GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2006 🢒 TopKick C-Series (Conventional) C6/C7/C8 🢒 HVAC 🢒 HVAC Systems - Manual 🢒 DTC P0646 or P0647

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 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

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

A/C Relay Coil Ignition

2

2

--

--

A/C Relay Switch B+

2

2

--

--

A/C Compressor Clutch Relay Control

P0646

P0647

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

Circuit/System Description

Ignition voltage is supplied directly to the A/C compressor clutch relay. The engine/powertrain control module (ECM/PCM) for the RPO L18/LLY, controls the relay by grounding the A/C clutch relay control circuit via an internal solid state device called a driver. The primary function of the driver is to supply the ground for the component being controlled. The driver has a fault line which is monitored by the ECM/PCM. When the ECM/PCM is commanding a component ON, the voltage of the control circuit should be near 0 volts. When the ECM/PCM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be near battery voltage.

Conditions for Running the DTC
    • The ignition voltage is between 9-18 volts.
    • The engine speed is more than 80 RPM.
    • The ECM/PCM A/C compressor clutch relay control transitions between ON to OFF or from OFF to ON.

Conditions for Setting the DTC
    • A short to ground on the A/C compressor clutch relay control circuit
    • A short to voltage on the A/C compressor clutch relay control circuit
    • An open circuit on the A/C compressor clutch relay control circuit or relay
    • An internally shorted or excessively low resistance A/C clutch relay coil

Action Taken When the DTC Sets
    • The A/C clutch relay is commanded OFF.
    • The conditions for which the DTC was set will be stored in the Failure Records data only. No information will be stored as Freeze Frame data.

Conditions for Clearing the DTC

The history DTC will clear after 40 fault-free ignition cycles.

Reference Information

Schematic Reference

HVAC Schematics

Connector End View Reference

HVAC Connector End Views

Description and Operation

    •  Air Temperature Description and Operation
    •  Air Delivery Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Output Controls
    •  Scan Tool Data Definitions

Circuit/System Verification

Ignition ON, command the A/C relay ON and OFF with a scan tool. Observe the A/C compressor operation. The A/C compressor clutch should engage and disengage when changing between the commanded states.

Circuit/System Testing
  1. Ignition OFF, disconnect the A/C compressor clutch relay.
  2. Ignition ON, verify that a test lamp does not illuminate between the relay coil control circuit terminal 85 and ground.
    3. ⇒If the test lamp illuminates, test the control circuit for a short to voltage.
  3. Verify that a test lamp illuminates between the relay coil ignition circuit terminal 86 and ground.
    4. ⇒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 fuse/breaker is open, test or replace the A/C compressor clutch relay.
  4. Connect a test lamp between the relay coil ignition circuit terminal 86 and the relay coil control circuit terminal 85.
  5. Command the A/C relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
    6. ⇒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.
  6. If all circuits test normal, test or replace the A/C compressor clutch relay.

Component Testing
  1. Ignition OFF, disconnect the A/C compressor clutch relay.
  2. Test for 60-180 ohms between terminals 85 and 86.
    3. ⇒If the resistance is not within the specified range, replace the relay.
  3. Test for infinite resistance between the following terminals:
    4. • 30 and 86
    • 30 and 87
    • 30 and 85
    • 85 and 87
    ⇒If not the specified value, replace the relay.
  4. Install a 30-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms between terminals 30 and 87.
    5. ⇒If greater than the specified range, replace the relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Compressor Relay Replacement
    •  Control Module References for ECM/PCM replacement, setup, and programming