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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2006 🢒 Vibe - FWD 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Electrical/Accessory Load Idle-Up Signal Diagnosis

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 electric rear window defogger
    • The exterior lighting system
    • The A/C system--When A/C is selected from the HVAC control unit the PCM receives a 12 volt signal. Once the PCM is aware of the request for A/C, the PCM determines whether engagement of the A/C compressor clutch is OK. In order to enable compressor clutch engagement the PCM provides a ground for the control circuit of the A/C compressor clutch relay. The PCM also increases engine idle speed in order to prevent 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 rear defogger or parking lamp (taillight) circuit may cause erratic electrical load idle-up circuit performance. Check for a properly operating rear defogger or taillight system.
    • A PSP switch that does not close, or loss of ground at the PSP switch may cause the engine to stall when the power steering loads are high.
    • The scan tool will display A/C Request Signal YES when A/C is selected and the HVAC blower switch is ON at any speed. The scan tool will display A/C Clutch ON whenever the PCM is allowing A/C compressor engagement. Therefore the scan tool will also display A/C Clutch ON when any defrost mode is selected.

An intermittent malfunction may be caused by a fault in any one of the idle-up signal 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. The Diagnostic System Check-Vehicle 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.

  2. This step verifies whether the PCM responds to the operation of the rear defogger. The IAC Motor Command Cycle percentage and the idle speed will increase when the rear defogger is turned ON if the idle-up system is OK.

    3.

  3. This step verifies whether the PCM responds to the operation of the taillights. The IAC Motor Command Cycle percentage and the idle speed will increase when the taillights are turned ON if the idle-up system is OK.

    4.

  4. This step verifies the operation of the A/C idle circuit. If the engine idle speed and IAC Motor Command cycle increase when the A/C is turned ON, the A/C idle circuit is functioning normally.

    5.

  5. This step verifies the operation of the power steering pressure (PSP) switch circuit. If the engine idle speed and IAC Motor Command increase when the steering wheel is turned to 90 percent, the PSP switch idle circuit is functioning normally.

    6.

  6. This step tests the PSP switch signal circuit for voltage from the PCM.

    7.

  7. This step tests the operation of the PSP switch.

    8.

  8. The A/C compressor clutch should not be engaged with the A/C switch Off. The PCM will not increase engine idle speed when the A/C compressor is supposed to be Off.

    9.

  9. If the A/C compressor clutch engages, the compressor clutch, the relay, and the control circuit are all OK. The PCM will then need to be replaced because there was no increase in the IAC motor command percentage during A/C operation in Step 4.

    10.

  10. This step tests for a short to ground in the Diagnostic Request circuit. A short to ground in the Diagnostic Request circuit can cause the PCM to ignore the idle-up signals to the PCM. A short in the Diagnostic Request circuit will cause the IAC Motor Command and the idle speed to remain fixed.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

Important: Check that the rear defogger and the parking lamps operate correctly before using this table.

  1. Install a scan tool.
  2. Start the engine.
  3. Run vehicle at idle with the accessories OFF, until normal operating temperature is reached.
  4. Monitor the IAC Motor Command parameter on the scan tool while turning the rear defogger switch ON and OFF.

Does the scan tool indicate that the IAC Motor Command increased the specified amount when the rear defogger switch was turned ON?

1-3%

Go to Step 3

Go to Step 18

3

Monitor the IAC Motor Command parameter on the scan tool while turning the parking lamp switch ON and OFF.

Does the scan tool indicate that the IAC Motor Command increased the specified amount when the parking lamp switch was turned ON?

1-3%

Go to Step 4

Go to Step 20

4

  1. Monitor the IAC Motor 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 IAC Motor Command increase within the specified range when the A/C is turned ON?

4-14%

Go to Step 5

Go to Step 10

5

  1. Monitor the IAC Motor Command parameter on the scan tool.
  2. Turn the steering wheel to the right and to the left.
  3. Return the steering wheel to the center, straight ahead position.

Did the scan tool indicate that the IAC Motor Command increased the specified amount when the steering wheel was turned all the way in either direction?

2-10%

System OK

Go to Step 6

6

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

Is 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 on the switch side with the test lamp.
  3. Start the engine.
  4. Observe the test lamp while turning the steering wheel to the right and to the left.

