For 1990-2009 cars
Engine Controls Components
Cell 21: Idle-Up Signals
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

When the A/C compressor clutch is ON, the A/C compressor control module sends an A/C idle-up signal to the powertrain control module (PCM). 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 airflow travels through the idle air passage. The increase in the idle speed prevents a rough idle or a stalling condition.

### Diagnostic Aids

Check for a faulty electrical connection to the PCM.

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

• | Backed out terminals

• | Incorrect mating of terminals

• | Broken electrical connector locks

• | Incorrectly formed terminals or damaged terminals

• | Faulty terminal-to-wire connections

• | Physical damage to the wiring harness

• | A broken wire inside the insulation

• | Corrosion of electrical connections, of splices, or of terminals

If a condition was not found in the A/C idle-up circuit check, refer to Symptoms .

### 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 to store the applicable freeze frame data on the scan tool. This creates an electronic copy of the data that was taken when the fault occurred. The scan tool stores the information for later reference.

2. This step checks for an increase in the idle speed when you turn ON the A/C.

3. The A/C idle-up signal circuit at the PCM should indicate 0.0 volts when you turn ON the A/C.

4. The A/C idle-up signal circuit at the PCM should indicate B+ when you turn OFF the A/C.

5. The A/C cut-out signal circuit at the PCM should indicate 0.0 volts when you turn OFF the A/C .

6. The A/C cut-out signal circuit at the PCM should indicate B+ when you turn ON the A/C.

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 it reaches the normal operating temperature.
4. Turn OFF all accessories.
5. Monitor the A/C parameter on the scan tool.
6. Turn ON the A/C switch, and turn the blower speed selector switch to any position except OFF.

Does the scan tool indicate ON when you are operating the A/C?

--

Go to Diagnostic Aids

Go to Step 3

3 |

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

Is the voltage at the specified value?

0 V

Go to Step 4

Go to Step 6

4 |

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

Is the voltage at the specified value?

B+

Go to Step 7

Go to Step 5

5 |

1. Turn OFF the ignition.
2. Disconnect the PCM electrical connector C3.
3. Back out the A/C idle-up signal wire from the PCM electrical connector terminal C3-19.
4. Reconnect the PCM electrical connector C3.
5. Turn ON the ignition, leaving the engine OFF.
6. Backprobe through the PCM electrical connector slot C3-19 to the PCM terminal with a DMM that is connected to ground.

Is the voltage at the specified value?

B+

Go to Step 6

Go to Step 12

6 |

1. Check for an open or for a short in the A/C idle-up signal circuit, between the PCM and the A/C compressor control module.
2. Repair the circuit 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 to ground.
4. Measure the voltage.

Is the voltage at 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 terminal C3-17 at the PCM connector with the DMM that is connected to ground.
4. Measure the voltage.

Is the voltage at the specified value?

B+

Go to Diagnostic Aids

Go to Step 9

9 |

1. Check for an open or for a short to ground in the A/C cut-out signal circuit, between the PCM and the A/C compressor control module.
2. Repair the circuit 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 to 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 5 of the A/C compressor control module with a DMM.

Is the voltage at 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 HVAC System Check in HVAC.

Is the action complete?

--

Go to Step 13

--

12 | Replace the PCM. Is the action complete? | -- | Go to Step 13 | --
13 | Operate the vehicle within the conditions under which the original symptom was specified. Does the system operate correctly? | -- | System OK | Go to Diagnostic Aids