HVAC Systems - Automatic: Other Pontiac Aztek I

DTCP1546: Short To Ground, Open Circuit Or Short To Battery Voltage On A/C Clutch Relay Control Circuit

Ignition voltage is supplied directly to the A/C compressor clutch relay. The powertrain control module (PCM) controls the relay by grounding the control circuit of the A/C clutch relay through an internal solid state device. This device is called a driver. The driver supplies the ground for the component that is being controlled. Each driver has a fault line which is monitored by the PCM. When the PCM commands a component ON, the voltage of the control circuit should be near 0 volts. When the PCM commands the control circuit to a component OFF, the voltage potential of the circuit should be near battery voltage. If the fault detection circuit senses an unexpected voltage, this DTC will set.

The PCM will monitor the control circuit to the A/C clutch relay for the following conditions

1. A short to ground
2. A short to voltage
3. An open circuit
4. An open relay coil
5. A relay coil that is shorted internally or which has a very low resistance

If the PCM detects any of these malfunctions, this DTC is set, and the affected driver is disabled.

Intermittent

Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to TESTING FOR INTERMITTENT AND POOR CONNECTIONS .

Tools Required

J 38778 Door Trim Pad Clip Remover

Scheme 90

Scheme 91

1. Open the hood.
2. Disconnect the electrical connector from the ambient air temperature sensor.
3. Use J 38778 to disconnect the wiring harness retainer which secures the ambient air temperature sensor to the hood latch support.
4. Remove the ambient air temperature sensor from the wiring harness retainer.
5. Remove the ambient air temperature sensor from the vehicle.

Scheme 92

1. Install the ambient air temperature sensor on the wiring harness retainer.
2. Install the wiring harness push-in retainer which secures the ambient air temperature sensor to the hood latch support.
3. Connect the electrical connector to the ambient air temperature sensor.

Scheme 93

1. Remove the windshield garnish molding. Refer to «GARNISH MOLDING REPLACEMENT -- WINDSHIELD PILLAR»(ref-180885-S31131957902005073000000) . Important: Remove the I/P upper trim panel slowly, the retainer clips could possibly fall into the I/P.
2. Lift the I/P upper trim panel up approximately 51 mm (2 in) and pull rearward to release I/P upper trim panel retainers.
3. Lift the I/P upper trim panel to access sun load sensor.
4. Disconnect the sunload sensor electrical connector.
5. Remove the sun load sensor.

Scheme 94

Scheme 95

1. Install sun load sensor.
2. Connect the sunload sensor electrical connector.
3. Align the I/P upper trim locating pins and the retainers to the slots in the I/P.
4. Push forward and down on the I/P upper trim panel to lock into place.
5. Install the windshield garnish molding. Refer to «GARNISH MOLDING REPLACEMENT -- WINDSHIELD PILLAR»(ref-180885-S31131957902005073000000) .

HVAC Control Module

The HVAC control module is a class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The body control module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features

Scheme 96

Mode Actuator

The mode actuator is a 5 wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5 volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5 volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5 volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts.

The HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

Recirculation Actuator

The recirculation actuator is a 3 wire bi-directional electric motor. Ignition 3 voltage, ground and control circuits enable the actuator to operate. The control circuit is either grounded or opened during normal operation. If the control circuit is grounded, then the shaft rotates in the opposite direction than it would if the circuit was opened. The actuator shaft rotates until the limit is reached. If the module needs to reverse the direction of rotation, then the control circuit's state is reversed. The open circuits are held at ignition 3 voltage from the actuator. The module will maintain the control circuit's state until a new position is needed.

Blower Motor Control Processor

The blower motor control processor is an interface between the HVAC control module and the blower motor. The blower motor speed control, Blower motor speed signal, battery positive voltage and ground circuits enable the control processor to operate. The HVAC control module sends the commanded blower motor speed to the control processor through a 0 to 12-volt linear ramped blower motor speed control circuit. As the blower motor speed control voltage increases, the blower motor speed increases. The control processor sends the actual blower speed to the control module through a 0 to 12-volt blower motor speed signal circuit. The processor supplies 12volts to the blower motor through the blower motor voltage supply circuit. The control processor uses the blower motor ground as a low side control to adjust the blower motor speed.

Air Speed

The blower motor forces air to circulate within the vehicle's interior. The vehicle operator determines the blower motor's speed by placing the blower motor switch in a desired speed position or by selecting automatic operation. In manual operation, once a blower speed is selected, the blower speed remains constant until a new speed is selected. In automatic operation, the HVAC control module will determine what blower speed is necessary in order to achieve or maintain a desired temperature.

OFF Mode

Press the passenger temperature knob to turn off the HVAC control module. When the vehicle is moving, air flowing over the vehicle increases the air pressure just ahead of the windshield. This forces air into the ambient air into the HVAC module and out through the floor and windshield outlets. Since the A/C compressor is not running, the incoming air may be warmed but not cooled. Recirculation is not adjustable in OFF mode.

