HVAC System - Manual

DTC P0646 or P0647

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0646
- Air Conditioning (A/C) Clutch Relay Control Circuit Low Voltage

DTC P0647
- Air Conditioning (A/C) Clutch Relay Control Circuit High Voltage

Diagnostic Fault Information






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

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

1. Ignition OFF, disconnect the A/C relay.
2. Ignition ON, verify a test lamp illuminates between the relay coil B+ circuit terminal and ground.

• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

3. Connect a test lamp between the relay coil B+ circuit terminal and the A/C compressor clutch relay control circuit terminal.
4. 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 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.

5. If all circuits test normal, test or replace the A/C relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests