For 1990-2009 cars
A/C Controls and Idle-Up Signals
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

The A/C compressor control module sends an A/C idle-up signal to the powertrain control module (PCM) when the A/C compressor clutch is on. The PCM sends a signal to the idle air control (IAC) valve in order to increase the flow through the idle air passage. The engine idle speed increases as more air flows through the idle air passage. The increase in the idle speed prevents a rough idle or stalling condition.

### Diagnostic Aids

Observe IAC Duty Cycle parameter on a scan tool while operating the A/C. The IAC Duty Cycle percentage should increase 4 to 12 percent when the A/C compressor is engaged. If the scan tool indicates that the IAC Duty Cycle is increasing when the A/C is turned On, but the idle quality is unsatisfactory, inspect the IAC system for proper operation.

Check for a faulty electrical connections at the A/C compressor control module and the PCM.

An intermittent malfunction may be caused by a fault in the A/C idle-up electrical circuit. Inspect the wiring harness and components for any of the following conditions:

• | Any backed-out terminals

• | Any improper mating of terminals

• | Any broken electrical connector locks

• | Any improperly formed or damaged terminals

• | Any faulty terminal-to-wire connections

• | Any physical damage to the wiring harness

• | A broken wire inside the insulation

• | Any corrosion of the electrical connections, the splices, or the terminals

### Test Description

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

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. This step checks whether the PCM is receiving the A/C ON signal from the A/C compressor control module.

3. The A/C idle-up signal circuit at the PCM should indicate 0.0 volts with the A/C On.

4. The A/C idle-up signal circuit at the PCM should indicate B+ with the A/C Off.

5. The A/C cut-out signal circuit at the PCM should indicate 0.0 volts with the A/C Off.

6. The A/C cut-out signal circuit at the PCM should indicate B+ with the A/C On.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Install a scan tool.
2. Start the engine.
3. Run the engine until a normal operating temperature is attained.
4. Turn OFF all accessories.
5. Monitor the A/C parameter on the scan tool.
6. Turn the A/C switch ON and the blower speed selector switch to any position except OFF.

Does the scan tool indicate ON with the A/C operating?

--

System OK

Go to Step 3

3 |

1. Ensure the engine is running.
2. Turn ON the A/C.
3. Backprobe the A/C idle-up signal circuit at the PCM connector with a digital multimeter (DMM) connected to ground.
4. Measure the voltage.

Does the voltage equal the specified value?

0 V

Go to Step 4

Go to Step 6

4 |

1. Ensure the engine is running.
2. Turn OFF the A/C.
3. Backprobe the A/C idle-up signal circuit at the PCM connector with the DMM connected to ground.
4. Measure the voltage.

Does the voltage equal the specified value?

B+

Go to Step 7

Go to Step 5

5 |

1. Turn OFF the ignition.
2. Disconnect the PCM electrical connector C1.
3. Back out the A/C idle-up signal wire from the PCM electrical connector.
4. Reconnect the PCM electrical connector C1.
5. Turn ON the ignition, leaving the engine off.
6. Backprobe through the PCM electrical connector slot to the PCM terminal with a DMM connected to ground.

Does the voltage equal the specified value?

B+

Go to Step 6

Go to Step 12

6 |

1. Check for an open or a short in the A/C idle up signal circuit between the PCM and the A/C compressor control module.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 13

Go to Step 11

7 |

1. Start the engine.
2. Turn OFF the A/C.
3. Backprobe the A/C cut-out signal circuit at the PCM connector with the DMM connected to ground.
4. Measure the voltage.

Does the voltage equal the specified value?

0 V

Go to Step 8

Go to Step 12

8 |

1. Start the engine.
2. Turn ON the A/C.
3. Backprobe the A/C cut-out signal circuit at the PCM connector with the DMM connected to ground.
4. Measure the voltage.

Does the voltage equal the specified value?

B+

Go to Diagnostic Aids

Go to Step 9

9 |

1. Check for an open or short to ground in the A/C cut-out signal circuit between the PCM and the A/C compressor control module.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 13

Go to Step 10

10 |

1. Disconnect the electrical connector from the A/C compressor control module .
2. Turn ON the ignition, leaving the engine off.
3. Measure the voltage of the A/C cut-out signal circuit at terminal 8 of the A/C compressor control module with a DMM.

Does the voltage equal the specified value?

B+

Go to Step 12

Go to Step 11

11 |

1. The A/C compressor control module is malfunctioning. Inspect the A/C compressor control module and the circuitry for a fault.
2. Repair as necessary. Refer to Symptoms - HVAC Systems - Manual in HVAC.

Is the action complete?

--

Go to Step 13

--

12 | Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | Go to Step 13 | --
13 | Operate the vehicle within the conditions under which the original symptom was noticed. Does the system now operate properly? | -- | System OK | Go to Diagnostic Aids