Normal Operation

WARNING: This page is about a different car, the 2009 Mercury Milan, 2009 Lincoln MKZ, and 2009 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.

Under normal operation, when A/C is requested, in Electronic Automatic Temperature Control (EATC) or Dual Automatic Temperature Control (DATC) system, a message is sent over the Medium Speed Controller Area Network (MS-CAN) bus to the Instrument Cluster (IC), then from the IC through the High Speed Controller Area Network (HS-CAN) bus to the PCM; or a voltage is sent to the IC through circuit CH434 (GY/YE), then a message is sent from the IC through the HS-CAN bus to the PCM in Electronic Manual Temperature Control (EMTC) systems.

Ignition voltage for the A/C clutch relay switch is provided through circuit SBB25 (RD). When the PCM energizes the relay, ignition voltage is supplied to the A/C clutch through circuit CH401 (VT/WH). Ground is supplied for the A/C clutch through circuit GD121 (BK/YE).

This pinpoint test is intended to diagnose the following:

Fuse
Wiring, terminals or connectors
PCM
HVAC module - EATC
HVAC module - DATC
HVAC module - EMTC
A/C evaporator discharge temperature sensor
A/C pressure transducer
A/C compressor clutch field coil
A/C control relay
A/C clutch air gap