A/C Fan won't spin - 2004 A4 1.8t - New FCM - New fan - HELP

[Bennie121]

Hello Fellas,

I've been working on my A/C fan the last couple of days. I've changed the fan itself with a new unit, as well as the FCM with a new unit both from ECS tuning. When I try to force the fan on with VCDS, nothing happens. It makes me sad.

I verified the A/C fan works, jumping it with the battery, the large fan works and seems to work correctly when tested from the engine module with VCDS

When i check for voltage at the 4 pin plug i get
brown - ground
Red/Blue- constant 12v
thin
red/white- signal?
red/gray - acc 12v
The fuse at S4 is good at 5 amps. I have a 60 amp and a 30 amp under the dash. They're both good.

The signal coming from the red/white wire does seem to produce a "sine wave" when i output test with VCDS-lite in the HVAC module. I say "seems" because unless i unhook the battery, I see the same wave pattern. Even after the entire output test on that module has concluded. Take this with a grain of salt as my o-scope is the more of a demonstration piece. I also tested it with a craftsman pro multimeter in the duty cycle setting. Basically same result, 0 to 100% duty cycle in a "sine wave" like pattern after the test concluded. When i test on AC setting on the multimeter, I never see more than 1 volt or so. Not totally sure this is normal...

I noticed the same sine wave when probing the ACC 12v red/gray wire. Completely confused as to what this means. I'm not an expert when it comes to using the scopes. It could be noise?


After reading some more forums, i decided to pull the ecu cover. Dry as a bone. One interesting thing I found in there was a blown 40 amp mega fuse. I have no clue what it is for. Everything seemed to work when i parked the car to work on it. I replaced the fuse and hurried to test with VCDS and still no AC fan. The radiator fan works with or without that fuse. I got sad at this point again. I will note that the fuse could have been blown when i tested a junkyard set of fans and FCM... the A/C fans were dead on my car, and the junkyard ones...

Changed the ECU with the original from the car, no change.

Only things i can think is bad HVAC module? I'd love to know where the signal wire terminates. Anyone know if it is in the ECU? or is it in the HVAC module?
Or a bad new FCM? I'd hate to exchange my FCM to find it was something in the car.
Or bad relays? Not sure where to test there... Most people have the opposite problem when it comes to relays and fans, theirs seems to run and not stop

Does anyone know any specifics about the "PWM" signal on the signal wire? I probably have the tools to emulate it.

Anyone have any suggestions? I'm extremely close to crying for mommy... My cheese has slid completely off my cracker... I've been reading, reading and reading... Haven't found a set of circumstances like mine

Am i using VCDS wrong, maybe?

Thanks in advance!

[EuroxS4]

A/C fan only comes on when A/C is on. If econ buttom is lit up a/c fan will not come on. Also nother thing to check if the charge in the a/c system and the pressure switch they are notorious for leaking right through the connector. Checking pressures in the A/C system is important. If the compressor is shot A/C fan wount come on either. Although withoutput test through VCDS it should come on. I would check to see if its getting power supplied to it when needed.