Idle Air Control (IAC) System Diagnosis

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Schematic:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) controls the Idle Air Control (IAC) valve in order to regulate the air flow through the idle air bypass passage. The amount of air flowing through the idle air bypass passage determines the idle speed. The PCM directs the IAC valve according to changes in engine load. A stalling or poor idle concern could be caused by an IAC valve that is not operating properly.

DIAGNOSTIC AIDS
Any condition that can affect engine performance at idle, can affect the performance of the IAC system. A careful visual and/or physical inspection of the engine is necessary when the scan tool indicates that the engine speed or the IAC duty cycle readings are out of range.

A warm engine that is idling too slow, with a high IAC Duty Cycle of 50 percent or more, may have an open in the ignition positive voltage circuit to terminal 5.

A warm engine that is idling too fast, with a low IAC Duty Cycle of 15 percent or less, may have an open in the ignition positive voltage circuit to terminal 2.

An intermittent IAC system concern may be caused by poor electrical connections in the IAC valve circuits. Thoroughly check any circuitry that is suspected of causing an intermittent complaint for the following conditions:
^ Any backed-out terminals
^ Any improper mating of terminals
^ Any broken electrical connector locks
^ Any improperly formed or damaged terminals
^ Any faulty terminal-to-wire connections
^ Any physical damage to the wiring harness
^ A broken conductor inside the wire insulation
^ Any corrosion of the electrical connections, the splices, or the terminals

Refer to the following procedures in Diagrams
^ Testing for Intermittent and Poor Connections
^ Circuit Testing
^ Testing for Intermittent and Poor Connections
^ Wiring Repairs
^ Connector Repairs

Thoroughly check any circuitry that is suspected of causing an intermittent complaint for the following conditions:
^ A poor connection
^ Rubbed through wire insulation
^ A broken wire inside the insulation
^ Physical damage to the wiring harness Backed out terminals
^ Improper mating of terminals
^ Broken electrical connector locks
^ Improperly formed or damaged terminals
^ Improper terminal to wire connections
^ Corrosion of electrical connections, splices, or terminals

Refer to the following procedures in Diagrams:
^ Testing for Intermittent and Poor Connections
^ Circuit Testing
^ Testing for Intermittent and Poor Connections
^ Wiring Repairs
^ Connector Repairs

IAC Valve Terminal (Pin) Location:

Use the table in order to locate the correct terminals to test during diagnosis. The table layout corresponds to the pin location of the IAC valve electrical terminals.

TEST DESCRIPTION
The numbers below refer to the step numbers in the diagnostic table.
1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
4. This step tests whether the PCM can operate the IAC valve with the scan tool. The IAC valve can be commanded to increase and/or decrease engine speed by using the scan tool RPM Control function. The RPM Control function of the scan tool is disabled when any DTCs are set, and when the engine is less than the normal operating temperature of 80°C (176°F).
5. This step checks the operation of the electrical load idle-up circuits. If there is no increase in the IAC Duty Cycle percentage when the accessory is turned on, the idle-up circuit may be inoperative or always on.
6. This step checks the operation of the A/C idle-up circuits. If there is no increase in the IAC Duty Cycle percentage when the A/C compressor is engaged, the idle-up circuit may be inoperative or always ON.
10. This step checks the electrical circuits between the IAC valve and the PCM.
11. This step determines whether the PCM is attempting to control the IAC valve. Each IAC coil control circuit should indicate a high voltage of 12 volts or more, and then a low voltage of 0.0 - 1.0 Volts as the PCM commands changes in the engine speed. Remember to clear any DTCs and run the engine until the engine reaches a normal operating temperature before attempting RPM Control with the scan tool.