With an O'scope and 31 years experience in these matters. And one direction does not do the trick because the PTCM has to know if the FCM is working, otherwise it would not know the FCM is "gone" and would not know enough to light the light.

The "PWM" line from the PTCM to the FCM is a digital bidirectional bus. The "tickle" pulses from the PTCM get "fatter" as it wants the fan to turn faster. Very short pulses (I cant remember, I think something like 5 ms) are the baseline. These are "I'm here watching you" pulses only. Once you turn the A/C on, the pulses get a lot wider, indicating that the fan should turn at "minimal" speed to assure the condenser can dissipate heat. Let the engine warm up good and you will see the pulses get even wider.

The return pulses from the FCM did not appear to get wider, no matter what, it would appear the PTCM is happy just knowing the FCM is still 'talking' and assumes in such cases that the FCM is operating the fan as it is being told to.


I've lost my notes, I did the homework two residences ago on my first Crossfire (the original red roadster, the one missing one pedal).