GM Service Manual Online
For 1990-2009 cars only

Object Number: 394060  Size: SF
TP/IAC/PCV - Comp Loc Figure
Cell 21: VSS and Cruise Controls
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The control module controls idle speed with an idle air control (IAC) valve. The desired RPM is based on sensor inputs and the actual engine RPM. Four circuits are used in order to control the IAC valve, allowing a precise amount of air into the intake manifold. In order to increase the idle RPM, the control module will command the valve pintle to move away from its seat, allowing more air to bypass the throttle plate. To decrease RPM, the control module commands the IAC pintle to move closer to its seat, causing a lower airflow to bypass the throttle plate. A scan tool will display the control module commands to the IAC valve in counts. The higher the counts, the more air is allowed to bypass the throttle plate. Lower counts indicate that less air is commanded by the control module.

This DTC is designed to determine if a high engine RPM is not responding to a decrease in IAC counts.

A high idle is 100 RPM more than the desired idle.

Conditions For Running DTC

    • No active TP sensor DTCs
    • No active VS sensor DTCs
    • No active ECT DTCs
    • No active MAP sensor DTCs
    • The ECT is more than 50°C (122°F)
    • System voltage is between 10-16 volts
    • The IAT is more than -25°C (-13°F)
    • The engine has been running for more than 30 seconds
    • The BARO is more than 70 kPa
    • The MAF is between 17.5-50 g/s
    • Vehicle speed is between 25-85 mph
    • Any change in TP is less than 1%
    • Any change in engine speed is less than 50 RPM
    • The IAC valve was commanded to move a specified number of counts
    • The above conditions present for more than 2 seconds

Conditions for Setting the DTC

The change in MAF is more than 3 g/s.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) if a failure is detected during 2 consecutive key cycles.
    • The control module sets the DTC and records the operating conditions at the time the diagnostic failed. The failure information is stored in the scan tool Freeze Frame/Failure Records.

Conditions for Clearing the MIL or DTC

    • The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed.
    • A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
    • Use a scan tool in order to clear the DTCs.

Diagnostic Aids

This DTC may be caused by an IAC valve which has stopped and is not responding to the VCM, a throttle stop screw that has been tampered with, or a damaged throttle or throttle linkage. A slow, unstable, or fast idle may be caused by a non - IAC valve system problem that cannot be overcome by the IAC valve. Out of range control IAC scan tool counts will be above 60 if idle is too low and zero if idle is too high. Check for the following conditions:

    • Inspect engine mounts - damaged mounts may cause tension on throttle linkage.
    • Fuel system too lean or too rich.
    • Throttle body - inspect bore for foreign material.
    • Refer to Rough, Unstable, or Incorrect Idle and Stalling .
    • Gasket leak.
    • IAC reset is done with the scan tool.

An intermittent may be caused by any of the following conditions:

    • A poor connection
    • Rubbed through wire insulation
    • A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description

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

  1. The IAC Reset function used in this step only energizes the circuit for 2 seconds, so this function will have to be used repeatedly during this step. Also it is recommended that the J 35616 connector test adapter kit is used during this procedure to prevent terminal damage to the IAC harness connector. This step can also be performed with the ignition ON and engine OFF.

  2. If the test lamp remained ON at any of the IAC valve harness connector terminals during the IAC valve reset function the circuit is shorted to voltage. If these circuits are checked before or after the IAC valve reset function it is possible that any combination of IAC circuits could light a test lamp, this would be a normal condition.

Step

Action

Value(s)

Yes

No

1

Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame and Failure Records for reference. The control module's data is deleted once the Clear Info function is used.

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Set the parking brake and block the drive wheels.
  2. Install the scan tool.
  3. Start the engine.
  4. Turn OFF the accessories.
  5. Command the engine speed to the first specified value and then to the second specified value using the scan tool.

Does the engine speed correspond within 100 RPM of the commanded speeds?

1200 RPM

500 RPM

Go to Diagnostic Aids

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the IAC valve harness connector.
  3. Measure the resistance across the IAC valve terminals A and B.
  4. Measure the resistance across the IAC valve terminals C and D.

Are the resistances across terminals A and B, and terminals C and D within the specified value?

40-80 ohms

Go to Step 4

Go to Step 13

4

  1. Measure the resistance across the IAC valve terminals B and C.
  2. Measure the resistance across the IAC valve terminals A and D.

Are the resistances across terminals B and C, and terminals A and D at the specified value?

Go to Step 5

Go to Step 13

5

    Caution: Avoid contact with moving parts and hot surfaces while working around a running engine in order to prevent physical injury.

  1. With the IAC valve still disconnected, start the engine.
  2. Probe each of the IAC valve harness connector terminals using a J 35616 connector test adapter kit and a test lamp connected to a ground.
  3. Important:: Command ON the Reset IAC function for each circuit tested. This function will only flash each circuit for 2 seconds.

  4. Use the IAC Reset function while the test lamp is connected to each IAC connector terminal. The test lamp should flash on each terminal.

Does the test lamp flash ON and OFF at all terminals?

--

Go to Step 9

Go to Step 6

6

Did the test lamp remain ON at any of the terminals during the IAC valve reset function?

--

Go to Step 8

Go to Step 7

7

Test for an open or short to ground in the IAC valve circuit that did not flash the test lamp.

Did you find a problem?

--

Go to Step 11

Go to Step 10

8

Test for a short to voltage in the IAC valve circuit where the test lamp remained ON.

Did you find a problem?

--

Go to Step 11

Go to Step 10

9

  1. Turn OFF the ignition.
  2. Remove the IAC valve. Refer to Idle Air Control Valve Replacement .
  3. Inspect the IAC valve for deposits.
  4. Inspect the throttle body passages for carbon buildup.

Did you find a problem?

--

Go to Step 12

Go to Diagnostic Aids

10

Test for a poor connection at the VCM.

Did you find a problem?

--

Go to Step 11

Go to Step 14

11

Repair the circuit as necessary. Refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 15

--

12

Clean the IAC valve and passages. Refer to the Cleaning and Inspection Procedure in the Idle Air Control Valve Replacement .

Is the action complete?

--

Go to Step 15

--

13

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

Is the action complete?

--

Go to Step 15

--

14

  1. Replace the VCM.
  2. Program the new VCM. Refer to VCM Replacement/Programming .
  3. Perform the Passlock Reprogramming Procedure. Refer to PASSLOCK Reprogramming Seed and Key in Theft Deterrent.
  4. Perform the CKP System Variation Learn procedure. Refer to Crankshaft Position System Variation Learn .

Is the action complete?

--

Go to Step 15

--

15

  1. Using the scan tool, clear the DTCs.
  2. Start the engine.
  3. Allow the engine to idle until the engine reaches normal operating temperature.
  4. Select DTC and the Specific DTC function.
  5. Enter the DTC number which was set.
  6. Operate the vehicle, with the Conditions for Setting this DTC, until the scan tool indicates the diagnostic Ran.

Does the scan tool indicate the diagnostic Passed?

--

Go to Step 16

Go to Step 2

16

Does the scan tool display any additional undiagnosed DTCs?

--

Go to the applicable DTC table

System OK