Originally Posted by
pizzaguy
The PTCM sends a tickle pulse to the FCM every few milliseconds. The FCM responds with a pulse. As long as that happens, the light stays out. If the PTCM does not get a pulse from the FCM, it lights the light to tell you that you have a cooling system problem.
The width of the pulses from the PTCM tell the FCM how fast to run the fan. The FCM sends pulses of 12 volts to the fan motor (your meter will average these pulses and imply to you the fan is getting 4 or 5 6 volts, but that is not the case). DC motors do not do well on low voltage, they do better on pulses of full, normal voltage to control speed. I have a switch in my car that breaks the PWM wire connection - this makes the light come on, and it ALSO means that the FCM does not see the pulses from the PTCM - this makes the FCM go into emergency mode (because it no longer is being told what to do) - and the FCM puts full voltage on the fan motor and it runs like a turbine!
How did you determine the PTCM sends a digital signal to the FCM? Since the FCM looks to contain the parts necessary for PW modulation, I am thinking a variable input (or ground) voltage could signal the FCM what to output. (I suggested ground voltage since that grey wire is the ONLY connection to pin 39 of the PTCM.) I suppose that FCM could be "smart" enough to decode/deliver bi-directional information from the PTCM but it SEEMS that one-directional variable DC voltage would do the trick.
I couldn't find a DETAILED description of the actual FCM function. I'm just an ex EE major that jumped over the CS in the 80's. You know...the kind that makes wild guesses out the wazoo! :-)
FWIW, I replaced the 50 amp control block fuse today and it's working property for now.
(In my 89 Corvette, the ECM triggers a simple relay for the 1-spd main fan. The aux fan is triggered via engine block sensor. Ahh....for simplicity! LOL Now that I know about these 50 amp "MIDI" fuses, maybe I'll use a couple of them to redo my Corvette's fan circuit. I want to convert to a more powerful 2-spd puller fan after building the stroker motor. The pusher non-shrouded aux fan seems fairly worthless.)
Thanks for the help guys!