Air Distribution

The HVAC control module controls the mode actuator in order to distribute airflow to a desired outlet. The mode switch provides the vehicle operator with the ability to override the automatic setting. When the mode door is moved to the defrost position, the A/C compressor clutch engages and the recirculation actuator will be moved to the outside air position. Regardless of the mode setting, a small amount of air will be diverted to the defrost ducts to reduce windshield fogging. This vehicle may be equipped with a passenger compartment air filter. When VENT is pressed, the following will occur

1. The recirculation actuator will be placed in the outside air position
2. The A/C compressor will be commanded off
3. The mode is adjustable

After a malfunction occurs to the mode actuator it is driven to the Defrost position. On start-up, the HVAC control module will place the mode door in the last selected position.

The HVAC control module works in a dual role operating both air temperature and A/C operation from the drivers side switch. The HVAC control module is a class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The body control module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features

Scheme 97

Air Temperature Actuator

The air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts. The HVAC control module uses a range of 0-255 counts to index the actuator position. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

Inside Air Temperature Sensor

Replacement of the inside air temperature sensor means replacing the HVAC control module. The inside air temperature sensor is an integral part of the HVAC control module. The HVAC control module monitors the passenger compartment inside air temperature through an inside air temperature sensor. When the air is cold, the sensor resistance and the signal voltage are high. When the air is warm, the sensor resistance and the signal voltage are low. The HVAC control module requests A/C compressor clutch engagement and controls the air temperature actuator door positions in order to maintain the selected air temperature. The HVAC control module sends out a 5-volt reference signal to the inside air temperature sensor over the 5-volt reference circuit. A thermistor varies the reference voltage back to the HVAC control module through the low reference circuit.

A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts.

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high or low pressure condition exists. The PCM disables the compressor clutch under the following conditions

1. A/C pressure is more than 2979 kPa (432 psi). The clutch will be enabled after the pressure decreases to less than 1510 kPa (219 psi).
2. A/C pressure is less than 186 kPa (27 psi). The clutch will be enabled after the pressure increases to more than 207 kPa (30 psi).

Evaporator Temperature Sensor

The A/C system is protected by the evaporator temperature sensor. The sensor located on the evaporator core provides the HVAC control module with the surface temperature of the evaporator core. If the evaporator temperature sensor reads a temperature of 0°C (32°F), the HVAC control module will turn off the A/C compressor clutch until evaporator temperatures reach 2°C (36°F).

Ambient Air Temperature Sensor

The ambient air temperature sensor is a 2-wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors

1. Ambient
2. Inside

A 5-volt reference and signal circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases.

The sensor operates within a temperature range of -40°C (-40°F) to 60°C (140°F). The HVAC control module receives a class 2 message from the instrument panel cluster (IPC). The ambient air temperature sensor is mounted in the forward engine area of the vehicle. In this position, it is exposed to the airflow through the grill before it reaches the radiators. The sensor signal varies between 0 to 5 volts. The HVAC control module converts the signal to a working temperature range.

If the HVAC control module detects a faulty class 2 signal, then the control module software will use a defaulted air temperature value.

The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the system is fixed.

The ambient air temperature is updated and displayed under the following conditions

Scheme 98

Sunload Sensor

The sunload sensor is a 2-wire photo diode. Low reference and signal circuits enable the sensor to operate. As the light shining upon the sensor gets brighter, the sensor conductance increases. The sensor signal decreases as the conductance increases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts.

The sunload sensor provides the HVAC control module a measurement of the amount of light shining on the vehicle. Bright, or high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If the HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted sunload valve. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed. A resistance check of the sunload sensor should not be performed as it will damage the sensor.

Engine Coolant

Engine coolant is the essential element of the heating system. The thermostat controls the normal operating temperature of the engine. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation.

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. The HVAC module distributes heated air to the passenger compartment for consistent passenger comfort.

Opening or closing the HVAC module temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and is recirculated back through the engine cooling system.

A/C Cycle

Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

A Mitsubishi scroll compressor is used on this model year vehicle. The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C system. The A/C system is mechanically protected with the use of a high pressure relief valve. If the high pressure switch were to fail or if the refrigerant system becomes restricted and refrigerant pressure continued to rise, the high pressure relief will pop open and release refrigerant from the system.

Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line, to the TXV.

The TXV is located at the evaporator inlet. The TXV is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the TXV, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to boil at the TXV. The TXV also meters the amount of liquid refrigerant that can flow into the evaporator.

Refrigerant exiting the TXV flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs the moisture and heat from the ambient air. The refrigerant exits the evaporator through the suction line and back to the compressor, in a vapor state, and completing the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.

The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment will also change form, or condense, and is discharged from the HVAC module as water.

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Scheme 100