Circuit Description
The powertrain control module (PCM) controls the air entering into the engine with an idle control (IAC) valve. In order to increase the idle RPM, the PCM commands the pintle inside the IAC valve away from the throttle body seat. This allows more air to bypass through the throttle blade. In order to decrease the RPM the PCM commands the pintle towards the throttle body seat. This reduces the amount of air bypassing the throttle blade. A scan tool will read the IAC valve pintle position in counts. The higher the counts, the more air that is allowed to bypass the throttle blade.
Conditions for Running the DTC
| • | No active MAP DTC's |
| • | No active IAT DTC's |
| • | No active ECT DTC's |
| • | No active TP DTC's |
| • | No active Oxygen Sensor DTC's |
| • | No active Fuel Trim DTC's |
| • | No active Injector Control DTC's |
| • | No active Misfire DTC's |
| • | No active CKP DTC's |
| • | No active CMP DTC's |
| • | No active EVAP DTC's |
| • | No active Fuel Tank Pressure DTC's |
| • | No active VSS DTC's |
| • | No active PCM Memory DTC's |
| • | Engine has been running more than 20 seconds. |
| • | BARO is more than 72 kPa. |
| • | Engine Coolant Temperature ( ECT) is more than 40°C (104°F). |
| • | The Idle Speed has beed stabilized for 5 seconds or more. |
| • | The battery voltage is between 10 volts and 17 volts. |
Conditions for Setting the DTC
| • | Engine RPM is below desired RPM by more than 100 RPM for 19 seconds. |
| • | IAC command is more than 145 counts. |
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
| • | Inspect the IAC valve electrical connection for proper mating. |
| • | Inspect the wiring harness for damage. |
| • | Inspect the throttle stop screw for signs of tampering. |
| • | Inspect the throttle linkage for signs of binding or excessive wear. |
| • | A slow or unstable idle may be the cause of one of the following conditions: |
| - | Fuel system to rich or to lean |
| - | Foreign material in the throttle body bore or in the air induction system |
| - | A leaking or restricted intake manifold |
| - | An excessive engine overloading: Check for seized pulleys, pumps, or motors on the accessory drive. |
| - | Too high a viscosity engine oil |
| • | Refer to Rough, Unstable, or Incorrect Idle and Stalling . |
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 step numbers on the Diagnostic Table.
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The Powertrain OBD System Check prompts you to complete some of the basic checks and to store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data captured when the malfunction occurred. The scan tool stores this data for later reference.
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A normally operating IAC system will be able to be extended and retracted by a scan tool and change the engine idle RPM. The valve movement is verified by an engine RPM change.
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If the scan tool was able to command the IAC valve smoothly, a malfunction may still exist internally within the IAC valve. You can check this by checking the internal resistance of the IAC valve.
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The IAC circuits always have ground or voltage signals on them in pairs. If the test light illuminates on more or less than 2 terminals, 1 of the circuits is shorted to voltage or open.
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The IAC circuits always have ground or voltage signals on them in pairs. If the test light illuminates on more or less than 2 terminals, 1 of the circuits is shorted to ground or open.
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The IAC circuits are constantly switched between ground and voltage. The test light should blink on all circuits when connected to ground.
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A test light that remains ON constantly indicates that the circuit is shorted to the voltage.
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Program the replacement PCM and perform the Crankshaft Position System Variation Learn Procedure. Refer to the latest Techline information for PCM programming.
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If no malfunctions have been present at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||||||||
Does the engine RPM change smoothly when the engine is commanded by the scan tool? | 900-2,000 RPM | |||||||||
Are the resistances across terminals A and B and terminals C and D within the specified value? | 40-80ohms | |||||||||
4 |
Are the resistances across terminals B and C and terminals A and D infinite? | -- | ||||||||
Does the test lamp illuminate on 2 terminals? | -- | |||||||||
Does the test lamp illuminate on 2 terminals? | -- | |||||||||
7 |
Was a repair necessary? | -- | ||||||||
Does the test lamp flash ON and OFF for all terminals? | -- | |||||||||
9 |
Was a repair necessary? | -- | ||||||||
10 |
Was a repair necessary? | -- | ||||||||
11 |
Was a problem found? | -- | ||||||||
Did the test lamp remain ON constantly for the terminals that did not blink? | -- | |||||||||
13 |
Is the action complete? | -- | -- | |||||||
|
Important:: The replacement PCM must be programmed.Refer to Powertrain Control Module Replacement/Programming . Replace the PCM. Is the action complete? | -- | -- | ||||||||
15 |
Does the scan tool indicate that this diagnostic has ran and passed? | -- | ||||||||
Check to see if any additional DTCs are set. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK Go to Diagnostic Aids |
