Compressor Controls Circuit

The compressor for the air conditioning is clutch regulated and driven by the engine by means of a belt drive. The clutch allows the compressor to be disengaged when compressor operation is not required.

The battery supplies voltage to the following components.
- The HVAC Control Module (when the ignition switch is in the RUN position.)
- The A/C Compressor Clutch Relay Coil (when the ignition switch is in the RUN position.)
- The normally open A/C Compressor Clutch Relay contact.

At the HVAC control module, B+ voltage is supplied by CKT 141 (BRN) to C3 pin 5 and to C2 pin G at all times from the 25 amp Fuse 12, HTR-AC, at the IP Fuse Block. Battery voltage is also supplied at all times to the A/C Compressor Clutch Relay from the 10 Amp A/C Fuse located in the Underhood Fuse Block through CKT 1240 (ORN). Ignition voltage is supplied to the A/C Compressor Clutch Relay from the 10 Amp fuse, IGN E from the Underhood Fuse Block through CKT 639 (PNK). The HVAC Control Module grounds through CKT 150 (BLK) to G105 and the A/C Compressor Clutch Relay grounds through CKT 150, G200.

To drive the A/C Compressor Clutch, the Powertrain Control Module (PCM) on diesels or the Vehicle Control Module (VCM) for gasoline engines, provides a ground on CKT 459 (DK GRN/WHT) which in turn activates the coil in the A/C Compressor Clutch Relay which is fed voltage through CKT 639 (PNK). This activates the A/C Compressor Clutch Relay which is fed voltage to CKT 59 (DK GRN) to the A/C Compressor Clutch, which grounds through CKT 150 (BLK), G105. Diode D104 in the harness is reverse-biased and prevents current flow bypassing the clutch coil to ground as well as serves as a means of de-energizing the magnetic field when the relay coil is de-energized.

For Gasoline Engines, the VCM will provide a ground path when a B+ voltage signal is received on CKT 762 (DK GRN/WHT). This voltage signal is provided by the HVAC Control Module when the A/C Push Button Switch is depressed or if defrost is selected through the Mode Dial Defrost Detent, and feeds B+ voltage through CKT 66 (LT GRN) to the A/C Compressor High Pressure Cutout Switch. Provided that the switch is closed, the signal is then supplied to CKT 762 (DK GRN/WHT). The A/C Compressor Clutch can also be turned ON/OFF by the VCM if a voltage drop is detected on CKT 604 (DK BLU) which feeds the A/C Compressor Refrigerant Pressure Switch and grounds through CKT 451 (BLK/WHT) to G104 or if a voltage drop is detected on CKT 603 (DK GRN) which feeds the A/C Compressor Cycling Switch and grounds on CKT 451 (BLK/WHT). These switches are normally closed.

For Diesel Engines, the signal is fed inn the same way as Gasoline Engines with exception of an A/C Compressor High Pressure Cutoff Switch and an A/C Compressor Cycling Switch is included in the circuit. CKT 66 (LT GRN) is fed B+ voltage from the HVAC Control Module, the A/C Compressor Cycling switch must be closed to continue the voltage path through CKT 603 (DK GRN) to the A/C Compressor High Pressure Cutoff Switch, which feeds the PCM through CKT 762 (DK GRN/WHT).

When the Front Blower Switch is in the OFF position, the HVAC Control Module feeds B+ voltage to CKT 71 (DK GRN) from C2, pin G to C3, pin 3. This voltage signal provides the HVAC Control Module with an input to the Compressor Logic Controller internal to the HVAC Control Module to prevent B+ voltage being fed to CKT 66 (LT GRN), preventing A/C Operation when the system is turned off.

When either the Re-circulate switch or MAX A/C is selected, CKT 1641 (DK GRN) provides a ground for the Recirculation Door Motor through either the VCM for a gasoline engine or through the A/C Compressor Low Pressure Cutout Switch to CKT 150 (BLK) to G105 for a diesel engine. The Recirculation Door Motor's normal position is for outside air. When the Recirculate Switch is depressed, this provides a ground on CKT 1641 (DK GRN) at the HVAC Control Module, that has an internal 1 K resistor, to activate the Recirculation Door Motor. When the VCM on a gasoline engine or the A/C Compressor Refrigerant Pressure Switch on a diesel engine closes or provides a no-resistance ground on CKT 1641 (DK GRN), the Recirculation Door Motor moves to recirculation position.