Control System Logic
Set the blower motor knob to any speed other than 0 to operate the system.
Controls and Compressor Operation
All customer commands for the EMTC climate control system come through the HVAC control head.
A/C Request
When the A/C button is pressed, the HVAC control module uses a dedicated wire to send the A/C request message to the BCM. The BCM sends the A/C request message through the HS-CAN to the PCM.
When the A/C request message is received by the PCM, the PCM engages the A/C clutch relay when all of the following conditions are met:
- The PCM does not detect excessively high or low refrigerant pressure from the A/C pressure transducer.
- The PCM does not detect engine coolant temperature excessively high.
- The PCM does not detect an ambient air temperature below approximately 0°C (32°F).
- The PCM does not detect a WOT condition present.
- The PCM does not receive a message from the BCM detecting an evaporator discharge air temperature below approximately 1°C (33.8°F).
- The PCM does not detect temperatures from the Mass Airflow/Intake Air Temperature sensor that does not correlate with other temperature sensor readings at ignition ON. (Most Mass Airflow (MAF) sensors have an integrated Intake Air Temperature (IAT) sensor).
The PCM monitors multiple temperature sensors for correlation (AAT CACT, CHT, ECT, IAT, IAT2, MAF, MAPT, TCBP, TCIPT as applicable). The PCM runs this logic after an engine off and a calibrated soak period, typically 6 to 8 hours. This soak period allows the Ambient Air Temperature (AAT) sensor (if equipped) and the other temperature sensors to stabilize and not differ by greater than a calibrated value, typically 18°C (32.4°F). If a sensor input is found to be reporting a temperature imbalance the PCM does not allow the A/C clutch to engage. For more information on PCM sensors, Refer to Engine Controls - Description & Operation (Except Diesel & Hybrid) or Engine Controls - Description & Operation (Diesel) .
Compressor Clutch
Ground is provided to the A/C clutch relay coil from the PCM, energizing the A/C clutch relay. When the PCM energizes the relay, voltage is supplied to the A/C compressor clutch field coil from the relay.