Engine Speed and Position Sensors

Engine speed (RPM), crankshaft position (CKP) & camshaft position (CMP) sensors are two basic types, inductive or Hall effect. Inductive sensors may be mounted on the camshaft housing, crankcase or in the distributor.

Inductive type:-

These sensors contain a permanent magnet partly enclosed in a soft iron core and partly in a field winding. The tip of the sensor is positioned close to the flywheel ring gear, a separate toothed trigger wheel, or a toothed wheel attached to the crankshaft pulley. In certain applications the sensor(s) are incorporated in the ignition distributor or in a separate unit driven by the camshaft.

Operation:-

As the teeth or pins of the trigger wheel pass the sensor the magnetic field changes, inducing a voltage signal which is sent to the ECM.

Hall Effect:-

The Hall effect generator consists of a semi-conductor material, through which a current flows, enclosed by a magnetic field at right angles to the current flow.

Operation:-

When the ignition is switched on, a small voltage is generated across the semi- conductor (the "Hall" voltage).

A soft iron trigger vane attached to the distributor shaft is positioned to pass between the poles of the magnet.

This diverts the magnetic field and has the effect of reducing the Hall voltage to practically zero.

As the vane passes the sensor, the Hall trigger is switched "on" when it is opposite a gap and "off" when there is no gap.

Throttle Position Switch (TPS)

The throttle position switch assembly is mounted on the throttle body and attached to the throttle vale shaft. It incorporates an idle switch and wide open throttle switch.

Operation:-

The idle switch governs fuel shut-off the overrun and the wide open throttle switch signals the ECM to provide full load enrichment.

Throttle Position Sensor (TPS)

Used as an alterative to a throttle switch assembly, the throttle position sensor is a variable resistance (potentiometer) connected to the throttle valve shaft. In the case of single point injection systems, load sensing may be by means of this sensor, avoiding the need for a MAP sensor.

Operation:-

It is supplied with a reference voltage and the output signal voltage is used by the ECM to interpret the exact throttle position.

Valves

Evaporative Emission (EVAP) Canister Purge Valve:-

A reduction in pollution of the atmosphere can be achieved by fitting an evaporative control system. Fuel vapour from the tank is stored in a carbon filter when the engine is stopped.

Operation:-

When the engine approaches normal operating temperature, the solenoid valve on the carbon filter opens, allowing the fuel vapour to be drawn out of the carbon filter into the inlet manifold.

Exhaust Gas Recirculation (EGR) Valve:-

To reduce the oxides of nitrogen emitted by the exhaust, a proportion of the exhaust gas can returned to the inlet manifold when the engine is above idle speed, but not under full load. The recirculated exhaust gasses displace some of the intake charge, but do not bum. This results in a major cooling of the combustion process and a significant reduction in NOx emissions.An incidental, but important side effect is an improvement in fuel consumption, resulting from the additional ignition timing advance permitted due to the lower combustion chamber pressures and increased pumping efficiency associated with EGR.

Operation:-

The exhaust gas recirculation valve is operated mechanically or by vacuum to control the flow of exhaust gas into the inlet manifold.

Heated Oxygen Sensor (HO2S)

Sometimes referred to as a Lambda sensor. Fitted in the exhaust system upstream of the catalytic converter, where it can measure the exhaust gases evenly from all cylinders. Where necessary (e.g. on a V-engine with a dual exhaust system) two sensors may be fitted.

Operation:-

The sensor reacts to the oxygen content of the exhaust gasses and uses the voltage generated (200-800 mV) to signal the ECM to modify the mixture (air/fuel) ratio.

If the oxygen content is at the ideal value (Lambda=1), the sensor signal voltage will be500 mV.

If the mixture is too weak the voltage will be 200-450mV (Lambda=1.10).

If it is too rich the voltage will be 550-80mV (lambda=0.96).

In practice the oxygen sensor continually signals the ECM to alter the fuelling by a small amount, to maintain the mixture strength as near as possible (+- 1%) to the theoretical ideal (stoichiometric) ratio of 14.7 parts air to 1 part fuel, which is the optimum for the catalytic converter to work efficiently.

The sensor will not begin to operate until it has reached 482-572C, but most now incorporate a heater element, which enables the closed loop system to come into operation soon after a cold start.

Engine coolant/air temperature sensor

The engine coolant (ECT) sensor probe is immersed in coolant and changes its resistance in proportion to engine coolant temperature changes.The intake air temperature (IAT) sensor probe is either incorporated in the air flow/mass meter or is mounted on the air stream of the intake system. Its resistance changes in proportion to intake air temperature changes.

Operation:-

In most cases the sensor incorporates a negative temperature coefficient (NTC) resistor, which decreases in resistance as the coolant temperature rises.