A/C Signal: Description and Operation

PURPOSE
This signal is a communication link between the A/C control head, Engine Control Module (ECM) and the Transmission Control Module (TCM).
The signal is connected to pin C 10 of the ECM and is bidirectional (PIN C 10 - INPUT + OUTPUT).

OPERATION
Signal flow to or from the ECM is as follows:

SIGNAL IN
^ The A/C compressor clutch relay supplies voltage to pin C 10 via terminal 87A 140 milliseconds before the A/C clutch is engaged. The ECM increases the idle air control valve opening to prevent an rpm drop when the compressor clutch engages.

SIGNAL OUT
^ The ECM sends a voltage signal to the A/C control head when the throttle valve is opened quickly (acceleration) while in first gear (recognized from vehicle speed and engine speed). The A/C compressor is switched OFF for 3 to 12 seconds to aid in initial acceleration.