97 PCM functions explanations.

[davebb]

Does anyone know if there's a way to look up what each input and output of the PCM are for? Also how the PCM handles the information? I posted another thread here (Cruise Control self test.) about this and now I really need to find out what the brake light circuit input is for. The PCM is feeding about 8 volts to the brake light circuit with the key on and this makes the cruise control self test fail. I've disconnected the brake circuit at the PCM and now the cc self test passes. So do I really need the brake input to the PCM?
Thank you!!!!!!

 

[J_C]

IDK the significance of the 8V, but you do need the brake input to the PCM because it uses that to disengage the torque converter lockup so the engine doesn't stall when braking.

 

[davebb]

IDK the significance of the 8V, but you do need the brake input to the PCM because it uses that to disengage the torque converter lockup so the engine doesn't stall when braking.

Well that makes sense. I wish I didn't have to guess about the 8V. Might mean a new PCM is in order.

I just found the answer Here (Why - Brake Lights have 8 volts on all the time? - Ford Truck Enthusiasts Forums)
The 8V seems to be inherent from the PCM and the led bulbs in the brake lights don't provide enough of a load to dissipate the small current from the PCM so the self test in the cruise control thinks the brake pedal is depressed. I just changed the brake light bulbs back to incandescent ones and no more self test fail.