Camshaft position (CMP) sensor

Camshaft position (CMP) sensor

The CMP sensor is used in conjunction with the CKP sensor to inform the ECM of the position of the engine in the 4 stroke cycle. Using the CKP sensor alone, the ECM is unable to determine whether a cylinder is on its compression or exhaust stroke.

The sensor uses the principle of magnetic induction to generate the signal. The cam wheel has four lobes which pass in close proximity to the CMP sensor as the camshaft rotates. The lobes disturb the magnetic field of the sensor and induce a voltage in the sensor coil.

In the event of a sensor failure, the ECM will continue to operate sequential fuel injection using the CKP sensor signal. It is possible that the injection timing will be one engine revolution out of sequence.

Mass air flow (MAF) sensor

The MAF sensor is used to measure the quantity of air being drawn into the engine and hence give an indication of the quantity of fuel to be injected to provide a stoichiometric mixture strength.

The MAF sensor is an anemometer located in the inlet air flow and consisting of two wires, one heated and one not. The non heated wire is at ambient temperature and has a resistance which is used as a reference. The heated wire is maintained at a constant temperature difference above the non heated wire. As air flows over the heated wire, current is applied to the wire to maintain the temperature difference, the faster the air flow, the greater the cooling effect and the greater the current required to maintain the temperature difference. The current supplied to the hot wire is converted to a voltage signal and sent to the ECM. The ECM uses the voltage signal to calculate the quantity of air being drawn into the engine.

If the sensor fails, the ECM calculates a value dependent on throttle position, engine speed and air temperature.