CIRCUIT OPERATION
DEFENDER 90 NAS
7
Crankshaft position (CKP) sensor
The CKP sensor signal is used as the basis for fuel injection timing. It informs the GEMS
that the engine is turning, the speed at which it is turning and its position in the 4 stroke
cycle.
The sensor uses the principle of magnetic induction to generate the signal. A reluctor ring,
attached to the engine flywheel, has a series of teeth spaced at 10 ° intervals, with one
tooth missing at 20 ° after TDC. The reluctor ring rotates with the engine, in close
proximity to the CKP sensor. As each tooth of the reluctor ring passes the sensor, it
disturbs the magnetic field of the sensor and a voltage is induced in the sensor coil. The
ECM calculates engine speed by counting pulses per second from the CKP sensor.
Engine position is calculated by counting pulses after missing pulse.
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 compression stroke 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
(chemically correct ratio) mixture strength.
The MAF sensor is an anemometer located in the inlet air flow, upstream of the throttle
body, which uses the Hot Wire principle to determine air flow. A single metering wire is
maintained at a constant temperature. As air flows over the wire, current is applied to the
wire to maintain the temperature at its reference temperature, the faster the air flow, the
greater the cooling effect and the greater the current required to maintain the temperature.
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.