For 1990-2009 cars

### Circuit Description

The powertrain control module (PCM) receives an idle-up signal when certain engine electrical loads or engine accessory loads are present. The PCM increases the flow of air through the idle (bypass) air passage of the idle air control (IAC) valve when receiving an idle-up signal. The additional air flowing through the idle air passage causes an increase in the idle speed. The idle speed is increased in order to accommodate the increased engine electrical or engine accessory load. The PCM monitors the following systems for idle-up signals:

• | The exterior lighting system

• | The HVAC system blower motor

• | The electric rear window defogger

• | The A/C system--When A/C is selected from the HVAC control unit the A/C compressor control module sends an A/C On signal to the PCM. Once the PCM is aware of the request for A/C, the PCM will determine whether engagement of the A/C compressor clutch is OK. The A/C compressor control module requires a signal from the PCM in order to allow operation of the A/C compressor. The A/C compressor control module will not turn on the A/C compressor clutch relay unless the PCM pulls the A/C Cutout Control circuit low. The increase in the idle speed prevents a rough idle or a stalling condition when the A/C compressor is engaged.

• | The power steering system--The power steering pressure (PSP) switch is normally open to ground. Turning the steering wheel increases the power steering oil pressure. Increased power steering oil pressure puts an additional load on the engine that may be noticeable at idle. The PSP switch closes when the power steering system oil pressure is high enough to cause an undesirable idle condition. The voltage signal from the PSP switch to the PCM indicates less than one volt when the PSP switch is closed. The voltage signal is near battery voltage when the PSP switch is open.

### Diagnostic Aids

Check for any of the following conditions:

• | The High Electrical Load parameter on the scan tool can indicate if the PCM has received the rear defogger or parking lamps ON signal. The scan tool should indicate Yes when the rear defogger or parking lamps are turned ON and engine speed should increase slightly. Check for a malfunction in the IAC system or a short to ground in the Diagnostic Request circuit if the engine speed does not increase.

• | A malfunctioning parking lamp (taillight) circuit may cause erratic performance of the electrical load idle-up circuit.

• | A PSP switch that is shorted can cause the engine idle speed to remain high even when driving straight ahead.

• | A PSP switch that is incorrectly calibrated can cause the engine idle speed to remain low when steering pump hydraulic pressure is high.

• | A faulty ground in the PSP switch can cause erratic switch operation. Inspect that the PS pump and bracket are secure, insuring a good path to ground. Inspect the PSP switch for a tight installation.

• | The scan tool will display A/C Relay Command On when the air conditioning control is turned ON, the blower switch is also ON, and the A/C compressor is operating.

• | Observe the IAC Motor Command on a scan tool while operating the A/C. The IAC Motor Command 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, but the idle quality is unsatisfactory, inspect the IAC system for proper operation.

• | If the scan tool indicates that the IAC Motor Command is increasing, but the idle quality is unsatisfactory, inspect the IAC system for proper operation. Refer to Idle Air Control (IAC) System Diagnosis .

An intermittent malfunction may be caused by a fault in any one of the idle-up circuits. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

### Test Description

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

1. A Diagnostic System Check-Engine Controls 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 responding to the electrical load idle-up inputs by commanding the IAC valve to increase the IAC Motor Command percentage. The IAC Motor Command parameter on the scan tool should increase 2-3 percent when operating the accessory. If the scan tool does not indicate an increase in the IAC Motor Command when the accessory is turned ON, the PCM may not be receiving the idle-up signal.

3. This step checks whether the PCM is responding to idle-up input signals from the A/C compressor control module. The PCM commands the IAC valve to increase the IAC Motor Command percentage when receiving the idle-up signal. The IAC Motor Command parameter on the scan tool should increase 4-11 percent when operating the A/C. If the scan tool does not indicate an increase in the IAC Motor Command when the A/C is turned ON, the PCM may not be receiving the idle-up signal.

4. This step checks whether the PCM is responding to the electrical load idle-up input of the PSP switch. The PCM commands the IAC valve to increase the IAC Motor Command percentage when receiving the PSP idle-up signal. The IAC Motor Command parameter on the scan tool should increase 2-3 percent when increasing the power steering system pressure. If the scan tool does not indicate an increase in the IAC Motor Command when the accessory is turned ON, the PCM may not be receiving the idle-up signal.

5. This step checks for voltage from the PCM or for an open or a short to ground in the PSP switch signal circuit.

6. This step checks for an inadequate ground connection at the power steering pump or a faulty PSP switch.

7. This step checks whether the PCM is receiving the A/C ON signal from the A/C compressor control module. If the scan tool indicates ON, but the A/C compressor is not operating, there may be a short to ground in the A/C Cut-Out Control circuit. Following the No diagnostic path will locate the problem.

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

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

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

11. 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
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform A Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 | Important: If the headlights, the HVAC blower motor, or the rear defogger do not operate properly, diagnose those components first.

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 Engine Speed and the IAC Motor Command parameters on the scan tool while turning each of the following electrical accessories ON and OFF:

• | Headlights

• | HVAC blower motor

• | Rear window defogger

Did the scan tool indicate an increase in the IAC Motor Command when each accessory was turned ON?

--

Go to Step 3

Go to Step 20

3 |

1. Turn OFF all accessories.
2. Turn the A/C switch ON and the blower speed selector switch to any position except OFF.
3. Monitor the Engine Speed and the IAC Motor Command parameters on the scan tool while operating the A/C system.

