HVAC System - Compressor Clutch Controls: Overview Chevrolet Malibu V facelift

MODEL IDENTIFICATION

Note. Metro, Prizm and Tracker, and Saturn are not covered in this article. For A/C compressor clutch diagnosis on Metro, Prizm, Tracker, and Saturn, see appropriate MANUAL A/C-HEATER SYSTEMS article.

MODEL IDENTIFICATION (CARS)

MODEL IDENTIFICATION (LIGHT TRUCKS & VANS)

Description

Powertrain Control Module (PCM) receives an A/C request signal from Instrument Panel Cluster (IPC) over UART serial data line. When A/C is requested, the PCM provides a ground path to A/C clutch relay control circuit. When relay circuit is grounded, A/C clutch relay is energized. PCM delays grounding relay circuit for .3 second. This allows PCM to adjust engine idle RPM for additional load.

PCM will temporarily de-energize A/C clutch relay when one or more of the following conditions occur

1. A hot engine restart.
2. Wide open throttle (TPS position over 90 percent).
3. If engine RPM is greater than 6000 RPM.
4. During Idle Air Control (IAC) valve reset

A/C clutch relay will remain de-energized when one or both of the following conditions occur

1. DTC P0530 or U1016 are set.
2. When there is no A/C request signal to PCM.

For additional trouble shooting information, go to MANUAL A/C-HEATER SYSTEMS - CAVALIER & SUNFIRE article.

Powertrain Control Module (PCM) receives an A/C request signal from Body Control Module (BCM) over UART serial data line. When A/C is requested, the PCM provides a ground path to A/C clutch relay control circuit. When relay circuit is grounded, A/C clutch relay is energized. PCM delays grounding relay circuit for .3 second. This allows PCM to adjust engine idle RPM for additional load.

PCM will temporarily de-energize A/C clutch relay when one or more of the following conditions occur

1. A hot engine restart.
2. Wide open throttle (TPS position over 90 percent).
3. If engine RPM is greater than 6000 RPM.
4. During Idle Air Control (IAC) valve reset

A/C clutch relay will remain de-energized when one or both of the following conditions occur

1. DTC P0530 or U1016 are set.
2. When there is no A/C request signal to PCM.

For additional trouble shooting information, go to MANUAL A/C-HEATER SYSTEMS - CAVALIER & SUNFIRE article.

Powertrain Control Module (PCM) receives an A/C request signal from Body Control Module (BCM) over Class 2 serial data line. When A/C is requested, PCM provides a ground path to A/C clutch relay control circuit. When relay circuit is grounded, A/C clutch relay is energized. The PCM delays grounding relay circuit for .3 second. This allows PCM to adjust engine idle RPM for additional load.

The PCM will temporarily de-energize A/C clutch relay when one or more of the following conditions occur

1. A hot engine restart.
2. Wide open throttle (TPS position over 90 percent).
3. If engine RPM is greater than 6000 RPM.
4. During Idle Air Control (IAC) valve reset.

A/C clutch relay will remain de-energized when one or both of the following conditions occur

1. DTC P0530 or U1016 are set.
2. When there is no A/C request signal to PCM.

For additional trouble shooting information, go to appropriate MANUAL A/C-HEATER SYSTEMS article.

Description (A/C Request Circuit)

For proper A/C system operation, the A/C request circuit monitors selected A/C mode, A/C system pressure and throttle position. When A/C mode is selected with A/C system pressure and throttle angle at a predetermined value, Engine Control Module (ECM) energizes A/C clutch relay through A/C clutch relay control circuit. The ECM de-energizes A/C clutch relay during wide open throttle, when engine coolant temperature or A/C system pressure exceeds a predetermined threshold, or when a non-A/C mode is selected. If system does not operate as specified, see DIAGNOSTIC SYSTEM CHECK .

Description (A/C Load Signal Circuit)

The ECM receives a signal from the A/C load signal circuit. A normally open pressure switch connected to battery voltage monitors A/C system operating pressures. If pressures reach approximately 175 psi (12.2 kg/cm 2 ) the switch will close, sending a signal to the ECM. The ECM will raise idle speed around 50 RPM to compensate for the additional load. If system does not operate as specified, see DIAGNOSTIC SYSTEM CHECK .

Description (A/C Compressor Control Circuit)

The ECM controls the ground side of the A/C clutch relay to energize the A/C compressor clutch. When component is commanded on by the ECM, voltage on the control circuit should be at or near zero volts. When component is commanded off by the ECM, voltage on the control circuit should be at or near battery voltage. If system does not operate as specified, see DIAGNOSTIC SYSTEM CHECK .

