Air Mass Sensors

Applications:-

The air mass sensor measures the mass of air inducted by the engine (air flow mass) with great precision. The signal produced by the air mass sensor is used to calculate the amount of fuel injection and in the case of diesel engines also to control the recirculation of exhaust. It is an important component in both the reduction of exhaust and air supply. A defective or dirty air mass sensor can deliver false input signals to the engines central control unit, which in turn sends false information to other components. In case of turbo diesels the air mass sensor is subject to particularly high burden due to the fact the both air flow rate and air speed are very high.

Description of functions:-

The complete air mass sensor consists of a flow channel (pipe) in which the inducted air-flow is directed past the actual sensor. Depending on the type of use and vehicle, the air mass sensor is either completely integrated in a tube made of synthetic material, or with the actual sensor as a separate individual plug in module. Both versions (with tube/separate) are termed air mass sensor.

Earlier Models were fitted with a hot-wire sensor element. The hot-wire was burn free by polluants following the short increase in heat after turning off the engine. Newer Models work using a film like heating resistor on a support, in which case the burn-through procedure does not apply.The heat-film sensor is heated to a temperature of approx 120-180 degrees Celsius above induction temperature (depending on vehicle manufacturer) the inflow of air cools the heat-film sensor. This cooling down is compensated by heating current via the electronic controller unit. This heating current is directly proportional to the amount of air induced.

Defects and possible causes:-

Defective and dirty air mass sensors deliver faulty signals. This can have the following consequences:

Black Smoke

Power Shortage

Minimal operation

Possible causes of damage:

If the air intake tube is porous, dirt particles can find their way into the inducted air. These collide at high speed with the air mass sensor, destroying the sensitive sensor element. Excessive oil spray from the crankshaft ventilator can lead to fouling-up of the sensor. Bad servicing, for instance un-cleanliness when changing air filters, use of wrong or substandard air filters can be the cause of dirt and damage to the air mass sensor. Splashed water, for instance from heavy rainfall, can force its way into the clean-air side and damage or dirt-clog the sensor. Salt water caused by winter gritting and melting snow increases this effect. Oil particles from oil-covered sport air filters can damage or dirt-clog the sensor.

Other sources can also cause an intact air mass sensor to deliver a false signal:

defective exhaust-gas recirculation valves

defective fuel tank ventilation valves

porosity of induction tract

clogged air filters

Air flow measurement

Volume air flow (VAF) sensor:-

The VAF sensor is fitted between the air filter and throttle body. It measures the amount of air drawn into the engine by the deflection of a flap inside the sensor.

Operation:-

The flap is connected to a potentiometer which sends a signal voltage to the ECM.

This is the primary signal which, in conjunction with those from the other system sensor, is used to determine the fuelling requirements of the engine.

In many applications the air temperature sensor is incorporated in this type of air flow sensor

The MAF sensor has no moving parts and provides very little restriction to the air flow. It is fitted between the air filter and throttle body.

The MAF sensor consists of venture with a resistance wire exposed to the incoming air.

An air temperature sensor or compensating wire is also exposed to the air flow.

The ECM senses changes in temperature caused by variation in air mass passing through the sensor body and increases or decreases the current flowing through the measuring wire.

An internal electronic circuit maintains the approximate tempreture of the measuring wire at a constant 120 C, above the intake air temperature.

As the mass of air passing the wire increases, so the current required to maintain the temperature also increases.

This increase in current is sensed by the ECM.

This is the primary signal, used in conjunction with signals from the other system sensors, for determining the correct amount of fuel to be supplied to the engine.

A "burn-off" facility operates when the system is switched off, to remove dirt from the filament.

Mass air flow (MAF) sensor hot-film type:-

Similar to the hot-wire type, but with the measuring element incorporated in ceramic plate. This arrangement obviates the need for a "burn-off" facility.

Manifold absolute pressure (MAP) sensor:-

Connected by vacuum hose to the engine inlet manifold. Usually located in the engine bay, or built into the ECM casing.

Operation:-

The sensor consists of a diaphragm and piezo-electric circuit which varies the resistance in proportion to the pressure in the manifold.

The sensor has a 5 volt supply and sends a signal voltage to the ECM which is proportional to the manifold pressure.

The ECM uses the voltage signals from the MAP sensor to adjust the signals to the injectors.

Idle air control (IAC) valve:-

The auxiliary air valve allows air to by-pass the throttle valve when the engine is cold, to increase idle speed during the warm-up period. Idle speed control devices are usually fitted into a throttle by-pass system, allowing extra air to enter the engine. This extra air complements the mixture enrichment during a cold start and the subsequent warm-up period. Some single point systems have a motorised actuator, which opens or closes the main throttle valve, by very small amounts, to control he idle speed.

Operation:-

The valve is mounted on the engine in a position where it is in direct contact with engine temperature.

The valve incorporates an electrically heated bi-metal strip which determines the cut-off point as the engine reaches normal operating temperature.

Idle air control (IAC) valve solenoid type:-

The auxiliary air valve allows air to by-pass the throttle valve when the engine is cold, to increase idle speed during the warm-up period.

Operation:-

The valve is mounted on the engine in a position where it is in direct contact with engine temperature.

The valve incorporates an electrically heated bi-metal strip which determines the cut-off point as the engine reaches normal operating temperature.

The solenoid idle air control actuator allows air to by-pass the throttle valve to govern the idle speed, under all conditions.

Operation:-

It is controlled directly by ECM, which interprets signals from sensors such as RPM and coolant temperature to maintain the idle speed at a pre-determined level. The solenoid acts against a spring, either with a rotary or linear action.

Idle air control (IAC) valve rotary type:-

The rotary idle air control actuator allows air to by-pass the throttle valve to govern the idle speed, under all conditions.

Operation:-

The device consists of rotary valve, in the by-pass air way, mounted on the shaft of a special electric motor.

The motor has two sets of armature windings and is limited to an angular movement of 90C.

The ECM supplies an alternating voltage to the two windings, depending on signals from sensors such as engine RPM and coolant temperature.

The alternating voltages exert opposing forces on the armature resulting in the valve taking up a position corresponding to the voltage radio.