Does the test lamp illuminate when the steering wheel is turned to the right and to the left?

--

Go to Step 22

Go to Step 9

8
  1. Check for an open or a short to ground in the PSP switch signal circuit between the PCM and the PSP switch.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 25

Go to Step 24

9
  1. Check for a faulty ground connection at the power steering pump.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 25

Go to Step 23

10

  1. Start the engine.
  2. Turn ON the HVAC blower switch to any speed.
  3. Turn the A/C switch OFF.

Is the A/C compressor clutch engaged?

--

Go to Step 14

Go to Step 11

11
  1. Continue to run the engine with HVAC blower ON and the A/C OFF.
  2. Backprobe the A/C Clutch Relay Control circuit at the PCM connector with a DMM connected to ground.
  3. Measure the voltage.

Does the voltage equal the specified value?

B+

Go to Step 12

Go to Step 17

12
  1. Turn ON the A/C.
  2. Backprobe the A/C Clutch Relay Control circuit at the PCM connector with a DMM connected to ground.
  3. Measure the voltage.

Did the voltage drop to within the specified value when the A/C was turned ON?

0.0-1.0 V

Go to Step 13

Go to Step 24

13

Did the A/C compressor engage when the A/C was turned ON?

--

Go to Step 24

Go to Step 16

14
  1. Test for a short to ground in the A/C Clutch Relay Control circuit between the PCM and the A/C compressor clutch relay.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

B+

Go to Step 25

Go to Step 15

15
  1. Turn OFF the ignition.
  2. Disconnect the PCM C3 connector.
  3. Start the engine.
  4. Turn ON the HVAC blower switch to any speed.

Is the A/C compressor engaged?

--

Go to Compressor Relay Replacementin Heating, Ventilation, and Air Conditioning

Go to Step 24 PCM

16
  1. There is a fault in an HVAC electrical circuit or component.
  2. Inspect and repair as necessary. Refer to Air Conditioning (A/C) System Performance Test in Heating, Ventilation, and Air Conditioning.

Did you complete the repair?

--

Go to Step 25

--

17
  1. There is no ignition positive voltage at the A/C Clutch Relay Control circuit. Inspect for any of the following conditions:
    2. • Test for an open in the A/C Clutch Relay Control circuit between the PCM and the A/C compressor clutch relay.
    • Test for a faulty A/C compressor clutch relay or repair the cause of no ignition positive voltage to the relay. Refer to Air Conditioning (A/C) System Performance Test in Heating, Ventilation, and Air Conditioning.
  2. Repair as necessary.

Did you complete the repair?

--

Go to Step 25

--

18
  1. Turn ON the ignition, with the engine OFF.
  2. Backprobe the Electrical Load Idle-Up Signal Defogger circuit at the PCM connector with a test lamp connected to ground.
  3. Turn ON the rear defogger switch.

Does the test lamp illuminate?

--

Go to Step 22

Go to Step 19

19
  1. Turn OFF the ignition.
  2. Test the Electrical Load Idle-Up Signal Defogger circuit for an open or a short.
  3. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 25

Go to Diagnostic Starting Point - Stationary Windows in Stationary Windows

20
  1. Turn ON the ignition, with the engine OFF.
  2. Backprobe the Electrical Load Idle-Up Signal Taillights circuit at the PCM connector with a test lamp connected to ground.
  3. Turn ON the parking lamp switch.

Does the test lamp illuminate?

--

Go to Step 22

Go to Step 21

21
  1. Turn OFF the ignition.
  2. Test the Electrical Load Idle-Up Signal Taillights circuit for an open or a short.
  3. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 25

Go to Diagnostic Starting Point - Lighting Systems in Lighting Systems

22

  1. Test for a short to ground in the Diagnostic Request circuit from the PCM and the data link connector (DLC).
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 25

Go to Step 24

23

Replace the PSP switch. Refer to Power Steering Pressure Switch Replacement in Power Steering Systems.

Did you complete the replacement?

--

Go to Step 25

--

24

Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 25

--

25

Operate the vehicle within the conditions that the concern was first noticed.

Does the system now operate properly?

--

System OK

Go to Diagnostic Aids