GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Prizm 🢒 Engine 🢒 Engine Controls - 1.8L 🢒 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 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 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 the air conditioning control is turned ON and the blower switch is also ON. The scan tool will display A/C Relay Command On only when the PCM is allowing A/C compressor engagement.

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-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.

  2. This step checks 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 checks 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 checks 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 checks 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 checks the PSP switch signal circuit for voltage from the PCM.

    7.

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

    8.

  8. The A/C On Signal circuit at the PCM should indicate 0.0 volts with the A/C on.

    9.

  9. The A/C On Signal circuit at the PCM should indicate B+ with the A/C off.

    10.

  10. The A/C Cutout Control circuit at the PCM should indicate 0.0 volts with the A/C off.

    11.

  11. The A/C Cutout Control circuit at the PCM should indicate B+ with the A/C on.

    12.

  12. This step checks 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

Refer to Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

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 19

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 21

4

  1. Install a scan tool.
  2. Start the engine.
  3. Run the engine until a normal operating temperature is reached.
  4. Turn OFF all accessories.
  5. Monitor the IAC Motor Command 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 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. Install a scan tool.
  2. Start the engine.
  3. Run the engine until a normal operating temperature is reached.
  4. Turn OFF all accessories.
  5. Monitor the IAC Motor Command parameter on the scan tool.
  6. Turn the steering wheel to the right and to the left.
  7. 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 digital multimeter (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 23

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.

Was a repair necessary?

--

Go to Step 26

Go to Step 25

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 26

Go to Step 24

10

  1. Start the engine.
  2. Turn ON the A/C.
  3. Backprobe the A/C On Signal circuit at the PCM connector with a digital multimeter (DMM) connected to ground.
  4. Measure the voltage.

Is the voltage within the specified range?

0.0-1.0 V

Go to Step 11

Go to Step 13

11

  1. Start the engine.
  2. Turn OFF the A/C.
  3. Backprobe the A/C On Signal circuit at the PCM connector with a DMM connected to ground.
  4. Measure the voltage.

Does the voltage equal the specified value?

B+

Go to Step 14

Go to Step 12

12
  1. Turn OFF the ignition.
  2. Disconnect the PCM electrical connector.
  3. Back-out the A/C On Signal wire from the PCM electrical connector.
  4. Connect the PCM electrical connector.
  5. Turn ON the ignition, with the engine OFF.
  6. Backprobe the A/C On Signal circuit through the empty slot in the PCM electrical connector with a DMM connected to ground.

Does the voltage equal the specified value?

B+

Go to Step 13

Go to Step 25

13
  1. Check for an open or a short in the A/C On 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 26

Go to Step 18

14

  1. Start the engine.
  2. Turn OFF the A/C.
  3. Backprobe the A/C Cutout Control 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 15

Go to Step 25

15

  1. Start the engine.
  2. Turn ON the A/C.
  3. Backprobe the A/C Cutout Control 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 16

16
  1. Check for an open or a short to ground in the A/C Cutout Control 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 26

Go to Step 17

17
  1. Disconnect the electrical connector for the A/C compressor control module .
  2. Turn ON the ignition, with the engine OFF.
  3. Measure the voltage of the A/C Cutout Control circuit at terminal 3 of the A/C compressor control module with a DMM.

Does the voltage equal the specified value?

B+

Go to Step 23

Go to Step 18

18
  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 Diagnostic Starting Point - HVAC Systems - Manual in HVAC.

Is the action complete?

--

Go to Step 26

--

19
  1. Turn ON the ignition, with the engine OFF.
  2. Access the PCM electrical connector C3.
  3. Backprobe the Electrical Load Idle-up signal of the defogger circuit at the PCM connector with a test lamp connected to ground.
  4. Turn ON the rear defogger switch.

Does the test lamp illuminate?

--

Go to Step 23

Go to Step 20

20
  1. Turn OFF the ignition.
  2. Disconnect the PCM electrical connector C3.
  3. Check the Electrical Load Idle-up Defogger circuit for an open or a short between junction block 2 and the PCM.
  4. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 26

Go to Diagnostic Starting Point - Stationary Windows in Stationary Windows

21
  1. Turn ON the ignition, with the engine OFF.
  2. Access the PCM electrical connector C3.
  3. Backprobe the Electrical Load Idle-up signal of the taillight circuit at the PCM connector with a test lamp connected to ground.
  4. Turn ON the parking lamp switch.

Does the test lamp illuminate?

--

Go to Step 23

Go to Step 22

22
  1. Turn OFF the ignition.
  2. Disconnect the PCM electrical connector C3.
  3. Check the Electrical Load Idle-up Taillights circuit for an open or a short between junction block 2 and the PCM.
  4. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 26

Go to Diagnostic Starting Point - Lighting Systems in Lighting Systems

23

  1. Check for a short to ground in the Diagnostic Request circuit from the PCM connector C3 to the TE1 terminal of the underhood diagnostic link connector.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 26

Go to Step 25

24

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

Is the action complete?

--

Go to Step 26

--

25

Replace the PCM. Refer to Powertrain Control Module Replacement .

Is the action complete?

--

Go to Step 26

--

26

Perform the scan tool Clear DTC Information function and road test the vehicle within the conditions that set the DTC.

Does the system now operate properly?

--

System OK

Go to Diagnostic Aids