Contents: Description of the scheme ↧ Checking the Idle Air Control System… ↧ Initial setting of idle system… ↧
Description of the scheme
The ECM controls engine idle speed using the Idle Air Control (IAC) valve. To increase idle speed, the ECM moves the IAC rod out of its socket, allowing more air to flow through the throttle body. A scanner will read the ECM's commands to the IAC valve by the number of pulses.
More pulses means more bandwidth (increase in rotation speed).
The IAC valve is mounted on the throttle body to control engine idle speed as commanded by the engine control module (ECM). The ECM sends voltage pulses to the windings of the IAC valve stepper motor to move the valve needle in or out a specified distance (step) with each pulse. The movement of the needle controls the valve air flow, which in turn controls the engine idle speed. The desired idle speed for all operating conditions is set in the ECM calibrations.
Parameters that affect idle speed adjustment:
- coolant temperature,
- park/Neutral switch position,
- car speed,
- battery voltage,
- air conditioning system pressure (if such is installed on the car).
The ECM "remembers" the appropriate IAC valve positions to maintain stable idle speed when the engine is warm under various combinations of the above parameters. This data is stored in the ECM ROM. The data is retained even after the ignition is turned off. The remaining IAC valve positions are calculated based on this stored data.
As a result of deviation due to wear and deviation in the minimum position of the throttle valve (within acceptable limits) have no effect on idle speed. This also means that if the ECM is disconnected from power supply in idle mode, (battery removal) idle speed control may fail, which will require additional pressure on the accelerator pedal during engine start-up until the ECM detects idle speed parameters.
The minimum throttle position is set at the factory with the upper screw. This setup allows enough air to pass through the throttle valve, which causes the throttle valve needle to move the required number of steps away from the socket, at a "preset" idle speed. The minimum position of the throttle valve' on the engine should not be considered as setting the minimum idle speed, as on other types of fuel-injected engines. The upper throttle screw is capped at the factory after adjustment.
Checking the Idle Air Control System (1.5L SOHC)
If the idle speed is too high, stop the engine.
Pull the IAC idle air control valve handle out fully using the IAC actuator.
Start the engine. If the idle speed is above 800 rpm:
- find and repair vacuum leaks,
- check if the throttle valve is sticking or not closing tightly enough,
- check that the basic idle speed parameters are set correctly.
Inspect IAC lines for blockages.
Measure the resistance between the terminals of these IAC valves. The resistance between terminals C and D, A and B should be (from 40 to 80 Ohm).
Check for an open or short in the wiring between the following terminals:
- terminal D of the IAC valve connector and terminal A28 of the ECM connector;
- terminal C of the IAC valve and terminal AZO of the ECM connector;
- terminal B of the IAC valve connector and terminal A13 of the ECM connector;
- terminal A of the IAC valve connector and terminal A29 of the ECM connector.
Initial setting of idle system parameters
Each time the battery cable or connector is disconnected or replaced, the following information gathering procedure must be performed.
Turn on the ignition for 5 sec.
Turn off the ignition for 10 sec.
Turn on the ignition for 5 sec.
Start the engine in Park/Neutral position.
Allow the engine to run until the coolant reaches 85°C.
Turn on the air conditioner for 10 seconds, if installed.
Turn off the air conditioner for 10 seconds, if installed.
If the vehicle has an automatic transmission, apply the parking brake. While pressing the brake pedal, move the gearshift lever to position D.
Turn on the air conditioner for 10 seconds, if installed.
Turn off the air conditioner for 10 seconds, if installed.
Turn off the ignition. The idle speed parameter setting procedure is complete.
The original article can be found on the resource «ChevyMan»