the right hand side.

I assumed therefore it must be a wiring issue but all of the wires going from the BCM to the light connectors measure 0 ohms and the contacts have been cleaned and checked AOK. So I purchased a BCM from a breaker yard but have the same problem. It seems there is a 10v signal missing from pin 4 on the inner cluster and pin 5 on the outer. I know this because it is present on the right hand side cluster. The 2 pins are connected together according to the wiring diagram so it seems the BCM is not supplying this signal. I wondered if I should have cleared the codes when I installed the new BCM? Perhaps a stored code in the gateway is telling the BCM not to switch on these lights?

vcds code 00984 - lamp for tail light, left (M4) 012  - electrical error in the circuit Ambient conditions: error status: 01101100 on voltage 11.70V off on off off off

Any help or suggestions would be greatly appreciated!

[Steve Q]

Yep I'd recommend getting the code cleared. Plus the bcm might need coding to the car. 

[Phil A3 8p7]

I’m going to refit the new BCM this weekend and clear the codes and check the coding is the same, fingers crossed. I’ll report back on how that goes. 
 

btw have you had the same problem Steve?

[Phil A3 8p7]

Refitted the new second hand BCM, coded to match the original and cleared the codes but still no left tail lights unfortunately! I’m thinking that maybe it’s a ground problem, perhaps where the grounds are spliced together in boot lid near the wiring boot somewhere? The ground points do measure ok but maybe one of them is hanging on by a strand. really clutching a straws now. 

Edited March 7, 2023 by Phil A3 8p7

[Steve Q]

I'd recommend getting an auto electrican to take a look 

[Phil A3 8p7]

Just to let you know that I have resolved the issue. So with the new BCM installed I  checked if the light worked in the right hand side but this time it didn’t, so I tried the right hand cluster in the left and it worked. Weird especially as the left worked in the right with the old BCM. Therefore I was pretty sure now that cluster was defective. So I tried the right cluster in the left and it worked confirming my hypothesis. But still a bit puzzled I decided to compare the voltages on the outer cluster pins 2, 3 & 4. 
 

left outer connector (old BCM)

pin 3 - 0.48v

pin 5 - 0v

pin 4 - 0.7v

pin 2 - 0.7v

Right outer connector

pin 3 - 0.48v

pin 5 - 0v

pin 4 - 0.48v

pin 4 - 0.48v 

With the new BCM the voltages on the left where the same as the right 0.48 V which concluded that the BCM and light cluster were both defective. 

On close inspection the left light has some water in it which probably caused the LED circuit to short out which induced a fault on the BCM. However I believe this was intermittent because it worked on the right hand side during my initial fault finding and that’s what made the whole fault finding process that much more complicated.

so I picked up a new cluster from a breaker and hey presto it’s now working. 

This would have been a lot easier to troubleshoot if I knew what the voltages were supposed to be on the tail light connectors so I hope this may help someone else out one day.

 

 

[cliffcoggin]

The solution would have been obvious if the water in the lamp had been spotted initially. Water is by far the most common cause of lamp faults, whether in the form of a short circuit or corrosion of terminals.

[Phil A3 8p7]

Not necessarily because the light worked on the right hand side initially and fault finding during the dark nights was not easy. But I hear what you’re saying.