• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
• | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
• | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
---|---|---|---|---|
A/C Relay Switch B+ | 2 | 2 | -- | -- |
A/C Relay Coil Ignition | 2 | 2 | -- | -- |
A/C Request Signal | 2 | 2 | 1 | -- |
A/C Compressor Clutch Relay Control | P0645, P0646, P0647 | P0645, P0646, P0647 | P0645, P0646, P0647 | -- |
A/C Relay Controlled Output | 2 | 2 | 1 | -- |
A/C Compressor Clutch Ground | -- | 2 | 2 | -- |
1. A/C Clutch Always Engaged 2. A/C Clutch Inoperative |
Ignition voltage is supplied to the mode switch from the blower motor switch. The blower motor switch must be in any position except for OFF for A/C operation. The mode switch must be in one of the following positions for an A/C request:
• | MAX A/C |
• | A/C |
• | Bi-Level |
• | Defog |
• | Defrost |
Once the mode switch is in one of these positions, the mode switch applies ignition voltage to the A/C high pressure switch. If the A/C high pressure switch is closed, the voltage will then be applied to the powertrain control module (PCM), for the 8.1L (RPO L18), or engine control module (ECM), for the 6.6L (RPO LLY). The voltage applied to the ECM/PCM is an A/C request signal. The ECM/PCM then monitors the voltage on the A/C low pressure switch signal. If the voltage is low near ground, then the ECM/PCM will enable the A/C compressor clutch relay. The ECM/PCM provides a ground for the relay so it can close it internal contacts.
The following conditions must be met in order for the ECM/PCM to engage the A/C compressor clutch:
• | A/C high side pressure is less than 2964 kPa (430 psi) |
• | A/C low side pressure is above 303 kPa (44 psi) |
• | Throttle position is less than 100 percent |
• | ECM/PCM does not detect excessive torque load |
• | ECM/PCM does not detect insufficient idle quality |
Control Module References for scan tool information
⇒ | If the reading does not change while switching between the commanded states, refer to A/C Request Malfunction. |
⇒ | If not the specified value, refer to Low Pressure Switch Malfunction. |
⇒ | If the A/C compressor does not engage or does not disengage, refer to A/C Compressor Malfunction. |
⇒ | If the test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal refer to Blower Motor Malfunction . |
⇒ | If not the specified value, test the signal circuit terminal A for a short to voltage. If the circuit tests normal, replace the ECM/PCM. |
⇒ | If not the specified value, test the signal circuit for an open/high resistance. If the circuit tests normal, replace the ECM/PCM. |
⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
⇒ | If not the specified value, test the signal circuit terminal B for short to ground. If the circuit tests normal, replace the ECM/PCM. |
⇒ | If not the specified value, test the signal circuit for short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM/PCM. |
⇒ | If the test lamp illuminates, test the control circuit for a short to voltage. |
⇒ | If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ignition circuit breaker/fuse is open, test or replace the A/C compressor clutch relay. |
⇒ | If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. |
⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
⇒ | If the A/C compressor clutch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, replace the A/C compressor. |
⇒ | If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM/PCM. |
⇒ | If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM/PCM. |
⇒ | If the resistance is not within the specified range, replace the relay. |
• | 30 and 86 |
• | 30 and 87 |
• | 30 and 85 |
• | 85 and 87 |
⇒ | If not the specified value, replace the relay. |
⇒ | If greater than the specified range, replace the relay. |
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
• | Control Module References for ECM/PCM replacement, setup, and programming |
• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
• | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
• | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
---|---|---|---|---|
A/C Relay Switch B+ | 2 | 2 | -- | -- |
A/C Relay Coil Ignition | 2 | 2 | 1 | -- |
A/C Request Signal | 2 | 2 | 1 | -- |
A/C Relay Coil Control | -- | 2 | 2 | -- |
A/C Relay Controlled Output | 2 | 2 | 1 | -- |
A/C Low Pressure Switch Ground | -- | 2 | 2 | -- |
A/C Compressor Clutch Ground | -- | 2 | 2 | -- |
1. A/C Clutch Always Engaged 2. A/C Clutch Inoperative |
Ignition voltage is supplied to the mode switch from the blower motor switch. The blower motor switch must be in any position except for OFF for A/C operation. The mode switch must be in one of the following positions for an A/C request:
• | MAX A/C |
• | A/C |
• | Bi-Level |
• | Defog |
• | Defrost |
Once the mode switch is in one of these positions, the mode switch applies ignition voltage to the A/C high pressure switch. If the A/C high pressure switch is closed, the voltage will then be applied to the A/C compressor relay and the engine control module (ECM). The voltage applied to the ECM is an input, this ensures idle quality. For the relay to close its internal contacts to supply the A/C compressor clutch with battery positive voltage, the A/C low pressure must be closed to provide a path to ground for the relay coil through the A/C low pressure switch signal circuit.
The following conditions must be met to engage the A/C compressor clutch:
• | A/C high side pressure is less than 2964 kPa (430 psi) |
• | A/C low side pressure is above 303 kPa (44 psi) |
Control Module References for scan tool information
⇒ | If the reading does not change while switching between the commanded states, refer to A/C Request Malfunction. |
⇒ | If the A/C compressor does not engage or does not disengage, refer to A/C Compressor Malfunction. |
⇒ | If the test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal refer to Blower Motor Malfunction . |
⇒ | If not the specified value, test the signal circuit terminal B for a short to voltage. If the circuit tests normal, replace the ECM. |
⇒ | If not the specified value and the A/C high side pressure is within specifications, test the signal circuit for an open/high resistance. If the circuit tests normal, replace the ECM. |
⇒ | If the test lamp illuminates, test the control circuit for a short to voltage. |
⇒ | If the test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. |
⇒ | If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the control circuit terminal 87 for a short to ground. |
⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
⇒ | If the A/C compressor clutch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, replace the A/C compressor clutch. |
⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
⇒ | If not the specified value, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the A/C compressor clutch relay. |
⇒ | If the resistance is not within the specified range, replace the relay. |
• | 30 and 86 |
• | 30 and 87 |
• | 30 and 85 |
• | 85 and 87 |
⇒ | If not the specified value, replace the relay. |
⇒ | If greater than the specified range, replace the relay. |
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
• | Control Module References for ECM/PCM replacement, setup, and programming |