Diagnostic Instructions

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

DTC Descriptor

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

Conditions for Setting the DTC

Action Taken When the DTC Sets

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

Electrical Information Reference

Scan Tool Reference

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.

⇒	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.

⇒	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.

⇒	If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, 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 of resistance between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

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 of resistance between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.

Repair Instructions