Diagnostic Instructions

DTC Descriptor

DTC P0645: Air Conditioning (A/C) Clutch Relay Control Circuit

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	P0645, P0646	P0645, P0647	P0645, 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 3 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:

Once the mode switch is in one of these positions, the mode switch applies ignition 3 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 PCM/ECM 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.

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

Component Connector End Views

Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

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. Verify that a test lamp illuminates between the 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.

3. Connect a test lamp between the ignition circuit terminal 86 and the control circuit terminal 85.
4. 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, 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 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.

⇒	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 between terminals 30 and 87.

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

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.