AC clutch stays engaged. Bad FCM fix

My MKIV jetta FCM was bad and cause the ac clutch to always engages. It would drain the battery overnight.

What I found was the transistor in the FCM that drives the relay to send power to the AC clutch was bad. After replacing it with a more robust transistor, it fixed the problem. Many people replaced the FCM but ended up replacing it again after a few months. Maybe with a more robust transistor in place it will last longer or fixed permanently.




The link is to the bad transistor specification.
https://1drv.ms/b/s!AqcJSAsfoVPIgpQYAsBDpxEWW-_ldw

I replaced with a transistor pulled from a circuit board laying around. The new transistor doesn't have to be the one I used as long as the specs are better.
https://1drv.ms/b/s!AqcJSAsfoVPIgpQZh8IqwZoF94VW5A

Wow! I'm impressed. There's been reports of bad FCM's found on eBay that keeps clutch engaged. Glad you found the cause. Great soldering skills BTW.


A side question: based on your knowledge of how the FCM is wired up, there is a way to install a switch to trigger the fan on manually? I'd like to do that during stop-n-go traffic to hopefully keep my auto transmission fluid from overheating.

Feed 12v to Pin 7 for high speed or pin 8 low speed. See link for more details.https://forums.vwvortex.com/showthread.php?8708577-FCM-pinout-needed

very impressive troubleshooting, do you have a background in electronics?

I studied in school but now it's just a hobby.