The DMC-12's idle speed is controlled by an electronic system manufactured by Bosch. An Electronic Control Unit (ECU) monitors the engine speed at all times. The specified idle speed is 775 +/- 50 RPM.

Control principle Edit

The idle speed regulator contains a rotary valve which controls the air flow through a duct that bypasses the throttle plates. (During engine warm-up, for example, when the engine speed is reduced due to friction, the idle speed regulator opens to allow additional air to the engine, increasing RPM to specification.)

System operation Edit

The idle speed motor is similar to an electric motor. It controls a rotary valve mounted on its shaft. The regulator is mounted on the left side of the intake manifold. It has two connections for air: one intake, and out out to the inlet manifold below the throttle plates.

The idle speed control system is only in operation when the idle speed microswitch is closed. The microswitch is closed when the throttle plates are fully closed; the ECU senses engine speed from the tachometer and activates the idle speed regulator, changing the position of the rotary valve. The idle speed microswitch is located on the engine throttle linkage below the air box.

At engine coolant temperatures below 15°C (59°F), a thermistor signals the ECU module to switch the regulator to a wider open position (letting in more air). This increases idle speed during engine warm-up. The thermistor is located on the coolant distribution pipe.

ECU electrical circuit function Edit

The ECU module receives current through Fuse #1 from the main relay, when the ignition switch is in the ‘run’ or ‘start’ position. When the throttle plates are closed, the idle speed micro-switch provides ground for the ECU module to operate.

A diode is used to prevent feedback to the ECU module from another circuit which also uses the idle speed micro-switch to provide its ground. This idle speed diode is black and is located in the module compartment behind the driver's seat.

The thermistor activates another circuit in the ECU module when the coolant temperature is below 15°C (59°F); this circuit creates a fast idle.

The ECU module receives an RPM signal from coil terminal #1. When the engine idle speed is above or below specification, the ECU module activates the electric motor in the idle speed regulator: the signal from the module will run the motor (in a forward direction, which will open the rotary valve, or a reverse direction, which will close the rotary valve) in order to achieve proper engine idle speed.

See Also Edit

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