Circuit Description
The IAC system controls the idle speed by allowing a controlled amount of air to bypass the throttle blade via a passage in the IAC valve. The IAC valve consists of a rotating shutter that is held in a neutral position by opposing springs within the valve assembly (equal to a slightly elevated idle). Switched B+ is provided to the drive unit within the IAC valve. The ECM controls the valve via two control circuits, one to drive the valve open and the other to drive the valve closed. The ECM Pulse Width Modulates both control circuits simultaneously. The ratio of the frequency between the two PWM signals determines the direction and amount that the drive unit rotates the shutter within the valve. As the shutter closes, the bypass air flow is reduced and the idle speed decreases. In order to increase the idle speed, the ECM commands the shutter open, allowing more air to bypass the throttle plates. If the ECM senses an open circuit, a short to ground, or a short to voltage on either of the IAC valve control circuits, this DTC will set.
The ECM will monitor the IAC valve control circuit for the following:
| • | A short to ground |
| • | A short to voltage |
| • | An open circuit |
| • | An internally shorted or excessively low resistance IAC valve control circuit |
When the ECM detects any of the above malfunctions, this DTC will set and the affected driver will be disabled. The ECM will continue to test the circuit and if the fault goes away, the driver will be reactivated.
Conditions for Running the DTC
| • | The ignition voltage is between 9.0 and 17.0 volts. |
| • | The engine speed is greater than 80 RPM. |
Conditions for Setting the DTC
The ECM senses a short to ground, open, or short to voltage on either IAC valve control circuit.
Action Taken When the DTC Sets
| • | The ECM illuminates the Malfunction Indicator Lamp (MIL) and records the operating conditions in the Freeze Frame on the second consecutive drive trip that the diagnostic runs and fails. |
| • | The Closed loop control will be disabled. |
| • | The Long term fuel trim values will be reset to 0. |
Conditions for Clearing the MIL/DTC
| • | The ECM turns OFF the MIL after three consecutive drive trips that the diagnostic runs and passes. |
| • | A History DTC clears after forty consecutive warm-up cycles in which no failures are reported by this diagnostic or any other emission related diagnostic. |
| • | The ECM battery voltage is interrupted. |
| • | The scan tool clears the MIL/DTC. |
Diagnostic Aids
Use the Connector Test Adapter Kit J 35616 for any test that requires probing the ECM harness connector or a component harness connector. Using this kit will prevent damage to the harness connector terminals.
Check for the following conditions:
| • | Poor connections at the ECM or at the component. Inspect the harness connectors for any backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. Refer to Engine Control Module Diagnosis for the proper procedure. |
| • | Damaged harness. Inspect the wiring harness for any damage. If the harness appears to be OK, observe the scan tool while moving the related connectors and the wiring harnesses. A change in the display may help in order to locate the fault. |
Refer to Symptoms for the Intermittents.
Test Description
The number(s) below refer to the step number(s) on the Diagnostic Table.
-
Verifies whether or not the fault is currently present. As long as the IAC output control is selected on the scan tool, the engine RPM should cycle continuously up and down, from approximately 500 RPM to 1500 RPM.
-
The pulse width (percentage of duty cycle) is varied in order to control the IAC operation. If any duty cycle is measured, the circuit is probably operational. If the engine will not start, take the measurement while cranking.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System check? | -- | Go to Powertrain Powertrain On Board Diagnostic (OBD) System Check | |
The engine speed should alternately raise and lower as long as the IAC is being controlled. Is the engine speed controllable with the scan tool? | -- | Fault not present. Refer to Diagnostic Aids
| ||
3 |
Does the test light illuminate? | -- | ||
Is the duty cycle greater than the value specified present on each circuit? | 0% | |||
5 |
Does the test light illuminate? | -- | ||
6 |
Is continuity indicated? | -- | ||
7 | Check the harness for continuity between the affected control circuit and ground with the DVM J 39200 . Is continuity indicated? | -- | ||
8 | Check the harness for continuity between the two control circuits with the DVM J 39200 . Is continuity indicated? | -- | ||
9 | Repair the Switched B+ circuit. Is the action complete? | -- | -- | |
10 | Repair the control circuit shorted to B+. Is the action complete? | -- | -- | |
11 | Repair the open/high resistance control circuit. Is the action complete? | -- | -- | |
12 | Repair the control circuit shorted to ground. Is the action complete? | -- | -- | |
13 | Repair the control circuits shorted together Is the action complete? | -- | -- | |
14 | Check for poor connections/terminal tension at the IAC valve harness connector. Refer to Electrical Diagnosis in Engine Electrical, and Troubleshooting Procedures for the proper procedure. Also, refer to Checking Terminal Contact in Engine Control Module Diagnosis . Was a problem found and corrected? | -- | ||
15 | Replace the IAC valve. Is the action complete? | -- | -- | |
16 | Check for a poor connection or poor terminal tension at the ECM harness connector. Refer to Electrical Diagnosis in Engine Electrical, and Troubleshooting Procedures for the proper procedure. Also, refer to Checking Terminal Contact in Engine Control Module Diagnosis . Was a problem found and corrected? | -- | ||
17 |
Important: This vehicle is equipped with a Theft Deterrent System which interfaces with the Engine Control Module (ECM). Program the new ECM with the frequency code of the theft deterrent module that is currently on the vehicle. Replace the ECM. Refer to Engine Control Module Replacement/Programming . Is the action complete? | -- | -- | |
18 |
Is this DTC set? | -- | ||
19 | Were there any additional DTCs noted during the Powertrain OBD System Check that have not been diagnosed? | -- | Go to the applicable DTC table | The System is OK |