When A/C is selected through A/C control panel, a 12-volt signal is supplied to A/C request input terminal of PCM. A/C clutch relay is controlled through PCM which also monitors A/C refrigerant pressure. If A/C refrigerant pressure, and engine operating conditions are within a specific calibrated acceptable range, the PCM will enable A/C clutch relay. This is accomplished by providing a ground path for A/C clutch relay coil with PCM. When A/C clutch relay is enabled, battery voltage is supplied to compressor clutch coil.

The PCM will enable A/C compressor clutch whenever engine is running and A/C has been requested, unless

1. Throttle angle is greater than 90 percent.
2. A/C head pressure is greater than 414 psi (29.1 kg/cm 2 ) with A/C refrigerant pressure sensor reading greater than 4.27 volts.
3. A/C head pressure is less than 35 psi (2.5 kg/cm 2 ) with A/C refrigerant pressure sensor reading greater than .40 volts.
4. Ignition voltage is less than 10.5 volts.
5. Engine speed is greater than 4500 RPM for 5 seconds or at 5400 RPM for any amount of time.
6. Engine coolant temperature is greater than 257°F (125°C).
7. Intake air temperature is less than 41°F (5°C).

Compressor for air conditioning system is belt-driven by engine through A/C compressor clutch. Clutch allows compressor to engage for A/C and defroster operation, disengage when air conditioning is not requested and disengage when engine load must be reduced. Battery voltage is supplied to powertrain control module (PCM) when heater and A/C control mode selector knob is in MAX A/C, NORM A/C, BI-LEVEL, DEFOG or DEFROST modes. Voltage signal tells PCM that A/C compressor operation is requested. Under normal operating conditions, PCM supplies a ground to relay coil of A/C compressor control. Relay energizes. Voltage is supplied to A/C compressor clutch through relay center fuse. A/C compressor clutch engages. Compressor runs. If PCM determines engine load should be reduced, such as during full throttle conditions, PCM de-energizes A/C compressor control relay in order to disengage compressor clutch. De-energization occurs even though voltage signal from heater and A/C control is still present. Diode of A/C compressor clutch connects across terminals of A/C compressor clutch. Diode provides a path for current which results from voltage spikes. Voltage spikes generate from collapsing magnetic field of A/C compressor clutch coil. Voltage spikes occur every time coil de-energizes.

When A/C is selected through A/C controller, a 12.0 volt signal is supplied to A/C request input of PCM. A/C compressor clutch relay is controlled through PCM. PCM monitors A/C refrigerant pressure. If A/C refrigerant pressure and engine operating conditions are within a specific calibrated acceptable ranges, PCM will enable A/C compressor relay. This is accomplished by providing a ground path for A/C relay coil within PCM. When A/C compressor relay is enabled battery voltage is supplied to compressor clutch coil.

The PCM will disengage A/C compressor clutch if one or more of the following conditions occur

1. Throttle angle is greater than 96 percent.
2. A/C head pressure is greater than 440 psi (31 kg/cm 2 ) with A/C refrigerant pressure sensor reading greater than 4.27 volts.
3. A/C head pressure is less than 36 psi (2.5 kg/cm 2 ) with A/C refrigerant pressure sensor reading less than .35 volts.
4. Ignition voltage is less than 10.0 volts.
5. Engine speed is greater than 4700 RPM for 5 seconds or at 5400 RPM for any amount of time.
6. Engine coolant temperature is greater than 255°F (124°C).
7. Intake air temperature is less than 41°F (5°C).

When A/C is selected through HVAC controller, a 12 volt signal is supplied to Body Control Module (BCM). BCM in turn signals PCM through Class 2 serial data circuit that A/C has been requested. PCM then determines if conditions suitable for A/C operation are present. If PCM determines that engine operating conditions are within acceptable ranges, PCM will enable A/C relay. PCM provides a ground path for A/C relay coil. This action closes A/C compressor relay contacts and battery voltage is supplied to compressor clutch coil. Compressor clutch coil has its own ground path.

PCM will enable A/C unless any of the following conditions are met

1. Throttle position is more than 96 percent.
2. A/C pressure is more than 472 psi (4.9 volts) or less than 9 psi (0.1 volt) as determined by A/C pressure sensor.
3. Ignition voltage is less than 10 volts.
4. Engine speed is more than 4700 RPM.
5. Engine Coolant Temperature (ECT) is more than 255°F (124°C).
6. Intake Air Temperature (IAT) is less than 41°F (5°C).
7. Class II communication error occurs between HVAC controller and PCM.

The purpose of the air conditioning (A/C) system is to provide cool air and remove humidity from interior of vehicle. A/C system is engaged when HVAC control module is in any mode except OFF and VENT. A/C system can operate regardless of temperature setting, as long as ambient temperature is greater than 38°F (3°C). If A/C compressor is turned off due to low ambient temperatures, compressor will not be activated until temperatures reach 42°F (6°C). Vehicle passenger can adjust temperature offset by adjusting right air temperature switch assembly. Passenger temperatures can be set 8°F (4°C) cooler or warmer than drivers setting.

