Idle Maintenance
Illustrated Example: E32/M60 Idle Air Management:
The basic function of the air management system is to maintain and stabilize engine RPM for all operating conditions. With the throttle plate closed the main air intake path is closed. Idle air must reach the intake ports in order for the engine to continue to operate.
The idle system control is integrated in the ECM, with the fuel injection and ignition control systems (except EML equipped vehicles).
Mechanically this is possible by allowing air to bypass the throttle plate through an idle speed control valve.
The ECM must receive input signals of all the operating conditions. These signals are provided by various switches, sensors, etc. that convert each condition into an electrical/electronic signal that the ECM can interpret and process.
INPUTS THAT AFFECT IDLE AIR MANAGEMENT
- Battery charge logic (if equipped) provides an input of charging system voltage. This allows the ECM to compensate by raising the idle RPM to increase generator output
- The RPM sensor input allows the ECM to monitor engine speed because of "invisible" inputs that can cause idle fluctuations due to drag on the engine: power steering, thick oil (frictional forces), etc.
- Throttle position sensor "arms" the ECM to regulate idle RPM when the throttle position is at rest. Then when the throttle position is off idle, idle regulation ceases
- Cold engine temperature provides taster idle to raise temperature sooner.
- Vehicle speed informs ECM when the vehicle is stationary and requires idle maintenance.
- A/C on request from the climate control system and compressor engage acknowledgment. These two signals "arm" and stabilize RPM during air conditioner operation
- Headlights "on" (if equipped) to stabilize the idle RPM from increased electrical load on the generator.
- Range selector provides a park/neutral input to the ECM identifying when the vehicle is in a drive gear. This signal allows idle stabilization for the increased load on the engine.
The programmed values in the ECM allow regulation of idle RPM based on the above inputs, as well as compensation for mechanical wear.
IDLE SPEED CONTROL VALVE OUTPUT CONTROL
Two Wire Idle Control Valve:
The idle control valve of the M42 engine is a two wire control valve. The idle control valve is spring loaded closed. It will failsafe to a fixed opening to allow the engine to idle in the event of a power failure.
One lead of the valve is provided power. The other lead is provided a modulated ground signal which controls the valve's opening angle against return spring pressure.
Three Wire Idle Control Valve:
The balance of the engine control systems covered by this course utilize a three wire control valve. The idle speed control valve is an electric motor with dual windings, and the armature is mechanically rotates a gate valve.
The valve is electrically opened and closed, with preset positions for starting, idling, deceleration and engine shut off.
The ECM is equipped with two final stage transistors which will alternate positioning of the idle speed actuator.
The final stages are "pulsed" simultaneously by the ECM which provides ground paths for the idle speed actuator.
The duty cycle of each circuit is varied to achieve the required idle RPM.
The three wire idle speed control valve is used as part of the ASC+T system (if equipped) during MSR regulation to reduce the engine's braking effect during decel.