Camshaft Position (CMP) Sensor (Engine management system - EDC)

Camshaft Position (CMP) Sensor

The CMP sensor is located on top of the engine on the camshaft cover. This sensor is a Hall effect sensor producing one pulse for every camshaft revolution. The CMP sensor is only used on start up to synchronise the ECM programme with the CKP signal. This is to identify number one cylinder for correct injection timing. Once this has been achieved the input from the CMP sensor is no longer used in any of the ECM strategies.

Electrical input to the CMP sensor is supplied via the main relay located in engine compartment fuse box. One output is sensor earth, the other is the signal output to the ECM.

In the event of a CMP sensor signal failure the engine will crank but will not start.

Mass Air Flow/ Inlet Air Temperature (MAF/ IAT) Sensor

The MAF/ IAT sensor is located on the engine intake air manifold, it combines the two functions into one unit.

The MAF sensor works on the hot film principle. The MAF sensor has two sensing elements contained within a film. One element is at ambient temperature e.g. 25 °C (77 °F) while the other is heated to 200 °C (392 °F) above this temperature e.g. 225 °C (437 °F). As air passes through the MAF sensor it has a cooling effect on the film. The current required to maintain the 200 °C (392 °F) differential provides a precise, although non-linear, signal of the air drawn into the engine. The MAF sensor output is an analogue voltage proportional to the mass of the incoming air. The ECM utilises this data, together with information from the other sensors and the fuelling maps, to determine the correct fuel quantity to be injected into the cylinders. It is also used as a feedback signal for the EGR system.