HVAC control module is interface between vehicle operator and Instrument Panel Module (IPM). HVAC control module receives power from rear fuse block on ignition 3 voltage circuit. IPM is also powered from rear fuse block through ignition 3 voltage circuit along with battery positive voltage circuit. IPM and HVAC control module communicate back and forth over HVAC control module signal and HVAC control module clock signal circuits.

In order for Powertrain Control Module (PCM) to internally ground A/C compressor clutch relay control circuit, Dash Integration Module (DIM) and PCM must communicate with each other over class 2 serial data circuits. PCM monitors A/C refrigerant line pressure and engine coolant temperature. This information is placed on class 2 serial data circuit and is monitored by DIM. DIM looks at A/C line pressure, engine coolant temperature, battery voltage along with A/C request input from IPM. DIM will request A/C operation from PCM if these parameters are within normal operating limits and IPM is requesting A/C operation.

PCM turns on A/C compressor by providing a path to ground through A/C compressor clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally within underhood fuse block. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode provides a path for voltage spike resulting from collapsing magnetic field of the compressor clutch coil. Ground circuit provides a path to ground for compressor. A/C clutch relay control circuit is grounded internally within PCM.

Compressor for air conditioning system is belt-driven by engine through A/C compressor clutch. Clutch controls compressor in the following ways

1. Disengages when air conditioning is not requested.
2. Disengages when engine load must be reduced.

Heater and A/C control automatically requests A/C compressor clutch engagement with system operating in defrost mode.

Under normal operating conditions, the following events occur

1. On Bonneville and LeSabre with manual A/C, A/C-heater control sends an A/C operation desired message to Dash Integration Module (DIM), DIM sends a request A/C compressor clutch engagement to Powertrain Control Module (PCM)
2. On Bonneville and LeSabre with automatic A/C, Instrument Panel Module (IPM) sends an A/C operation desired message to Dash Integration Module (DIM), DIM sends a request for A/C compressor clutch engagement to Powertrain Control Module (PCM)
3. On all models, PCM supplies a ground to A/C compressor control relay coil.
4. A/C CLU relay energizes.
5. Voltage is supplied to A/C compressor clutch through A/C CLU fuse.
6. A/C compressor clutch engages and compressor runs.

PCM monitors A/C refrigerant line pressure and engine coolant temperature. This information is placed on Class 2 communication link and is monitored by DIM. DIM looks at A/C line pressure, engine coolant temperature and battery voltage. DIM will request A/C operation from PCM if these parameters are within normal operation limits and A/C-heater control is requesting A/C operation.

If PCM determines that engine load should be reduced, such as during full throttle conditions, PCM will de-energize A/C compressor control relay. This will disengage compressor, even though request for A/C compressor clutch engagement from A/C-heater control is still present.

A/C compressor clutch diode is connected across A/C compressor clutch terminals. This diode provides a path for current resulting from voltage spikes. Collapsing magnetic field of A/C compressor clutch coil generate these voltage spikes. Voltage spikes occur every time coil is de-energized.

Compressor for air conditioning system is belt-driven by engine through A/C compressor clutch. Clutch controls compressor in the following ways

1. Disengages when air conditioning is not requested.
2. Disengages when engine load must be reduced.

Heater and A/C control automatically requests A/C compressor clutch engagement with system operating in defrost mode.

Under normal operating conditions, the following events occur

1. A/C-heater control sends an A/C operation desired message to Body Control Module (BCM). BCM sends a request for A/C compressor clutch engagement to Powertrain Control Module (PCM).
2. PCM supplies a ground to A/C compressor control relay coil.
3. A/C CLU relay energizes.
4. Voltage is supplied to A/C compressor clutch through A/C CLU fuse.
5. A/C compressor clutch engages and compressor runs.

PCM monitors A/C refrigerant line pressure and engine coolant temperature. This information is placed on Class 2 communication link and is monitored by DIM. DIM looks at A/C line pressure, engine coolant temperature and battery voltage. DIM will request A/C operation from PCM if these parameters are within normal operation limits and A/C-heater control is requesting A/C operation.

If PCM determines that engine load should be reduced, such as during full throttle conditions, PCM will de-energize A/C compressor control relay. This will disengage compressor, even though request for A/C compressor clutch engagement from A/C-heater control is still present.

A/C compressor clutch diode is connected across A/C compressor clutch terminals. This diode provides a path for current resulting from voltage spikes. Collapsing magnetic field of A/C compressor clutch coil generate these voltage spikes. Voltage spikes occur every time coil is de-energized.

A/C system can operate regardless of temperature setting, as long as ambient air temperature is greater than 38°F (3°C). If A/C compressor clutch is turned off due to cold ambient air temperatures, compressor will not come back on until ambient air temperatures reach 41°F (5°C).

Regardless of selected A/C mode setting, a request is made to Powertrain Control Module (PCM) to turn on A/C compressor clutch. Request is made through A/C request signal circuit from HVAC control assembly. Power and ground are provided to HVAC control assembly by ignition voltage and ground circuits.

When vehicle operator makes an A/C request, a 12 volt request signal is sent out over A/C request signal circuit and grounded through PCM. When this request signal is grounded, PCM can activate A/C compressor clutch.

PCM turns on A/C compressor clutch by providing a path to ground through A/C compressor clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay from of underhood fuse block on ignition voltage circuit. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode prevents a voltage spike from entering vehicles electrical system. Ground circuit provides a path to ground for compressor. A/C compressor clutch relay control circuit is grounded internally within PCM. On vehicles with V8 option, an A/C compressor clutch status signal is sent to PCM on A/C compressor clutch supply voltage circuit.

When A/C request is selected, a signal is sent from Instrument Panel Module (IPM) to Powertrain Control Module (PCM). In order for PCM to internally ground A/C clutch relay control circuit, Dash Integration Module (DIM) and PCM must communicate with each other over class 2 serial data circuits. PCM monitors A/C refrigerant line pressure and engine coolant temperature. This information is placed on class 2 serial data circuit and is monitored by DIM. DIM looks at A/C line pressure, A/C refrigerant low temperature sensor, engine coolant temperature, battery voltage along with A/C request input from HVAC control module. DIM will request A/C operation from PCM if these parameters are within normal operating limits and IPM is requesting A/C operation. If there is a malfunction in A/C system, instrument panel will read SERVICE A/C SYSTEM to alert vehicle operator that there is a malfunction with A/C system.

PCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally of underhood fuse block. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode provides a path for voltage spike resulting from collapsing magnetic field of compressor clutch coil. Ground circuit provides a pathway to ground for compressor. A/C clutch relay control circuit is grounded internally within PCM.

HVAC control module is interface between vehicle operator and HVAC control processor. When an A/C request is selected, a signal is sent from HVAC control processor to Powertrain Control Module (PCM). PCM monitors A/C refrigerant line pressure, A/C refrigerant temperatures and engine coolant temperature. If all PCM signals are normal, A/C clutch relay control circuit will be grounded, allowing A/C compressor to engage. If there is a malfunction in A/C system, driver information center will read either SERVICE A/C SYSTEM or LOW RERFIG A/C OFF to alert vehicle operator that there is a malfunction with A/C system.

PCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay from underhood fuse/relay block on A/C low pressure switch signal circuit and both A/C pressure switches. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Ground circuit provides a path to ground for compressor. A/C clutch relay control circuit is grounded internally within PCM.

PCM will open A/C clutch relay control circuit when coolant temperatures get too hot or their is a loss of refrigerant. When engine coolant temperatures reach 259°F (126°C), PCM will open A/C clutch relay control circuit. When PCM reads wide open throttle, PCM will shut off compressor for 20 seconds.

The A/C clutch relay is PCM controlled to delay A/C compressor clutch engagement after A/C is turned on. This allows PCM to adjust engine RPM before the A/C compressor clutch engages. The PCM will engage compressor clutch any time A/C has been requested, unless one or more of the following conditions occur

1. Coolant temperature is high.
2. A/C system pressure is low.
3. A/C system pressure is high.
4. During wide open throttle.
5. High engine RPM.
6. Certain DTCs are set.

When A/C-heater control panel is placed in A/C mode, a 12-volt signal is sent to PCM. When PCM receives this signal, PCM will ground A/C clutch relay control circuit to energize A/C clutch relay. This is shown on scan tool as A/C REQUEST YES.

When an A/C request has been detected by PCM, the PCM will ground A/C clutch relay control circuit. As a result, relay contacts close, allowing current to flow through relay to compressor clutch. When A/C REQUEST has been detected by PCM, cooling fans will turn on when A/C system pressure is greater than a predetermined value.

Regardless of selected A/C mode setting, a request is made to Powertrain Control Module (PCM)/Vehicle Control Module (VCM) to turn on A/C compressor. Request is made through A/C request signal circuit from HVAC control assembly. Power and ground are provided to HVAC control assembly by ignition voltage, ground circuits and splice pack.

In order for PCM/VCM to internally ground A/C clutch relay control circuit, request signal needs to be grounded. A 12-volt request signal is sent out over A/C request signal circuit and grounded through PCM/VCM when vehicle operator makes an A/C request. When this request signal is grounded, PCM/VCM can activate A/C compressor clutch.

PCM/VCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally of underhood fuse block. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode prevents a voltage spike from burning up compressor clutch coil. Ground circuit and underhood fuse block provides a path to ground for compressor. A/C clutch relay control circuit is grounded internally within the PCM/VCM.

A/C system is protected by A/C refrigerant pressure sensor. Sensors output to PCM/VCM is variable and is dependent upon pressure inside line. A higher pressure results in a higher voltage output. A/C pressure is constantly monitored in order to allow A/C compressor clutch to disengage as needed. If line pressures climbs greater than 410 psi (28.8 kg/cm 2 ) or fall less than 30 psi (2.1 kg/cm 2 ), PCM will turn off A/C compressor clutch. When high side pressures drop back down between 150-250 psi (10.5-17.6 kg/cm 2 ), PCM/VCM will allow the A/C compressor to operate.

A/C clutch relay will remain de-energized when DTC P0530 is set, or if there is no A/C request signal due to an open A/C select switch circuit. For additional diagnosis, see appropriate MANUAL A/C-HEATER SYSTEMS article.

Powertrain Control Module (PCM) will operate A/C system without any driver input. PCM will command A/C compressor clutch on to help reduce moisture inside vehicle. A/C compressor is on, even in cool weather conditions, to help eliminate moisture from fogging windshield. A/C LED will not illuminate unless driver presses A/C request switch on HVAC control module. Otherwise, A/C system may be running without A/C LED indicator illuminated.

Regardless of selected A/C mode setting, a request is made to PCM to turn on A/C compressor on class 2 serial data circuit. Request is made by HVAC control module to Body Control Module (BCM) through A/C request signal circuit when A/C switch grounds 12-volt signal from BCM. BCM then sends a class 2 message to PCM on BCM class 2 serial data circuit. PCM grounds clutch relay control circuit and engages A/C compressor clutch. Power is provided to HVAC control module by console junction block through ignition voltage, which is routed through splice pack and battery positive voltage circuits.

PCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally through underhood junction block. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode, located in underhood junction block, prevents a voltage spike from entering vehicles electrical system. ground circuit provides a path to ground for compressor and relay. A/C clutch relay control circuit is grounded internally within PCM.

BCM or PCM will shut off A/C compressor under extreme vehicle loads. If system voltage falls below 10 volts, an A/C command will not be given until voltage is raised to 10.5 volts. If coolant temperature rises greater than 259°F (126°C) or throttle is opened 95 percent or more, compressor clutch will be turned off.

A/C compressor clutch is protected from an overheating condition by an A/C compressor temperature switch mounted at rear of compressor. Switch is wired internally to compressor and is not serviceable. Switch shuts off compressor when temperatures reach 311°F (155°C) and allows clutch to engage again when temperatures fall to 257°F (125°C).

A/C system is protected by A/C refrigerant pressure sensor. Sensors' output to PCM is variable and is dependent upon pressure inside line. A higher pressure results in a higher voltage output. A/C pressure is constantly monitored in order to allow A/C compressor clutch to disengage as needed. If line pressures climb greater than 432 psi (30.4 kg/cm 2 ), PCM will turn off A/C compressor clutch until pressure lowers to 219 psi (15.4 kg/cm 2 ). If line pressures fall below 27 psi (1.9 kg/cm 2 ), PCM will turn off A/C compressor clutch until pressure raises to 30 psi (2.1 kg/cm 2 ).

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

Compressor for air conditioning system is belt-driven by engine through A/C compressor clutch. Clutch allows compressor to perform the following functions

1. Engage for A/C and defroster operation.
2. Disengage when air conditioning is not requested.
3. Disengage when engine load must be reduced.

Battery voltage is supplied to Powertrain Control Module (PCM) when A/C control is activated or when A/C-heater control mode selector is placed in any of the following modes: MAX A/C, NORM A/C, BI-LEVEL, DEFOG or DEFROST. Voltage signal tells PCM that A/C compressor operation is requested. Under normal operating conditions, PCM supplies a ground to relay coil of A/C CLU relay, relay energizes, voltage is supplied to A/C compressor clutch coil through relay center fuse, A/C compressor clutch coil engages and compressor runs.

If PCM determines engine load should be reduced, such as during full throttle conditions, PCM de-energizes A/C CLU relay in order to disengage compressor clutch coil. De-energization occurs even though voltage signal from A/C-heater control is still present at PCM.

A/C CLU diode provides a path for high current that results from voltage spikes generated from collapsing magnetic field of A/C compressor clutch coil. Voltage spikes occur every time coil de-energizes.

Powertrain Control Module (PCM) will operate A/C system without any driver input. PCM will command A/C compressor clutch on to help reduce moisture inside vehicle. A/C compressor is on, even in cool weather conditions, to help eliminate moisture from fogging windshield. A/C LED will not illuminate unless driver presses A/C request switch on HVAC control module. Otherwise, A/C system may be running without A/C LED indicator illuminated.

Regardless of selected A/C mode setting, a request is made to PCM to turn on A/C compressor. Request is made by HVAC control module to PCM through A/C request signal circuit when A/C switch grounds 12 volt signal from PCM. PCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally of underhood junction block. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode, located in underhood junction block, provides a path for voltage spike resulting from collapsing magnetic field of compressor clutch coil. Ground circuit provides a path to ground for compressor and relay. A/C clutch relay control circuit is grounded internally within PCM.

PCM will shut off A/C compressor under extreme vehicle loads, such as wide open throttle. PCM will shut off A/C compressor under extreme vehicle loads. If system voltage falls below 10 volts, an A/C command will not be given until voltage is raised to 10.8 volts. If coolant temperature rises greater than 259°F (126°C) or throttle is opened 95% or more, compressor clutch will be turned off. Compressor operation will be allowed when throttle is backed off to 90%.

A/C compressor clutch is protected from an overheating condition by an A/C compressor temperature switch mounted at rear of compressor. Switch is wired internally to compressor and is not serviceable. Switch shuts off compressor when temperatures reach 311°F (155°C) and allows clutch to engage again when temperatures fall to 257°F (125°C).

A/C system is protected by A/C refrigerant pressure sensor. Sensors output to PCM is variable and is dependent upon pressure inside line. A higher pressure results in a higher voltage output. A/C pressure is constantly monitored in order to allow A/C compressor clutch to disengage as needed. If line pressures climb greater than 432 psi (30.4 kg/cm 2 ), PCM will turn off A/C compressor clutch until pressure lowers to 235 psi (16.5 kg/cm 2 ). If line pressures fall below 26 psi (1.8 kg/cm 2 ), PCM will turn off A/C compressor clutch until pressure raises to 30 psi (2.1 kg/cm 2 ). A 5 volt reference signal is sent out over 5 volt reference circuit, from PCM, to A/C refrigerant pressure sensor. PCM monitors A/C pressure by monitoring voltage drop in sensor on A/C pressure sensor signal circuit. This circuit is how PCM monitors HVAC pressures. Ground for A/C refrigerant pressure sensor is provided by low reference circuit.

A/C system is protected by evaporator temperature sensor. This sensor is mounted on evaporator core and provides HVAC control module with surface temperature of evaporator core. If evaporator temperature sensor reads a temperature of 32°F (0°C), HVAC control module will turn off A/C compressor clutch until evaporator temperatures reach 36°F (2°C). HVAC control module sends out a 5 volt reference signal to evaporator temperature sensor over 5 volt reference circuit. A thermistor varies reference voltage back to HVAC control module through low reference circuit.

Regardless of selected A/C mode setting, a request is made to Powertrain Control Module (PCM)/Vehicle Control Module (VCM) to turn on A/C compressor clutch. Request is sent to PCM/VCM through A/C request signal circuit from HVAC control assembly. Power and ground is provided to HVAC control assembly by ignition voltage and ground circuits.

PCM/VCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Ignition voltage circuit provides power to A/C compressor clutch relay control. Once relay contact closes its internal switch, power from battery voltage circuit is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode prevents a voltage spike from entering vehicles electrical system. Ground circuit provides a pathway to ground for compressor. Both A/C request signal and A/C clutch relay control circuits are grounded internally within PCM/VCM.

A/C system is protected by 2 pressure switches. A/C high pressure switch interrupts A/C request signal when A/C line pressure exceeds a predetermined value. A/C low pressure switch interrupts A/C low pressure switch signal when A/C line pressure drops below a predetermined value. When PCM/VCM stops receiving required signals, A/C compressor clutch relay control circuit is no longer grounded, thus shutting off compressor.

A/C system is engaged when HVAC switch is pressed. A/C system can operate regardless of temperature setting, as long as outside ambient temperature is greater than 40°F (4°C). If A/C is selected and A/C is not available, A/C LED will flash, indicating that A/C is not allowed. A/C compressor is activated when FROST DEFROST is selected. A/C will not be allowed if engine exceeds 5985 RPM.

HVAC control module receives power from instrument panel fuse block on ignition voltage circuit along with battery voltage circuit. HVAC control module communicates directly to Powertrain Control Module (PCM)/Vehicle Control Module (VCM) on class 2 serial data and PCM/VCM class 2 serial data circuits through splice pack.

Regardless of selected A/C mode setting, a request is made to PCM to turn on A/C compressor. Request is made through A/C request signal circuit, through A/C high pressure switch, from HVAC control module.

PCM/VCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally of underhood fuse block. Once A/C compressor clutch relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode prevents a voltage spike. Ground circuit and underhood fuse block provides a path to ground for compressor. A/C clutch relay control circuit is grounded internally within PCM/VCM.

A/C system is protected by 2 pressure switches. A/C high pressure switch interrupts A/C request signal circuit when A/C line pressures climb greater than 410 psi (28.8 kg/cm 2 ). A/C low pressure switch interrupts A/C low pressure switch signal circuit when A/C line pressures fall below 30 psi (2.1 kg/cm 2 ). When PCM/VCM sees an open in either signal, A/C clutch relay control circuit is no longer grounded, thus shutting off compressor. When high side pressures drop back down to 150-250 psi (10.5-17.6 kg/cm 2 ), PCM/VCM will allow A/C compressor to operate.

