For 1990-2009 cars
Engine Controls Components
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

The powertrain control module (PCM) controls the engine idle speed by adjusting the position of the idle air control (IAC) motor pintle. The IAC is a bi-directional motor driven by 2 coils. The PCM pulses current to the IAC coils in steps (counts) in order to extend the IAC pintle into a passage in the throttle body and decrease air flow. The PCM reverses the current pulses in order to retract the pintle and increase air flow. This method allows highly accurate control of the idle speed and quick response to changes in engine load.

### Diagnostic Aids

Inspect for the following conditions:

• | Inspect the wiring harness for damage.

• | A restricted air intake system. Inspect for a collapsed air intake duct, a restricted air filter element, or foreign objects blocking the air intake system.

• | The throttle body. Inspect for objects blocking the IAC passage or the throttle bore, excessive deposits in the IAC passage and on the IAC pintle, and excessive deposits in the throttle bore and on the throttle plate.

| Inspect for a sticking throttle plate. Also inspect the IAC passage for deposits or objects which do not allow the IAC pintle to fully extend.

• | A vacuum leak -- Inspect for conditions that cause vacuum leaks, such as:

- | Disconnected or damaged hoses

- | Leaks at the EGR valve and at the EGR pipe to the intake manifold

- | Leaks at the throttle body

- | A faulty or incorrectly installed PCV valve

- | Leaks at intake manifold

- | The brake booster hose disconnected, ect.

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

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.

• | Loose terminal connection

- | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.

- | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.

• | 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 connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

• | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

### Test Description

The numbers below refers to the step number on the Diagnostic Table:

1. This vehicle is equipped with a PCM which utilizes an electrically erasable programmable read-only memory (EEPROM). When the PCM is replaced, the new PCM must be programmed.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | 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. Start the engine.
2. Turn all accessories OFF (A/C, rear defroster, etc.).
3. Use a scan tool in order to command the RPM up to 1500, down to 650, and then up to 1500 while monitoring the Engine Speed on the scan tool.

Does the Engine Speed remain within the specified value of Desired Idle for each RPM command?

±50 RPM

System OK

Go to Step 3

3 |

1. Disconnect the IAC.
2. Install a J 37027-A to the IAC Harness connector.
3. With the engine running, command the RPM up to 1500, down to 650, and then up to 1500 while observing the IAC System Monitor node lights.

Does each node light cycle red and green (never off)?

--

Go to Step 5

Go to Step 4

4 |

1. Test the following circuits for an open, a short to voltage, or a short to ground:

• | IAC A Low

• | IAC A High

• | IAC B Low

• | IAC B High

2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go to Step 12

Go to Step 10

5 | Was a low idle symptom or rich condition noticed? | -- | Go to Step 6 | Go to Step 7
6 |

1. Visually/physically inspect for the following conditions:

• | Throttle body tampering

• | A restricted air intake system

• | A collapsed air intake duct

• | A restricted air filter element

• | Any foreign objects blocking the air intake system

• | Inspect the throttle body for objects blocking the IAC passage or the throttle bore, excessive deposits in the IAC passage and on the IAC pintle, and excessive deposits in the throttle bore and on the throttle plate

2. If a problem is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 11

Go to Step 8

7 |

1. Visually/physically inspect for the following conditions:

• | Any vacuum leaks. Refer to Emission Hose Routing Diagram .

• | Any throttle body tampering

• | Any binding in the throttle Plate or the throttle shaft

• | Misadjustment or binding in the accelerator and cruise control cables. Refer to Accelerator Control Cable Replacement .

• | A faulty, missing, or incorrectly installed PCV valve. Refer to DTC P1351 Ignition Coil Control Circuit High Voltage .

2. If a problem is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 11

Go to Step 8

8 | Inspect for a faulty connection at the IAC harness connector. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 9
9 | Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Did you complete the repair? | -- | Go to Step 12 | --
10 | Inspect for a faulty connection at the PCM harness connector. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 12 | Go to Step 11
11 | Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the replacement? | -- | Go to Step 12 | --
12 |

1. Clear any DTCs.
2. Start the engine.
3. Turn all of the accessories OFF (A/C, rear defroster, etc.).
4. Use a scan tool in order to command the RPM up to 1500, down to 650, and then up to 1500 while monitoring the Engine Speed on the scan tool.

Does the engine speed remain within the specified value of Desired Idle for each RPM command?

±50 RPM

System OK

Go to Step 3