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How it works

ABS Wheel Speed Sensors

An anti-lock brake system (ABS) wheel speed sensor measures road-wheel speed and direction. This information is used for anti-lock brake (ABS), traction control (TCS) or vehicle stability (ASP) system ECUs to prevent the vehicle’s wheels from locking or spinning. This increases the driver’s control of the vehicle.

How it works:
There are two basic types of wheel speed sensor, passive and active;

1. Passive sensors;
These are similar in operation to Inductive cam/crank sensors; they consist of a soft iron, permanent magnetic pin around which is wound a fine copper wire coil. The unit fits in close proximity to a rotating trigger wheel. The trigger wheel is mounted onto a rotating assembly, such as the disc, drum or hub. When the trigger-wheel rotates past the pick-up assembly the alternating teeth generate a small inductive voltage in the copper windings. This voltage signal is used by the ECU as wheel speed information. Passive sensors generally only have two wires, one signal and one earth/shield.

2. Active sensors;
The operation of the active sensor can be likened to the Hall type sensor found in distributors etc. The pick-up assembly has an inbuilt amplifier and thus relies on a supply voltage which is normally 5v but it can be 12v.

The rotating element consists of a multi-pole (north, south, north, south …) magnetic ring, which can be located onto a rotating assembly as with the passive sensor. There is an increasing trend to incorporate it into the wheel bearing seal and to use magnetic powder instead of fixed magnets.

The rotating, alternating, magnetic poles generate a magnetic flux within the sensor element which then amplifies and regulates the signal for the ECU to use as wheel speed information. The output of an active sensor is digital with a square wave signal, it is capable of sending wheel speed information down to 0mph, whereas the passive sensor’s accuracy is usually dubious below, 25mph. Active sensors generally have three wires; one power supply, one signal and one earth/shield.

Knock Sensor
The knock sensor responds to spark knock caused by over advanced timing. The knock sensor is mounted directly on to the side of the cylinder block, they “listen” for engine pinking and send an oscillating voltage signal to the ECU. The ECU uses this information to control the ignition timing.

Pinking is a phenomenon caused by the explosion in the combustion chamber taking place too early. When this occurs the flame front within the combustion chamber collides with the still rising piston causing the characteristic pinking or pinging noise which sounds like many marbles dropping onto a steel plate. In mild cases it results in decreased engine power, in severe cases it results in major engine damage.

The more advanced the ignition timing is set on an engine the higher the theoretical power, up to a limit. The limit is generally just before the point of pinking. It is the job of the Knock sensor, in conjunction with the ECU, to keep the ignition timing at this peak setting.

The ECU will advance the ignition timing until pinking is detected then retard it by, say 10 degrees, this process is repeated many times per second. Some ECUs have the capability of advancing and retarding the timing individually per cylinder, that is, a four cylinder engine may have four different ignition advance settings. The timing setting achieved is a variation on a timing “map” within the ECU, which takes into consideration parameters such as engine speed, load and temperature.

The Knock sensor can also have a secondary role. The point at which pinking occurs is also the point at which peak NOx emissions are generated. By accurately controlling the timing, the emissions of an engine can be reduced.

How they work:
The sensing element consists of a piezo-ceramic element and a seismic mass which is clamped into place by the locating bolt. The sensor has a specific exciting frequency which is matched to the frequency vibration band within which pinking occurs

Throttle Position Sensor (TPS) / Throttle Potentiometer

A throttle position sensor (TPS) (also called throttle potentiometer) is a sensor used to monitor the position of the throttle in an internal combustion engine. The sensor is usually located on the butterfly spindle so that it can directly monitor the position of the throttle valve butterfly.

The sensor is usually a potentiometer, and therefore provides a variable resistance dependent upon the position of the valve (and hence throttle position).

The sensor signal is used by the engine control unit (ECU) as an input to its control system. The ignition timing and fuel injection timing (and potentially other parameters) are altered depending upon the position of the throttle, and also depending on the rate of change of that position. For example, in fuel injected engines, in order to avoid stalling, extra fuel may be injected if the throttle is opened rapidly (mimicking the accelerator pump of carburetor systems).

More advanced forms of the sensor are also used, for example an extra closed throttle position sensor (CTPS) may be employed to indicate that the throttle is completely closed.

Some ECUs also control the throttle position and if that is done the position sensor is utilised in a feedback loop to enable that control.

Related to the TPS are accelerator pedal sensors, which often include a wide open throttle (WOT) sensor. The accelerator pedal sensors are used in "drive by wire" systems, and the most common use of a wide open throttle sensor is for the kickdown function on automatic transmissions.

