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 the ECM and is bidirectional.

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

SIGNAL IN
^ The A/C compressor clutch relay supplies voltage to the ECM 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.