For 1990-2009 cars

Refer to Engine Control Schematic

#### A/C Controls

Master Electrical Component List
Cruise Control
Power Distribution Schematics
Outside Rearview Mirror Schematics
Engine Cooling Schematics
Engine Cooling Schematics
Engine Cooling Schematics
.

### A/C Operation Description

The engine control module (ECM) controls the A/C clutch relay. ECM A/C operation is controlled by two input circuits, A/C request and A/C load, and one output circuit, the A/C compressor relay control.

The heater and A/C control sends an A/C request signal, approximately battery voltage, whenever the operator depresses the A/C selector switch on the control panel. When the ECM receives this signal, the ECM will turn the A/C clutch relay ON unless one of the following conditions is observed:

• | The engine is not running.

• | The ignition voltage is less than 9 volts.

• | A wide open throttle (WOT) condition is present.

The ECM will NOT detect the A/C request signal if the A/C system pressure is too high or too low. The A/C request circuit is in series with a portion of the A/C compressor refrigerant pressure switch. The pressure activated switch contacts will open if the A/C pressure is not correct for proper A/C operation. Refer to the electrical schematic diagrams for the specific operating pressures. The heater and A/C control monitors the A/C request circuit for shorts. A shorted condition will cause a heating, ventilation and air conditioning (HVAC) diagnostic trouble code (DTC) to set.

The ECM also monitors the A/C load switch circuit. This circuit is designed to increase the engine speed by about 50 RPM whenever a high temperature and pressure condition causes the A/C load switch contacts to close.

The ECM controls clutch operation by supplying a ground path for the A/C compressor relay. The A/C compressor relay control circuit is monitored for opens and shorts. An open or shorted condition will cause an ECM DTC to set. ECM control of the A/C clutch relay can be tested using a scan tool.

### Circuit Description

The A/C compressor relay switch contact is supplied battery power from fuse 33. When the ECM commands the A/C clutch relay ON, the relay switch contacts close allowing current to flow to the A/C compressor clutch coil and also to the auxiliary coolant fan relay, causing the auxiliary coolant fan to operate. The current flows through the A/C compressor clutch coil to the secondary cooling fan temperature switch. The current continues through the secondary cooling fan temperature switch to ground. The temperature activated contacts of the secondary cooling fan temperature switch will open, turning the A/C clutch OFF, if coolant temperature is too high for proper A/C system operation.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. You must perform the A/C Request Circuit Diagnosis prior to diagnosing the A/C compressor clutch circuit. This procedure contains important steps on checking the systems for diagnostic trouble codes.

2. The engine coolant must be below the specified temperature in order to properly test the HVAC compressor clutch circuit. When the temperature threshold is exceeded, the secondary cooling fan temperature switch contacts will open the ground path to the A/C clutch coil.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the A/C Request Circuit Diagnosis? | -- | Go to Step 2 | Go to Air Conditioning (A/C) Request Circuit Diagnosis
2 |

1. Turn ON the ignition, with the engine OFF.

Important: Engine coolant must be below the temperature specified before proceeding.

2. Command the A/C relay ON and OFF several times using the scan tool.
3. Observe the A/C compressor clutch.

Does the A/C compressor clutch engage and disengage when commanded ON and OFF?

100°C (212°F)

Go to Step 3

Go to Step 4

3 | ECM operation of the A/C compressor clutch is OK at this time. Is a concern about A/C operation still present? | -- | Go to Diagnostic Starting Point in HVAC | System OK
4 | Does the A/C compressor clutch remain engaged all the time? | -- | Go to Step 10 | Go to Step 5
5 |

1. Turn ON the ignition, leaving the engine OFF.
2. Command the A/C relay ON using the scan tool.
3. Observe the auxiliary coolant fan.

Does the auxiliary coolant fan operate when the A/C relay is commanded ON?

--

Go to Step 8

Go to Step 6

6 |

1. Turn ON the ignition, leaving the engine OFF.
2. Remove the A/C compressor relay.
3. Probe the A/C compressor relay switch positive voltage circuit with the test lamp connected to a known good ground.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 12

7 |

1. Connect a 30 amp fused jumper from the A/C compressor relay terminal 8 to terminal 4.
2. Observe A/C compressor clutch operation.

Does the A/C compressor clutch engage while the relay terminals are jumpered?

--

Go to Step 14

Go to Step 13

8 |

1. Disconnect the A/C compressor clutch connector.
2. Command the A/C relay ON using the scan tool.
3. Probe the A/C compressor power circuit with the test lamp connected to a known good ground.

Does the test lamp illuminate?

--

Go to Step 9

Go to Step 13

9 |

1. Probe the A/C compressor clutch coil power circuit with the test lamp connected to the A/C compressor clutch coil GROUND circuit terminal.
2. Ensure the A/C relay is still commanded ON.

Does the test lamp illuminate?

--

Go to Step 18

Go to Step 11

10 | Remove the A/C relay. Is the A/C compressor clutch still engaged? | -- | Go to Step 15 | Go to Step 14
11 |

1. Turn OFF the ignition.
2. Disconnect the secondary cooling fan temperature switch connector.
3. Test the GROUND circuit for an open or high resistance between the A/C compressor clutch coil connector and the secondary cooling fan temperature switch connector.
4. Repair the wiring as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found and repaired?

--

Go to Step 19

Go to Step 16

12 |

1. Repair the cause of no battery power to the A/C compressor clutch relay switch contacts. Refer to Wiring Repairs in Wiring Systems.
2. If the fuse was open, repair the short to ground or the cause of the excessive current draw.

Is the action complete?

--

Go to Step 19

--

13 | Repair the open A/C compressor clutch power circuit between the relay and the A/C compressor clutch coil connector. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 19 | --
14 | Replace the A/C compressor relay. Is the action complete? | -- | Go to Step 19 | --
15 | Repair the A/C compressor clutch power circuit shorted to B+. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 19 | --
16 |

1. Test the GROUND circuit for an open or high resistance between the secondary cooling fan temperature switch connector terminal 3 and ground. Refer to Engine Cooling for wiring views.
2. Repair the wiring as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found and repaired?

--

Go to Step 19

Go to Step 17

17 | Replace the secondary cooling fan temperature switch. Refer to Engine Cooling Fan Control Switch Replacement in Engine Cooling. Is the action complete? | -- | Go to Step 19 | --
18 | Replace the compressor clutch coil. Refer to Air Conditioning Clutch Coil Removal and Air Conditioning Clutch Coil Installation in Heating, Ventilation, and Air Conditioning. Did you complete the replacement? | -- | Go to Step 19 | --
19 |

1. Reconnect all disconnected components and connectors.
2. Clear DTC information from the ECM and the heater and A/C control.
3. Command the A/C relay ON using the scan tool.

Does the A/C clutch engage?

--

System OK

Go to Powertrain On Board Diagnostic (OBD) System Check