Circuit Description
The idle air control (IAC) valve is located in the throttle body of both TBI and MFI Systems. It consists of a movable pintle, driven by a small electric motor called a stepper motor. The stepper motor is capable of moving in exact, measured amounts called counts. The PCM uses the IAC valve to control idle RPM. It does this by changing the pintle position In the idle air passage of the throttle body. This varies the air flow around the throttle plate when the throttle is closed. The IAC valve motor is a two phase bi-polar permanent magnet stepper motor that provides highly accurate rotation, or movement, every time the polarity of a winding is changed. This change in polarity can be seen when observing a test lamp while the PCM is attempting to change engine RPM (the test lamp will flash red or green each time the polarity is changed). To determine the desired position of the IAC pintle at idle or during deceleration, the PCM refers to the following inputs: Engine RPM, Battery voltage, Air temperature, Engine Coolant Temperature, Throttle Position Sensor angle, Engine load, and vehicle speed. If engine speed can not be controlled by the PCM through the use of the IAC system, an idle speed DTC will be set.
Conditions for Running the DTC
| • | There are no active TP, ECT, MAP, IAT, MAF, CKP Sensor, EVAP, Misfire, EGR, VSS, Fuel Trim, or Injector Circuit DTCs. |
| • | The engine coolant temperature is more than 70°C (158°F). |
| • | The engine has been running for at least 2 minutes. |
| • | The system voltage is between 9.0-18.0 volts. |
| • | The vehicle speed is less than 3 mph. |
| • | The intake air temperature is more than -18°C (5°F). |
| • | The throttle is less than 1.5 percent. |
| • | The BARO is more than 65 kPa. |
Conditions for Setting the DTC
| • | The engine speed is more than 100 RPM less than Desired Idle. |
| • | The above condition is present for more than 15 seconds. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using a scan tool. |
Diagnostic Aids
Inspect for the following conditions:
| • | A restricted air intake system |
| • | The throttle body--Inspect for: |
| - | Objects blocking the IAC passage or throttle bore |
| - | Excessive deposits in the IAC passage and on the IAC pintle |
| - | Excessive deposits in the throttle bore and on the throttle plate |
| • | Large vacuum leaks--Inspect for a condition that causes a large vacuum leak, such as an incorrectly installed or faulty PCV valve or a brake booster hose that is disconnected. Refer to Visual/Physical Inspection in Symptoms . |
Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.
| • | Faulty terminal connections--Inspect the harness connectors for the following conditions: |
| - | Backed-out terminals |
| - | Improper mating |
| - | Broken locks |
| - | Improperly formed or damaged terminals |
| - | Faulty terminal-to-wire connections--Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. |
| • | A damaged harness--Inspect the wiring harness for damage. |
| If the harness inspection does not reveal a problem, observe the display on the scan tool while moving the connectors and the wiring harnesses that are related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. |
| • | Inspect for clean and secure connections at the PCM and the engine grounds. |
If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.
The number below refers to the step number on the diagnostic table:
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On Board (OBD) System Check? | -- | ||||||||||
2 | Are any other DTCs set? | -- | Go to other DTC first | |||||||||
3 |
Does Engine Speed remain within the specified value of Desired Idle for each RPM command? | ±100 RPM | Go to Diagnostic Aids | |||||||||
4 |
Does each node light cycle red and green, never OFF? | -- | ||||||||||
5 |
Did you find and correct the condition? | -- | ||||||||||
6 |
Did you find and correct the condition? | -- | Go to appropriate section for on-vehicle service | |||||||||
7 | Inspect for the following conditions:
Do any of the above require a repair? | -- | ||||||||||
8 | Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Is the action complete? | -- | -- | |||||||||
9 | Inspect for a faulty connection at the PCM harness connector. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | ||||||||||
|
Important : The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the replacement? | -- | -- | ||||||||||
11 |
Does Engine Speed remain within the specified value of Desired Idle for each RPM command? | ±100 RPM | System OK |
