Diagnostic Instructions
| • | 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. |
DTC Descriptor
DTC P0646: Air Conditioning (A/C) Clutch Relay Control Circuit Low VoltageDTC P0647: Air Conditioning (A/C) Clutch Relay Control Circuit High Voltage
Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
A/C Relay Coil B+ | P0646 | P0647 | P0647 | -- |
A/C Compressor Clutch Relay Control | P0646 | P0647 | P0647 | -- |
Circuit/System Description
The engine control module (ECM)/powertrain control module (PCM) controls the A/C compressor clutch based on an A/C request serial data message from the HVAC control module. The ECM/PCM monitors the A/C refrigerant pressure sensor and if pressure is within a specified range, will engage the A/C compressor by grounding the A/C compressor clutch relay control circuit.
B+ is applied to the A/C relay in the RUN/START position and with the A/C compressor clutch relay control circuit grounded by the ECM/PCM, current flows through the coil side of the A/C relay. With current flowing through the coil side of the relay, a magnetic field is created which pulls the switch side of the relay closed, sending current to the A/C compressor clutch, allowing the A/C compressor to be engaged.
Conditions for Running the DTC
An A/C request is received from the HVAC module.
Conditions for Setting the DTC
A short to voltage, short to ground, or open/high resistance is seen on the A/C compressor clutch relay control circuit.
Action Taken When the DTC Sets
DTC P0646 and DTC P0647 are Type C DTCs
Conditions for Clearing the DTC
DTC P0646 and DTC P0647 are Type C DTCs
Reference Information
Schematic Reference
Connector End View Reference
Description and Operation
Air Temperature Description and Operation
Electrical Information Reference
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Circuit/System Verification
Engine running, command the A/C Relay On and OFF with a scan tool. The A/C compressor clutch should engage and disengage.
Circuit/System Testing
- Ignition OFF, disconnect the A/C relay.
- Ignition ON, verify a test lamp illuminates between the relay coil B+ circuit terminal and ground.
- Connect a test lamp between the relay coil B+ circuit terminal and the A/C compressor clutch relay control circuit terminal.
- Engine running, command the A/C relay ON and OFF. The test lamp should turn ON and OFF when changing between the commanded states.
- If all circuits test normal, test or replace the A/C relay.
| ⇒ | If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. |
| ⇒ | 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. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Control Module References for ECM/PCM replacement, setup, and programming
