Automatic Climate Control

AIR CONDITIONING PROGRAMMER (ACM)
The ACM is attached to a bracket located next to the right end of the HVAC module assembly behind the glove box and can be identified by the electrical connectors attached to it. Within the case are the functional components described below. The ACM provides the controls for temperature regulation.

The ACM controls an actuator that adjusts the air mix valve position to blend cold and warm air. The motor is reversible and moves a rotary shaft which drives the air mix valve link. The motor also provides a position sensor signal to monitor actual valve position.

The ACM also responds to operate the four vacuum solenoids. The solenoids control vacuum to the various vacuum actuators which in turn control the air inlet (recirc.) valve, A/C DEFROST valve and the UP/DOWN valve. An additional function of the ACM is to control the Park Brake Release Actuator. The PCM sends a Class II message to the ACM. This message is sent when the transmission is shifted out of Park or Neutral. The ACM will then apply vacuum to the Park Brake Release Solenoid and release the park brake.


TEMPERATURE SENSOR INPUTS





In-Vehicle Air Temperature Sensor
The In-Vehicle air temperature sensor is located in the Instrument Panel (I/P) to the left of center. To provide interior air temperature sensing, a small amount of air is drawn into the sensor housing and passed over the thermistor. The air movement is accomplished by using an aspirator mounted on the top of the HVAC module. Air flow from the HVAC module creates a slight vacuum at one end of the aspirator which draws In-Vehicle air through the sensor housing and over the thermistor.

Outside Temperature Sensor
This sensor is located in front of the radiator. The sensor data is processed by the ACM and displayed on the HVAC control head.

During periods of city traffic and idle conditions, engine heat can surround the area of the outside temperature sensor and cause temperature readings that are higher than the actual outside air temperature. To help eliminate these "false" readings, programming techniques are used to minimize unwanted temperature variations.

A temperature "memory" feature is used in the ACM outside temperature programming to help provide greater accuracy under engine restart conditions. if engine coolant temperature is less than 10°C (18°F) above the outside temperature sensor reading, the actual sensed outside temperature is displayed. Moreover, if engine coolant is greater than 10°C (18°F) above the sensor reading, "memorized" outside temperature is displayed. This is the displayed temperature sensed when the engine was previously operating. At car speeds greater than 20 mph, the outside temperature display may be allowed to increase, but only after a built-in 80-second time delay which allows for outside air to cool the sensor. The 80-second time delay starts when vehicle speed reaches or maintains at least 20 mph. if the sensor reading is ever less than the displayed value or if the vehicle speed is 45 mph or greater, there is no delay and outside temperature changes are displayed as rapidly as possible.

Coolant Temperature Sensor
The coolant temperature sensor is located in a coolant passage of the intake manifold. Sensor resistance is monitored by the PCM which in turn sends the coolant temperature value to the ACM over the data circuit.

A/C High Side Temperature Sensor
The high side sensor is located in the high pressure refrigerant liquid line between the condenser and orifice tube. The ACM monitors the refrigerant temperature and transfers that data to the PCM to use in conjunction with the coolant temperature to determine the need for cooling fans, and to prevent the A/C compressor from operating at high discharge pressures.

A/C Low Side Temperature Sensor
The low side sensor is located in the low pressure refrigerant line between the orifice tube and evaporator. The ACM monitors this sensor to determine the low side pressure based on the pressure/temperature relationships of R-134a.

Sun Load Sensor
The Sun Load sensor uses a light sensitive photocell to control the signal voltage to the ACM. The ACM will see a low voltage in direct light and a higher voltage in shaded conditions.


LOW REFRIGERANT PRESSURE SWITCH
The low refrigerant pressure switch is located in the low pressure refrigerant line. When pressurized, the switch is closed and allows the compressor clutch relay to energize, feeding battery voltage to the compressor clutch coil. If the pressure drops below 2 psi, the switch opens and cuts off voltage to the clutch. There is a circuit to the ACM which monitors the state of the switch for diagnostic purposes.

HIGH REFRIGERANT PRESSURE SWITCH
The high refrigerant pressure switch is located in the compressor discharge line and monitors the compressor discharge pressure. This switch acts as a safeguard to prevent refrigerant "blow-off" from the compressor pressure relief valve and disengages the compressor in the case of refrigerant pressure above 2965 kPa (430 psi). in this function it supplements the A/C high side temperature sensor.

Should pressures reach that level, the switch will open cutting off the power supply to the relay and disengage the compressor clutch. As soon as the compressor shuts "OFF", the pressures will begin to drop. When they drop to 1379 kPa (200 psi), the switch will close and allow the compressor to engage. Compressor cycling due to pressure switch cycling can occur under conditions such as high Outside temperature, high humidity, extended idle, slow vehicle speed or tailwind, and is considered normal. Should cycling occur without the above conditions, a blocked line or condenser is possible and appropriate service should be performed.

AIR MIX VALVE LINKAGE
The air mix valve linkage consists of a plastic crank arm integral with the output shaft of the actuator, a nylon retainer, and a rod. One end of the rod is fastened to the air mix valve and the opposite end is retained by the nylon retainer.

The ACM uses an actuator(s) to drive the air mix valve(s).

ACTUATOR(S)
The air temperature is controlled by the movement of an actuator. There is one actuator that controls the temperature for a single zone equipped vehicle (C68) which is located on top of the HVAC module.

After an actuator has been replaced, the actuator needs to be recalibrated for proper travel. The ignition must be cycled ON for three minutes to allow the programmer and the actuator(s) to recalibrate for proper travel.