Automatic Temperature Control (ATC)

CIRCUIT OPERATION
Power for the ATC control head is supplied on circuit F20. This circuit is HOT in the RUN position only and protected by a 10 Amp fuse located in cavity 6 of the junction block.

Power for the fuse is supplied on circuit A22 from the ignition switch. Circuit A2 from the Power Distribution Center (PDC) supplies power to the ignition switch. This circuit is protected by a 40 Amp fuse located in cavity J of the PDC.

Illumination for the control is supplied on circuit E2. This circuit is controlled by the Body Control Module (BCM) and the dimmer control.

Ground for the control is provided on two circuits. They are the Z1 and Z2 circuits. Both of these circuits terminate at the Airbag Control Module (ACM) bracket.

The BCM is used with this system to control the operation of the various mode door actuators, blower speed, and A/C operation. When the operator requests a function in the system, a message is sent across the CCD Bus to the BCM on circuits D1 and D2.

The BCM then processes this information and activates the proper function. It also sends a message back to the ATC control to illuminate the proper displays.

CIRCUIT OPERATION
When the A/C is turned ON, or the Defrost is turned ON, a message is sent across the CCD Bus to the Body Control Module (BCM) requesting A/C operation.

The BCM then looks at the temperature of the evaporator through the evaporator temperature sensor, located in the A/C-Heater housing. The BCM uses circuit C12 to send a signal to the sensor and circuit C57 as the sensor return to the BCM.

If the evaporator temperature sensor is in the normal limits the BCM sends a signal to the Powertrain Control Module (PCM), across the CCD Bus, requesting A/C operation.

After receiving this input, the PCM activates the A/C compressor by grounding the C28 circuit, which is connected to the coil side of the A/C clutch relay This relay is located in the Power Distribution Center (PDC).

Power for the coil side of the relay is supplied on circuit F18. This circuit is protected by a 10 Amp fuse located in cavity 20 of the junction block. This fuse is HOT in the RUN position only

When the coil is energized, contacts in the relay close connecting circuits A16 and C2. Circuit A16 is protected by a 40 Amp fuse located in cavity D of the PDC and also supplies power for the radiator fan relays. The C2 circuit connects from the relay to the A/C compressor clutch.

The A/C compressor clutch receives this current and creates a magnetic field, energizing the clutch. The A/C compressor clutch is case grounded to the A/C compressor.

The A/C compressor clutch has a diode located in it. This diode is used to control the induced voltage resulting from the magnetic field collapsing when the clutch is disengaged.

The A/C compressor clutch relay is also used by the PCM to disengage the compressor in a Wide-Open Throttle (W.O.T.) condition.