Modern day sensors are Non Contact type, wherein a Magnet and a Hall Sensor is used. In the potentiometric type sensors, two metal parts are in contact with each other, while the butterfly valve is turned from zero to WOT, there is a change in the resistance and this change is resistance is given as the input to the ECU.

Non Contact type TPS work on the principle of Hall Effect, wherein the magnet is the dynamic part which mounted on the butterfly valve spindle and the hall sensor is mounted with the body and is stationary. When the magnet mounted on the spindle which is rotated from zero to WOT, there is a change in the magnetic field for the hall sensor. The change in the magnetic field is sensed by the hall sensor and the hall voltage generated is given as the input to the ECU. Normally a two pole magnet is used for TPS and he magnet may be of Diametrical type or Ring type or segment type, however the magnet is defined to have a certain magnetic field.

Oxygen Sensor / Lambda Sensor


The oxygen sensor (also called lambda sensor) sends a signal to the engine computer based on the amount of oxygen in the exhaust gas. This signal is used by the engine ECU to fine-tune the mixture to the optimum level for maximum catalyst efficiency and longevity. A worn-out oxygen sensor can cause excessive gasoline consumption, elevated exhaust emissions, accelerated catalytic converter damage failures and cause engine performance problems such as surging and hesitating.

Automotive oxygen sensors, colloquially known as O2 sensors, make modern electronic fuel injection and emission control possible. They help determine, in real time, if the air fuel ratio of a combustion engine is rich or lean. Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine. But when information from oxygen sensors are coupled with information from other sources, they can be used to indirectly determine the air-to-fuel ratio. Closed-loop feedback-controlled fuel injection varies the fuel injector output according to real-time sensor data rather than operating with a predetermined (open-loop) fuel map. In addition to enabling electronic fuel injection to work efficiently, this emissions control technique can reduce the amounts of both unburnt fuel and oxides of nitrogen from entering the atmosphere. Unburnt fuel is pollution in the form of air-borne hydrocarbons, while oxides of nitrogen (NOx gases) are a result of combustion chamber
tempuratures exceeding 2000 deg. F due to excess air in the fuel mixture and contribute to smog and acid rain. Volvo was the first automobile manufacturer to employ this technology in the late 1970s, along with the 3-way catalyst used in the catalytic converter.

Modern spark-ignited combustion engines use oxygen sensors and catalytic converters as part of an attempt by governments working with automakers to reduce exhaust emissions. Information on oxygen concentration is sent to the engine management computer or ECU, which adjusts the amount of fuel injected into the engine to compensate for excess air or excess fuel. The ECU attempts to maintain, on average, a stoichiometric air-fuel ratio by interpreting the information it gains from the oxygen sensor. The primary goal is to lower the levels of certain by-products in the exhaust stream, namely hydrocarbons (which are released when the fuel is not burnt, i.e. in when misfiring), carbon monoxide (which is the result of running rich) and NOx (which dominate when the mixture is lean). Failure of these sensors, either through normal aging, the use of leaded fuels, or fuel contaminated with silicones or silicates, for example, can lead to damage of an automobile's catalytic converter and expensive repairs.

Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental to emissions control and can even damage the vehicle. When the engine is under low-load conditions (such as when accelerating very gently, or maintaining a constant speed), it is operating in "closed-loop mode." This refers to a feedback loop between the ECU and the oxygen sensor(s) in which the ECU adjusts the quantity of fuel and expects to see a resulting change in the response of the oxygen sensor. This loop forces the engine to operate both slightly lean and slightly rich on successive loops, as it attempts to maintain a stoichiometric ratio on average. If modifications cause the engine to run moderately lean, there will be a slight increase in fuel economy, sometimes at the expense of increased NOx emissions, much higher exhaust gas temperatures, and eventual misfires at ultra-lean air-to-fuel ratios. If modifications cause the engine to run rich, then there will be a slight increase in power, but at the risk of decreased fuel economy, and an increase in unburned hydrocarbons in the exhaust which causes overheating of the catalytic converter. Long-term operation at very rich mixtures can cause catastrophic failure of the catalytic converter (see backfire). The ECU also controls the spark engine timing along with the fuel injector pulse width, so modifications which alter the engine to operate either too lean or too rich may result in inefficient fuel consumption whenever fuel is ignited too soon or too late in the combustion cycle.

When an internal combustion engine is under high load (e.g. wide open throttle), the output of the oxygen sensor is ignored, and the ECU automatically enriches the mixture to protect the engine. Any changes in the sensor output will be ignored in this open-loop state, as are changes from the air flow meter, which might otherwise lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.