Repair of the Fan controller module beside the passenger seat, which activates the S4/GT/GTS fans through the power stage:

I tried to investigate with my oscilloscope, also.... The tc 4051 and lm2902n were sending strange outputs in board, when removed lm2902 failed completely...I still have faulty chip and I'll tell you something as soon as I find datasheets. I changed 3 chip(tc4050 and tc4051 +lm2902n), 4 condenser (2 are10uF 63v, one is 220uF 25v and the last one is 100uF 35v) and 11 diodes (10 are bc337, one is bc327) for 5,50 euro. For this little expense who cares about the culprit, except you and me (obviously)?
Bad part of this refresh :that ugly White isolant , maybe silicone, is a nightmare when close to chips, you cannot clip the heatsink to every Pin.... So be careful with solder not to cook chips...
Francesco

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1989 s4
part# 928 618 418 00

stage I connections number 1 thru 8
stage II connections number 1 thru 8

Plug I
1=Green/White
2=Red
3=blank
4=Black
5=White/Black
6=Blue
7=Brown/Black
8=Green

Plug II
1=Blue/Green
2=White/Red
3=Brown/Blue
4=Brown/Gray
5=Green/Gray
6=Brown/Green
7=Gray
8=blank
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1990 928 S4