For 1990-2009 cars

### Circuit Description

The air conditioning (A/C) system uses an A/C refrigerant pressure sensor mounted in the high pressure side of the A/C refrigerant system to monitor A/C refrigerant pressure. The engine control module (ECM) uses this information to turn ON the engine coolant fans when the A/C refrigerant pressure is high and to keep the compressor disengaged when A/C refrigerant pressure is excessively high or low.

The air conditioning pressure (ACP) sensor operates like other 3-wire sensors. The ECM applies a 5-volt reference and a sensor ground to the sensor. Changes in the A/C refrigerant pressure will causes the ACP sensor input to the ECM to vary. The ECM monitors the ACP sensor signal circuit and can determine when the signal is out of the possible range of the sensor. When the signal is out of range for a prolonged period of time, the ECM will not allow the A/C compressor clutch to engage. This is done to protect the compressor.

### DTC Descriptors

This diagnostic procedure supports the following DTC:

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

### Conditions for Running the DTC

Engine is running.

### Conditions for Setting the DTC

• | Engine is running.

• | A/C clutch is engaged

• | Ignition voltage is between 11-16 volts.

• | An open or a short to ground condition in the A/C compressor relay circuit exists.

### Action Taken When the DTC Sets

• | The malfunction indicator lamp (MIL) will not illuminate.

• | The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.

### Conditions for Clearing the MIL/DTC

• | A history DTC will clear after 40 warm-up cycles without a fault.

• | DTCs can be cleared by using the scan tool.

### Diagnostic Aids

Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal-to-wire connection at the ECM.

Inspect the wiring harness for damage. If the harness appears to be OK, observe the A/C pressure display on the scan tool while moving the connectors and wiring harnesses related to the ACP sensor. A change in the A/C pressure display will indicate the location of the fault. If this DTC cannot be duplicated, reviewing the Failure Records vehicle mileage 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.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: HVAC Schematics Connector End View Reference: Component Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 |

1. Turn the ignition switch to OFF.
2. Disconnect the A/C compressor relay.
3. Measure the resistance between A/C compressor relay terminals 85 and 86.

Is the resistance within the specified value?

0 Ω

Go to Step 3

Go to Step 7

3 | With the test light connected to ground, probe the ignition feed circuit, at terminal 85 in the relay harness connector. Does the test light illuminate? | -- | Go to Step 5 | Go to Step 4
4 | Repair a short to ground between the ignition switch terminal and A/C compressor relay terminal 85. Did you complete the repair? | -- | Go to Step 9 | -
5 |

1. Disconnect the ECM.
2. With the test light connected to ground, probe the ignition feed circuit at terminal 86 in the relay harness connector.

Does the test light illuminate?

--

Go to Step 8

Go to Step 6

6 | Repair a short to ground between the ECM wiring connector terminal C35 and A/C compressor relay terminal 86. Is the repair complete? | -- | Go to Step 9 | --
7 | Replace the A/C compressor relay. Did you complete the replacement? | -- | Go to Step 9 | --
8 | Replace the ECM. Refer to Control Module References . Did you complete the replacement? | -- | Go to Step 9 | --
9 |

1. Using the scan tool, clear the DTCs.
2. Start the engine and idle at normal operating temperature.
3. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic has run and passed?

--

Go to Step 10

Go to Step 2

10 | Check if any additional DTCs are set. Are any DTCs displayed that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle | System OK