Engine management system - MEMS (Diagnostic connector): Description and operation (page 2 of 2)

Acceleration enrichment

When the throttle pedal is depressed, the ECM receives a rising voltage from the throttle position sensor and detects a rise in manifold pressure from the manifold absolute pressure sensor. The ECM provides additional fuel by increasing the normal injector pulse width and also provides a number of extra additional pulses on rapid throttle openings.

Over-run fuel cut-off

The ECM implements over-run fuel cut-off when the engine speed is above 1600 rpm with engine at normal operating temperature and the throttle position sensor in the closed position, i.e. when ECM senses that the vehicle is ’coasting’ with the throttle pedal released. The ECM indexes the idle air control valve open slightly to increase the air flow through the engine to maintain a constant manifold depression to keep emissions low.

Fuel is immediately reinstated if the throttle is opened. If engine speed drops below 1600 rpm on over-run, fuel is progressively reinstated.

Over-speed fuel cut-off

To prevent damage at high engine speeds the ECM will implement fuel cut-off at engine speeds above approximately 7000 rpm. Fuel is reinstated as engine speed falls.

Ignition switch off

In the first 10 seconds after ignition is switched off, the ECM drives the idle air control valve to its power down position (ready for the next engine start), and stores any required information.

The ECM then monitors the engine bay temperature using the ambient air temperature sensor. If the temperature is above a certain limit, the ECM will drive the engine bay fan for 8 minutes, and will then power down. If the engine bay temperature is below the limit the ECM will power down after 10 seconds.

Engine compartment ambient air temperature sensor

The ECM monitors the engine compartment temperature using the ambient air temperature sensor. When the temperature exceeds a certain limit, the engine bay fan relay is energised to run the fan. If the temperature continues to rise, and exceeds another higher limit, the engine bay warning lamp (in the instrument pack) is illuminated.

If the ambient air temperature sensor fails, the engine bay fan will run while the ignition is on and the warning lamp will be permanently lit.