For 1990-2009 cars
Engine Controls Components
Cell 20: 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 (engaged). The PCM sends a signal to the idle speed control (ISC) motor in order to command the ISC motor plunger to open the throttle plate. The engine idle speed increases when the throttle plate opens. The increase in the idle speed prevents a rough idle or stalling condition.

### Diagnostic Aids

Check for a faulty electrical connection to the PCM.

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

• | Backed out terminals.

• | Improper mating of terminals.

• | Broken electrical connector locks.

• | Improperly formed or damaged terminals.

• | Faulty terminal to wire connections.

• | Physical damage to the wiring harness.

• | A broken wire inside the insulation.

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

If no problem was 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 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 for an increase in the idle speed with the A/C ON.

3. The A/C idle-up signal circuit at the PCM should indicate 0.0 volts with the A/C ON (operating).

4. The A/C idle-up signal circuit at the PCM should indicate B+ with the A/C OFF (not operating).

5. The A/C cut-out signal circuit at the PCM should indicate 0.0 volts with the A/C OFF (not operating).

6. The A/C cut-out signal circuit at the PCM should indicate B+ with the A/C ON (operating).

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic 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 normal operating temperature is reached.
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?

--

Go to Diagnostic Aids

Go to Step 3

3 |

1. Start the engine.
2. Turn ON the A/C (A/C operating).
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 volts

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 connected to ground.

Is the voltage at the specified value?

B+

Go to Step 6

Go to Step 12

6 | 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. 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 to ground.
4. Measure the voltage.

Is the voltage at the specified value?

0 volts

Go to Step 8

Go to Step 12

8 |

1. Start the engine.
2. Turn ON the A/C (A/C operating).
3. Backprobe the A/C cut-out signal circuit, terminal C3-17, at the PCM connector with the DMM connected to ground.
4. Measure the voltage.

Is the voltage at the specified value?

B+

Go to Diagnostic Aids

Go to Step 9

9 | 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. 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 A/C compressor control module electrical connector.
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 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. Refer to Powertrain Control Module Replacement . 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 now operate properly? | -- | System OK | Go to Diagnostic Aids