GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2004 🢒 Epica 🢒 Engine 🢒 Engine Controls - 2.0L (L34 GMDAT) 🢒 Idle Air Control (IAC) System Diagnosis

Circuit Description

The engine control module (ECM) controls the engine idle speed by adjusting the position of the idle air control (IAC) valve pintle. The IAC valve is a stepper motor driven by 2 internal coils. The movement of the IAC valve is controlled electrically by 4 circuits. Drivers within the ECM control the polarity of the 2 windings inside the IAC valve through these circuits. The ECM, by commanding the correct polarity in sequence, is able to command the motor within the IAC valve to rotate clockwise or counter clockwise in steps. In order for the armature of the stepper motor of the IAC valve to turn one revolution, it needs to move approximately 24 steps. The IAC valve motor is linked by a gear drive to the IAC valve pintle. The electrical pulses sent to the IAC valve coils, by the ECM, allows the pintle to extend or retract into the passage in the throttle body. By retracting the pintle, air is allowed to bypass the throttle valve, which will increase air flow and raise engine speed. When the pintle is extended, bypass air is decreased which lowers engine speed. IAC valve movement is measured in counts on the scan tool. Each count is equivalent to a step of the IAC valve. When the IAC valve is fully extended, and seated in the throttle body bore, the scan tool displays zero and the engine speed is slow. As the IAC valve pintle retracts the counts will raise along with engine speed.

IAC Valve Reset

When the ignition switch is turned OFF for more than 10 seconds an IAC valve reset occurs. At that time, the ECM commands the IAC valve to extend for a certain period which allows the IAC pintle to seat in the throttle body bore. This position is then sensed as zero count position for the IAC valve by the ECM. It must be observed that the IAC valve position is only sensed by the ECM measuring counts or steps of the driver circuit, there is no direct sensing of its exact position. When this extend time period ends, the ECM then commands the IAC valve to retract a predetermined amount. This will allow for a high engine speed on the next ignition cycle. If for any reason the IAC valve pintle moves after this reset, before the next ignition cycle, the ECM will not be able to detect it and will affect the ability to control engine idle. When the IAC valve is removed or replaced for any reason the reset must be performed. Refer to Idle Learn .

Diagnostic Aids
    • A low, high, or unstable idle condition could be caused by a non-IAC system condition that cannot be overcome by the IAC valve. Refer to Symptoms - Engine Controls .
    • If the condition is intermittent, refer to Intermittent Conditions .

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connect End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important:  Ensure that the Engine Speed parameter stabilizes with each Commanded RPM change to determine if the engine speed stays within 100 RPM below the commanded RPM or within 200 RPM above the commanded RPM.

  1. Start the engine.
  2. Turn OFF all accessories.
  3. Slowly increment the engine speed to 1,800 RPM, then to 600 RPM, then to 1,800 RPM with a scan tool.
  4. Exit the RPM Control function.

Is the engine speed within 100 RPM below the commanded engine speed or within 200 RPM above the commanded engine speed?

Go to Diagnostic Aids

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Disconnect the idle air control (IAC) valve.
  3. Connect the J 37027-1A IAC Motor Driver to the IAC valve.
  4. Start the engine.
  5. Command the IAC valve in until 600 RPM is reached with the J 37027-1A .
  6. Command the IAC valve out until 1,800 RPM is reached with the J 37027-1A .
  7. Return the engine speed to the Desired Idle Speed parameter.

Did the Engine Speed parameter steadily decrease to 600 RPM and steadily increase to 1,700 RPM when the IAC valve was commanded in and out?

Go to Step 5

Go to Step 4

4

Did you observe an excessively high idle condition that could not be controlled with the IAC motor driver?

Go to Step 11

Go to Step 12

5

  1. Connect a test lamp between one of the IAC valve control circuits and a good ground.
  2. Start the engine.
  3. Observe the IAC Counts parameter with a scan tool.
  4. Command high RPM with the J 37027-1A until the IAC Counts parameter starts to increment.
  5. Command low RPM with the J 37027-1A until the IAC Counts parameter starts to increment.
  6. Return the Engine Speed to the Desired Idle Speed parameter.
  7. Repeat the above procedure for the other 3 IAC valve control circuits.

Did the test lamp remain ON, never flashing, while the IAC Counts were incrementing for any of the IAC valve control circuits?

Go to Step 9

Go to Step 6

6

Did the test lamp remain OFF and never flash while the IAC Counts were incrementing at any of the IAC valve control circuits during the above test?

Go to Step 8

Go to Step 7

7

  1. Connect a test lamp between the IAC coil A low control circuit and the IAC coil A high control circuit.
  2. Observe the IAC Counts parameter with a scan tool and the test lamp.
  3. Command high RPM with the J 37027-1A until the IAC Counts parameter starts to increment.
  4. Command low RPM with the J 37027-1A until the IAC Counts parameter starts to increment.
  5. Return the Engine Speed parameter to the Desired Idle Speed parameter.
  6. Repeat the above procedure with the test lamp connected between the IAC coil B low control circuit and the IAC coil B high control circuit.

Did the test lamp stay illuminated and never flash while the IAC Counts were incrementing during the above test?

Go to Diagnostic Aids

Go to Step 16

8

Test the IAC valve control circuit where the test lamp remained OFF for the following:

    • An open
    • A short to ground
    • A short to another IAC valve control circuit

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 12

9

Test the IAC valve control circuit where the test lamp remained illuminated for a short to voltage or a short to another IAC valve control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 14

10
  1. Remove the IAC valve. Refer to Idle Air Control Valve Replacement .
  2. Inspect for the following conditions:
    3. • The throttle body for damage and/or tampering
    • For vacuum leaks
    • For an incorrectly installed positive crankcase ventilation (PCV) valve and hose, if equipped
    • For a malfunctioning PCV valve
    • The throttle shaft for binding
    • The throttle linkage or cruise control linkage for binding
    • A skewed throttle position (TP) sensor signal

Did you find and correct the condition?

Go to Step 17

Go to Step 13

11
  1. Remove the IAC valve. Refer to Idle Air Control Valve Replacement .
  2. Inspect for the following conditions:
    3. • The throttle body for damage and/or tampering
    • For excessive deposits on the throttle plate
    • For excessive deposits in the throttle bore
    • For debris in the IAC passage
    • For excessive deposits on the IAC valve pintle
    • For a restricted air intake system

Did you find and correct the condition?

Go to Step 17

Go to Step 13

12
  1. Disconnect the engine control module (ECM).
  2. Test each IAC valve control circuit for a high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 14

13

Test for an intermittent and for a poor connection at the IAC valve. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 15

14

Test for an intermittent and for a poor connection at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 16

15

Important: If the IAC valve is removed or replaced, perform the Idle Learn .

Replace the IAC valve. Refer to Idle Air Control Valve Replacement .

Did you complete the replacement?

Go to Step 17

--

16

Important: If the IAC valve is removed or replaced, perform the Idle Learn .

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

Go to Step 17

--

17
  1. Reassemble the vehicle, as necessary.
  2. Clear any DTCs with a scan tool.
  3. Start the engine.
  4. Operate the system to verify the repair.

Did you correct the condition?

Go to Step 18

Go to Step 2

18

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK