Idle Speed/Throttle Actuator - Electronic: Testing and Inspection
Chart C-2C - Idle Air Control:
Wiring Diagram for Chart C-2C - Idle Air Control:
CHART C-2C - IDLE AIR CONTROL CHECK
Circuit Description:
The ECM will control engine idle speed by commanding IAC pintle movement to control the bypass of air around the throttle plate. It does this by sending voltage pulses to the proper motor winding to achieve the desired IAC motor position. This will cause the pintle to move "IN" or "OUT" of the motor a given distance for each pulse received. ECM pulses to the IAC motor are referred to as "counts".
To increase idle speed - ECM will command sufficient counts to retract the IAC pintle and allow more air to flow through the idle air passage and bypass the throttle plate until idle speed reaches the proper RPM. This may be observed as an increase in the IAC counts.
To decrease idle speed - ECM will command sufficient counts to extend the IAC pintle and reduce air flow through the idle passage bypassing the throttle plate. This may be observe as a reduction in the IAC counts.
Each time the engine is started and then the ignition is turned off, the ECM will reset the IAC by fully extending the pintle and then retracting it t a pre-determined "park position". Normal ECM control of the IAC will now start at this point. This is how the ECM monitors pintle position for a given idle RPM requirement. The IAC may also be reset, with engine stopped, by selecting "IAC MOTOR SET" (EO07) as an "ECM Output Cycling" function of the service diagnostics. This will result in a fully seated pintle to allow a proper minimum air rate adjustment to be made.
Do not apply battery voltage across the IAC motor terminals, or the motor will be permanently damaged.
The ECM uses the following information to control idle speed:
a. Battery Voltage
b. Coolant Temperature
c. Intake Air Temperature (MAT)
d. Power Steering Pressure Switch
e. Throttle Position Sensor
f. Engine Speed
g. Mass Air fFow (MAF)
h. A/C Clutch Command.
Test Description: The following numbered steps correspond with the step numbers on the diagnostic chart.
1. The engine RPM should be higher after disconnecting the IAC motor and restarting the engine. This is due to the pintle having been seated and then retracted to the "park position" allowing for more air flow and resulting in higher than normal idle speed.
2. Idle RPM should return to the approximate level noted at the beginning of step #1.
3. Be sure to disconnect the IAC motor prior to this test. The test light will confirm the ECM signals by a steady or flashing light.
4. Before replacing an ECM, be sure to check the resistance at the IAC motor windings. A reading of less than 20 ohms (shorted) may result in a repeat ECM failure.
Diagnostic Aids:
Engine idle speed may be adversely affected by:
Park/Neutral Switch - If the ECM interprets the transaxle as always being in neutral, the idle will not be controlled at specified RPM when in drive.
Leaking Injector(s) - Leaking injectors will cause fuel imbalance and poor idle quality. See Chart A-7.
Vacuum leaks - Vacuum leaks can result in higher than normal idle.
TPS Sticking - When the TPS or throttle shaft is binding or sticking in an open throttle position, the ECM does not interpret the vehicle as not moving and will not control idle.
EGR - In addition to the electronic control of EGR examine the EGR valve for proper seating.
Electrical Connections - Faulty battery, cables, or connection can result in voltage variations. The ECM will try to compensate, resulting in erratic idle speeds.
A/C Command - The ECM will compensate for AC compressor clutch load when functioning properly.
Power Steering Pressure Switch - If the ECM interprets the Pressure Switch as being closed the idle may be too high.