P0646

DTC P0646

CIRCUIT DESCRIPTION
Ignition voltage is supplied directly to the A/C compressor clutch relay. The engine control module (ECM) or powertrain control module (PCM) 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. If the fault detection circuit senses a voltage other than what is expected, a DTC will set.

The ECM/PCM will monitor the control circuit for the following:
- A short to ground
- A short to voltage
- An open circuit
- An open relay coil
- An internally shorted or excessively low resistance relay coil

When the ECM/PCM detects any of the above malfunctions, a DTC is set and the affected driver is disabled.

CONDITIONS FOR RUNNING THE DTC
- The ignition voltage is between 11.0-18.0 volts.
- The engine speed is less than 600 RPM.
- The ECM/PCM driver transitions from ON to OFF or from OFF to ON.

CONDITIONS FOR SETTING THE DTC

P0645
The ECM/PCM detects an open on the control circuit of the A/C compressor clutch relay when commanded off with the engine in crank or run status.

P0646
The ECM/PCM detects a short to ground on the control circuit of the A/C compressor clutch relay when commanded off with the engine in crank or run status.

P0647
The ECM/PCM detects a short to voltage on the control circuit of the A/C compressor clutch relay when commanded off with the engine in crank or run status.

ACTION TAKEN WHEN THE DTC SETS
- The ECM/PCM will not illuminate the malfunction indicator lamp (MIL).
- The ECM/PCM will store conditions which were present when the DTC set as Failure Records data only. This information will not be stored as Freeze Frame data.

CONDITIONS FOR CLEARING THE DTC
- A History DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction.
- The DTC can be cleared by using a scan tool.
- The DTC will become history if the ECM/PCM no longer detects a failure.

DIAGNOSTIC AIDS

IMPORTANT: Be sure to verify that the ECM/PCM engine grounds are secure and clean.

If DTC P0645, P0646 and P0647 cannot be duplicated, reviewing the Failure Records vehicle millage since the diagnostic test last failed may help determine how often the condition that caused the DTC to set occurs. This may assist in diagnosing the condition.

If the condition is not present, refer to Testing for Intermittent and Poor Connections in Diagnostic Aids.

TEST DESCRIPTION

Steps 1-11:

The numbers below refer to the step numbers on the diagnostic table.
2. Listen for an audible click when the A/C compressor clutch relay operates. Command both the ON and OFF states. Repeat the commands as necessary.
3. Tests for voltage at the coil side of the A/C compressor clutch relay. The 10-amp fuse supplies power to the coil side of the A/C compressor clutch relay.
4. Verifies that the engine control module is providing ground to the A/C compressor clutch relay.