Vehicle operator can activate A/C system by activating the following modes: MAX A/C, A/C BI-LEVEL, MIX-BLEND or FRONT DEFROST. A/C system can operate regardless of temperature setting, as long as ambient air temperature is greater than 38°F (3°C) degrees. If A/C compressor clutch is turned off due to cold ambient air temperatures, compressor will not come back on until ambient air temperatures reach 41°F (5°C).

Regardless of selected A/C mode setting, a request is made to Powertrain Control Module (PCM)/Vehicle Control Module (VCM) to turn on A/C compressor. Request is made through A/C request signal circuit from HVAC control assembly. Power is provided to HVAC control assembly by ignition voltage.

In order for PCM/VCM to internally ground A/C clutch relay control circuit, 2 separate request signals need to be grounded. A 12-volt request signal is sent out over A/C request signal circuit, through A/C high pressure switch, and grounded through PCM/VCM when vehicle operator makes an A/C request. A separate 12-volt request signal is sent out over A/C low pressure switch signal circuit, through A/C low pressure switch and ground circuit. When both these request signals are grounded, PCM/VCM can activate A/C compressor clutch.

PCM/VCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally of underhood fuse block. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode prevents a voltage spike from burning up compressor clutch coil. Ground circuit and underhood fuse block provides a path to ground for compressor. A/C clutch relay control circuit is grounded internally within PCM/VCM.

A/C system is protected by 2 pressure switches. A/C high pressure switch interrupts A/C request signal when A/C line pressures climb greater than 410 psi (28.8 kg/cm 2 ). A/C low pressure switch interrupts A/C low pressure switch signal when A/C line pressure fall below 30 psi (2.1 kg/cm 2 ). When the PCM/VCM sees an open in either signal, A/C clutch relay control circuit is no longer grounded, thus shutting off compressor. When high side pressures drop back down between 150-250 psi (10.5-17.6 kg/cm 2 ), PCM/VCM will allow the A/C compressor to operate.

A/C clutch relay is controlled by Vehicle Control Module (VCM) to delay A/C compressor clutch engagement after A/C is turned on. This allows VCM to adjust engine RPM before A/C compressor clutch engages. VCM will engage A/C compressor clutch any time A/C has been requested unless coolant temperature is high, A/C system pressure is low, A/C system pressure is high, wide open throttle, or engine RPM is high.

When A/C-heater control panel is placed in A/C mode, a 12-volt signal is sent to VCM. When VCM receives this signal, VCM will ground A/C clutch relay control circuit to energize A/C clutch relay. This is shown on scan tool as A/C REQUEST YES.

When an A/C request has been detected by VCM, VCM will ground A/C clutch relay control circuit. As a result, relay contacts close, allowing current to flow through relay to A/C compressor clutch. When A/C REQUEST has been detected by VCM, cooling fans will turn on when A/C pressure is greater than a predetermined value.

PCM controls operation of A/C clutch relay. When any A/C mode is selected on Heating, Ventilation and Air Conditioning (HVAC) control module, a 12-volt signal is sent through A/C high pressure switch and to PCM. If conditions are met, PCM will send another 12-volt signal to A/C compressor cycling switch, which is ground signal. When these 2 signals are received by PCM, PCM supplies a ground for A/C compressor clutch relay.

PCM controlled HVAC system consists of the following components

1. A/C high pressure switch.
2. A/C compressor cycling switch.
3. A/C compressor relay.
4. PCM

PCM will engage A/C compressor clutch unless one or more of the following conditions occur

1. Throttle angle at 100 percent.
2. A/C compressor cycling switch pressure is less than 18 psi (1.2 km/cm 2 ) or greater than 49 psi (3.4 km/cm 2 ).
3. A/C high pressure cut-out switch pressure is greater than 420 psi (29.5 km/cm 2 ).
4. Engine speed is greater than 5500 RPM.
5. Engine coolant temperature greater than 250°F (121°C).

Any condition described above will prevent operation of A/C compressor clutch.

Vehicle Control Module (VCM) control of A/C clutch improves idle quality and performance by performing the following conditions

1. Delaying clutch engagement until idle speed is increased.
2. Releasing clutch when idle speed is too low.
3. Providing additional fuel at the instant clutch is applied. Additional fuel smooths cycling of compressor.

Turning air conditioning on supplies battery voltage through pressure switches to VCM. When VCM receives voltage on A/C request signal, A/C enable relay circuit is grounded. A/C compressor clutch then engages.

A/C system can operate regardless of temperature setting, as long as outside ambient temperature is greater than 40°F (4°C). If A/C is requested and temperatures are too low, LED will flash 3 times to remind vehicle operator that A/C is not available.