Did the scan tool indicate an increase in the IAC Motor Command when the A/C compressor was engaged?

--

Go to Step 4

Go to Step 10

4 |

1. Turn OFF all accessories.
2. Monitor the Engine Speed and the IAC Motor Command parameters on the scan tool while turning the steering wheel all the way to the right and all the way to the left.

Did the scan tool indicate an increase in the IAC Motor Command when turning the steering wheel from the right to the left?

--

Go to Symptoms - Engine Controls

Go to Step 5

5 |

1. Turn OFF all accessories.
2. Monitor the power steering switch on the scan tool.
3. Turn the steering wheel all the way to the right and all the way to the left.

Did the scan tool indicate High Pressure when the steering wheel was turned all the way in either direction?

--

Go to Idle Air Control (IAC) System Diagnosis

Go to Step 6

6 |

1. Turn ON the ignition, leaving the engine OFF.
2. Disconnect the PSP switch electrical connector.
3. Measure the voltage from the PSP switch connector to ground, harness side, with a DMM.

Was the voltage near the specified value?

B+

Go to Step 7

Go to Step 8

7 |

1. Connect a test lamp to B+.
2. Probe the PSP switch connector, switch side, with the test lamp.
3. Start the engine.
4. Observe the test lamp while turning the steering wheel all the way to the right and all the way to the left.

Was the test lamp ON when the steering wheel was turned all the way to the right or left?

--

Go to Diagnostic Aids

Go to Step 9

8 |

1. Check for an open or short to ground in the PSP signal circuit between the PCM and the PSP switch.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 30

Go to Step 29

9 |

1. Check for a faulty ground connection at the power steering pump.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 30

Go to Step 28

10 |

1. Monitor the A/C Relay Command parameter on the scan tool.
2. Turn the A/C switch ON and the blower speed selector switch to any position except OFF.

Does the scan tool indicate ON when the A/C compressor is operating?

--

Go to Idle Air Control (IAC) System Diagnosis

Go to Step 11

11 |

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 on the idle-up signal circuit when the A/C compressor is operating.

Is the voltage at the specified value?

0 V

Go to Step 12

Go to Step 14

12 |

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 within the specified value?

4.5-5.0 V

Go to Step 15

Go to Step 13

13 |

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.

Is the voltage within the specified value?

4.5-5.0 V

Go to Step 14

Go to Step 29

14 |

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 30

Go to Step 19

15 |

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 16

Go to Step 29

16 |

1. Start the engine.
2. Turn ON the A/C (A/C operating).
3. Backprobe the A/C cut-out signal circuit 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 17

17 |

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 30

Go to Step 18

18 |

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 29

Go to Step 19

19 |

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 Symptoms - HVAC Systems - Manual in HVAC Systems with A/C-Manual.

Did you complete the repair ?

--

Go to Step 30

--

20 | Did the scan tool indicate an increase in the IAC Motor Command when the HVAC blower motor was turned ON? | -- | Go to Step 22 | Go to Step 21
21 |

1. Turn OFF the ignition.
2. Disconnect the PCM electrical connectors.
3. Turn ON the ignition, leaving the engine OFF.
4. Probe the harness side of the HVAC Blower Idle-Up Signal circuit, with a test lamp connected to B+.
5. Observe the test lamp while operating the HVAC blower motor switch.

Did the test lamp illuminate when the HVAC blower motor switch was turned ON?

--

Go to Step 29

Go to Step 25

22 | Did the scan tool indicate an increase in the IAC Motor Command when the rear window defogger switch was turned ON? | -- | Go to Step 23 | Go to Step 24
23 |

1. Backprobe the electrical load idle-up circuit of the exterior lighting system at the PCM connector, with a test lamp connected to ground.
2. Observe the test lamp while operating the headlight switch.

Did the test lamp illuminate when the headlight switch was turned ON?

--

Go to Step 29

Go to Step 26

24 |

1. Backprobe the electrical load idle-up circuit of the rear defogger switch at the PCM connector, with a test lamp connected to ground.
2. Observe the test lamp while operating the rear defogger switch.

Did the test lamp illuminate when the rear defogger switch was turned ON?

--

Go to Step 29

Go to Step 27

25 |

1. Inspect for the correct operation of the HVAC blower circuit. Refer to Symptoms - HVAC Systems - Manual and HVAC Schematics in HVAC.
2. Inspect the HVAC blower idle-up signal circuit for an open or a short.
3. Repair any faults found as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 30

Go to Step 29

26 |

1. Inspect for the correct operation exterior lighting circuit. Refer to Diagnostic Starting Point - Lighting Systems and Exterior Lights Schematics in Lighting Systems.
2. Inspect the exterior lighting idle-up signal circuit for an open or a short.
3. Repair any faults found as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 30

Go to Step 29

27 |

1. Inspect for the correct operation rear defogger circuit. Refer to Rear Window Defogger Description and Operation in Body and Accessories and Defogger Schematics in Stationary Windows.
2. Inspect the rear defogger idle-up signal circuit for an open or a short.
3. Repair any faults found as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 30

Go to Step 29

28 | Replace the PSP switch. Refer to Power Steering Pressure Switch Replacement . Did you complete the replacement? | -- | Go to Step 30 | --
29 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 30 | --
30 | 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