Interchanging Q7’s Battery Management Module with that of A4

[DeeWS]

I have two cars – an Audi A6 C6 2005 V6 3.2L and a Q7 2007 V8 4.2L.

Quite some time ago, my mechanic had replaced the battery management module in the Q7 with an additional one that I had lying around for the A6. Is the A6 module compatible for the Q7. The part number for the A6 is 4F0 915 181A/0 199 000 013 while the one originally in the Q7 was 4L0 915 181/0 199 000 017.

I am asking the question about compatibility because lately I am having a problem in the Q7. Whenever I start the Q7 engine, the battery management module gets the DTCs 02254 – “Interface to Generator 004 - No Signal/Communication” and 02479 – “Partial Consumer Shut-Off Active           000”.

Previously in the autumn of 2022, the Q7 had stopped twice while driving. As I could not immediately restart the car, while waiting, I had measured the battery voltage and found it to be around 11.2 volts. The DTCs would appear then. I had to wait for the voltage to rise above 12.5 volts before the car would start. I assumed therefore the battery was weak. I do not want to replace until I am sure that its not the battery management module nor the alternator. What causes this temporary loss of interface to the generator?  Bad alternator? Weak battery? Connection problem between alternator and J644? Or the J644 module itself? I measured actual voltages with multimeter and they seem ok except when the car had stopped and the voltage was 11.2 volts but that has not happened again.

In attempting to analyze the reason for the DTCs, I used VCDS Advanced Measuring Values for Battery Regulation 61 and noticed that whenever Generator Interface was Not OK then the Generator Voltage was 14.5 volts and when the Generator Voltage would rise higher that 14.5 volts e.g. 14.7 or 15.0, the Generator Interface became OK. Additionally, I tried to find out from Ross Tech Forums for VCDS where the Generator Voltages (Group 1 and Group 10) are measured to understand why both are not the same. No success from there. Obviously, they are measured at differed points for their values to be different.

Key Position	Engine Condition	Time Elapsed Marker	Group 1 Generator Voltage (specified) in Volts	Group 1 Battery Voltage (actual) in Volts	Group 1 Load Response Time in seconds	Group 10 Generator Voltage (specified)	Group 10 Dynamic Field Signal  %	Group 10 Generator Interface OK/Not OK	Group 10 Generator OK/Not OK
Accessory	Off	0	12.00	12.35 to 12.40	3	0.00	0	OK	OK
Start	Cranking	43	12.00	10.80	6	0.00	0	OK	OK
Start	Cranking	44	12.00	12.20	6	0.00	0	Not OK	OK
On	Running	44.96 to 2618	14.50 (constant)	Varies between 13.35 to 13.45	3	0.00	0	Not OK	OK
On	Running	2618.93 to 2645.64	15.00 (constant)	Varies between 13.95 to 14.10	3	15.00	51.6	OK	OK
On	Running	2646.51 to 2744.87	14.50 (constant)	14.10 falling to 13.35	3	15.00	41.9	Not OK	OK
On	Running	2745.83	14.70	13.45	3	14.30	41.9	OK	OK
On	Running	2746.78	15.00	14.00	3	15.00	41.9	OK	OK
On	Running	2747.84 to >> 3019	15.00 (constant)	Varies between 14.00 to 14.10	3	15.00	Varies between 41.9 to 48.4	OK	OK
Off	Stopped car

[Steve Q]

Yep I'd say incompatible. You'll need the q7 one 

[DeeWS]

Thanks Steve. Will get one for Q7 and replace. Have a good day!

[Steve Q]

On 3/22/2023 at 11:59 AM, DeeWS said:

Thanks Steve. Will get one for Q7 and replace. Have a good day!

Thank you you too 🙂