ECU causing fan failure?

[DeSha]

I am trying to solve a no fan issue on a 2005 A4 quattro sedan. neither works when requested with vag-com or when running.
I have power through 40a fuse to ro/bl wire on fan control module (FCM).
I have power through 5a fuse to/gr wire on FCM
I have 0 ohms resistance from ECU to FCM on ro/ws wire
I have good ground trough br wire.

I tried a new FCM still no fans.

I have no O-scope to test signal into FCM on ro/sw wire, but vag-com says both are being activated.

How can I test if the ECU is sending correct request on tested wire?
Anyone have a diagram of the FCM circuit, or signal description? The diagram doesnt even show the wires from FCM to fans, and I havent found signal discriptions, or discription of its function.
anyone heard of ECU fan request circuit failure?

Thanks for any help

[DeSha]

In addition, I noticed that the small fan tries to activate. It spins about 1-2 revolutions and then stops. This indicates that at least that fan is working.

anyone know if one fan failure can affect the other fan. I still don't understand the FCM total function.

[diagnosticator]

The FCM receives a PWM run speed signal from the ECU. This signal can be measured as variable voltage as the average value of the PWM. Based on the value of the PWM run speed signal, the FCM outputs a power supple voltage to the fan motors. The wiring for the FCM and fan motors is in the Bentley. The condenser cooling fan should be running whenever the A/C compressor is active. A DC motor that starts to run but stops, is most likely faulty. You can prove the function of the fan motors by connecting the motors to 12 volts DC, the fan should run full speed with 12 volts jumpered to the motor.

[DeSha]

I brought my other a4 1.8t in today and tried to compare signals from ECU to FCM. I tried measurung Hertz, AC voltage, and DC voltage. I found that though they didnt match each other there was def a change in sigal between running to Off..
I had tested the small fan during FCM R&R but not the big one. Both spun freely. I had assumed that it wasnt one fan since both fans failed to operate. I was wrong. The failed large fan must be preventing the small fan some how. I cut the new FCM out of the car again and tested both this time. big one sparked but didnt move.

What I learned. One bad fan can cause issues with total operation of FCM. I wish I understood how.

[DeSha]

Well to my frustration the bad fan wsnt it. With new fan and new fan controler I am still having issues with both fans. both fans ocassionally start to spin and then stop without making it to speed.

This brings me back to the original question. any body hear of ECU fan control issues? anyone heard of this kind of failure? any help is appreciated. This is getting frustrating.

[DeSha]

Final info: Turns out both fans were bad. The big one didnt run when jumped. However the little one did. Even though it wasnt stiff or locked up it was apparently pulling to many Amps and causing FCM to shut down.

So one fan can cause both to not opperate, and spinning fans can still be bad.