Regardless of selected A/C mode setting, a request is made to Powertrain Control Module (PCM) to turn on A/C compressor. request is sent to PCM through A/C request signal circuit from HVAC control module. Power and ground are provided to HVAC control module by fuse block through ignition voltage and ground circuits.

In order for PCM to internally ground A/C clutch relay control circuit, 2 separate request signals need to be grounded. A 12-volt reference signal is sent out over A/C request signal circuits, through A/C high pressure switch and grounded through HVAC control module when vehicle operator makes an A/C request. A separate 12-volt reference signal is sent out over A/C refrigerant low pressure cut-out switch signal circuit, through A/C low pressure switch and ground circuit. When both these reference signals are grounded, PCM can activate A/C compressor clutch.

PCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally of engine wiring harness junction block. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode provides a path for voltage spike resulting from collapsing magnetic field of compressor clutch coil. Ground circuit provides a pathway to ground for compressor. A/C clutch relay control circuit is grounded internally within PCM.

A/C system is protected by 2 pressure switches. A/C high pressure switch interrupts A/C request signal when A/C line pressure exceeds a predetermined value. A/C low pressure switch interrupts A/C refrigerant low pressure cut-out switch signal when A/C line pressure falls less than 21-25 psi (1.5-1.8 kg/cm 2 ). When PCM sees an open in either signal, A/C clutch relay control circuit is no longer grounded, thus shutting off compressor. Low pressure switch will close when pressure reaches 38-42 psi (2.7-3.0 kg/cm 2 ).

A/C system can operate regardless of temperature setting, however, recirculation is only available when HVAC control module is either in A/C or BI-LEVEL mode. Regardless of selected A/C mode setting, a request is made to Powertrain Control Module (PCM) to turn on A/C compressor. Request is sent to PCM through A/C request signal circuit from HVAC control module. Power and ground are provided to HVAC control module by left instrument panel fuse block through ignition voltage and ground circuits.

In order for PCM to internally ground A/C clutch relay control circuit, 2 separate request signals need to be grounded. A 12 volt reference signal is sent out over A/C request signal circuits, through A/C high pressure switch and grounded through HVAC control module when vehicle operator makes an A/C request. A separate 12 volt reference signal is sent out over A/C refrigerant low pressure cut-out switch signal circuit, through A/C low pressure switch and ground circuit. When both these reference signals are grounded, PCM can activate A/C compressor clutch.

PCM turns on A/C compressor by providing a path to ground through A/C clutch relay control circuit for A/C compressor clutch relay. Power is provided to A/C compressor clutch relay internally of engine wiring harness junction block. Once relay closes its internal switch, power from battery is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C compressor clutch diode prevents a voltage spike from entering vehicles electrical system. Ground circuit provides a pathway to ground for compressor. A/C clutch relay control circuit is grounded internally within PCM.

A/C system is protected by 2 pressure switches. A/C high pressure switch interrupts A/C request signal when A/C line pressure exceeds a predetermined value. A/C low pressure switch interrupts A/C refrigerant low pressure cut-out switch signal when A/C line pressure falls less than 21-25 psi (1.5-1.8 kg/cm 2 ). When PCM sees an open in either signal, A/C clutch relay control circuit is no longer grounded, thus shutting off compressor. Low pressure switch will close when pressure reaches 38-42 psi (2.7-3.0 kg/cm 2 ).

A/C system can operate regardless of air temperature setting. Regardless of selected A/C mode setting, a request is made to Powertrain Control Module (PCM) to turn on A/C compressor clutch. Request is sent to PCM through A/C request signal circuit and A/C high pressure switch from HVAC control assembly. On vehicles equipped with diesel engine, A/C request is sent to PCM through the A/C request signal circuit and A/C high and low pressure switches from HVAC control assembly. Power and ground are provided to HVAC control assembly by ignition voltage and ground circuits.

PCM turns on A/C compressor by providing a path to ground through A/C compressor clutch relay control circuit for A/C compressor clutch relay. Ignition voltage circuit provides power to A/C compressor clutch relay control. Once relay contact closes its internal switch, power from battery positive voltage circuit is provided to A/C compressor clutch through A/C compressor clutch supply voltage circuit. Whenever compressor is turned off, A/C clutch diode prevents a voltage spike from damaging compressor clutch coil. Ground circuit provides a pathway to ground for compressor and diode. Both A/C request signal and A/C clutch relay control circuits are grounded by ground circuits.

A/C system is protected by 2 pressure switches. A/C high pressure switch interrupts A/C request signal when A/C line pressure exceeds a predetermined value. A/C low pressure switch interrupts A/C low pressure switch signal when A/C line pressure drops below a predetermined value. When PCM stops receiving the required signals, A/C compressor clutch relay control circuit is no longer grounded, thus shutting off compressor.