An engine sensor is a device that changes resistance or voltage output with a change in a condition such as temperature, position, movement, etc. A modern electronic fuel injection system uses numerous sensors to improve efficiency. An Electronic Fuel Injection system might use many of the sensors. In the Fig below how’s some of the conditions sensed by one computer system. Note that it checks everything from charging voltage to air conditioning operation.
Throttle position sensor
A throttle position sensor is a variable resistor or multi position switch connected to the throttle valve shaft. When the driver presses on the gas pedal for more power, the throttle shaft and sensor are rotated. This changes the internal resistance of the sensor. The change in current signals the computer and the computer can alter the air-fuel ratio as needed. One type of throttle position sensor is illustrated in Fig.3.29. Note how it mounts on the throttle body and that it has several contacts to change output resistance.
Sensors, located in many different locations on engine, may be used to feed information to computer.
Note typical conditions sensed and controlled by electronic means. (Buick)
Throttle position sensor. A-Sensor is mounted on throttle body over throttle shaft. B-View of inside of sensor. As throttle shaft rotates, it turns plate which alters internal circuit connections and resistance, sending electric current signals to computer. (Toyota)
Engine Coolant Temperature Sensor
An engine coolant sensor monitors the operating temperature of the engine. It is mounted so that it is exposed to the engine coolant.
When the engine is cold, the sensor might provide a high current flow (low resistance). The computer would adjust for a richer air-fuel mixture for cold engine operation. When the engine warms, the sensor would supply information (high resistance for example) so that the computer could make the mixture leaner.
Water temperature sensor usually is threaded into opening in block or head where it will be in contact with coolant.
Inlet Air Temperature Sensor
Air flow meter shows air temperature sensor. Sensor uses thermistor which is extremely sensitive to temperature changes. As temperature rises, resistance of thermistor decreases. (Subaru)
An inlet air temperature sensor measures the temperature of the air entering the engine, Cold air is dense than warm air, requiring a little more fuel. Warm air is NOT as dense as cold air, requiring a little less fuel. The air temperature sensor helps the computer compensate for changes in outside air temperature and maintain an almost perfect air-fuel ratio.
Charge Temperature Sensor
unlike air temperature sensor which senses only coldness of air, charge temperature sensor checks temperature of the air-fuel mixture. It is located in intake port just before intake valve. (Chrysler)
A charge temperature sensor, similar to an inlet air temperature sensor, measures the temperature of the air-fuel mixture. It is installed in the intake port, in front of the engine intake valve.
Crankshaft Position Sensor
A crankshaft position sensor is used to detect engine speed. It allows the computer to change injector openings as engine speed changes. Higher engine speed generally requires more fuel.
This sensor can be located on the front, rear, or center of engine. Its tip is close to the crank so that it can sense the teeth or notches as they rotate past the sensor. The magnetic field around the sensor, and current flow through the sensor, change as the crank rotates, allowing the computer to measure engine rpm.
Flap Air Flow Sensor
A flap air flow sensor measures the air flow into the engine. This helps the computer determine how much fuel should be injected into the intake manifold.
The air flow sensor usually mounts ahead of the throttle body assembly in the air inlet duct system.
In the Fig. below shows how a typical air flow sensor operates. At idle, the sensor flap is nearly closed. Sensor resistance stays high. This tells the computer that the engine is idling and needs very little fuel.
Flap air flow sensor tells computer whether engine is idling or at higher speed by measuring amount of air moving into engine. A -At idle speed, air flap is nearly closed. Responding to small current, computer produces short injection pulse width for small amount of fuel. B-Throttle open for more power, air flap swings open and sends strong signal to computer. Computer then sends wide pulse width to injectors for richer fuel mixture. C-Potentiometer is attached to sensor shaft. Wiping arm moves across potentiometer as flap moves. Depending on flap position, potentiometer will send weak or strong signal to electronic control unit.
As engine speed and air flow increase, air forces the flap to swing open. This moves the variable resistor to the low resistance position. The increased current flow now tells the computer that more air is flowing into the engine. The computer then increases injector pulse width as needed.
In the Fig. below shows how the flap type air flow sensor and potentiometer (variable resistor) are connected. Note that a weak spring is used to return the flap to the closed, idle position.
Cutaway shows how air sensor flap connects to and controls potentiometer. Strength of signal to computer depends on where wiping arm rests on resistor of potentiometer. (Volkswagen)
Mass Air Flow Sensor
A mass air flow sensor performs about the same function as a flap type sensor, but it sends more precise information to the computer. In the Fig. below is a newer type sensor found on some late model cars.
Basically, the mass air flow sensor uses a small electrically energized, resistance wire to detect air flow. The wire's temperature drops as air flows over it. The greater the air flow, the lower its temperature. The drop in temperature changes the wire's resistance, signaling the computer of more air intake. The opposite is true for low air flow.
A mass air flow sensor, sometimes called a "hot wire" sensor, is desirable because it automatically compensates for changes in air temperature and atmospheric pressure. It eliminates the need for an air temperature sensor and air pressure sensor.
Modern electronic, multi-point fuel injection system. Mass air flow sensor is located in duct ahead of engine. It detects air flow volume with fine resistance wire. (Chevrolet)
Mass air flow sensor with resistance wire. Wire is heated by electric current and is very sensitive to temperature. The greater the air flow past it, the lower its temperature becomes. The lower its temperature, the more the signal to computer changes. (Chevrolet)
Oxygen sensor
Oxygen sensor detects amount of oxygen in engine's exhaust. (Oldsmobile)
The oxygen sensor or exhaust gas sensor measures the oxygen content in the engine's exhaust gases. The oxygen content is an excellent indicator of whether the air-fuel mixture is too rich or too lean. The oxygen sensor is one of the most important sensors in modern electronic fuel injection systems. It actually checks the efficiency of the fuel system with the engine running.
Oxygen Sensor Construction
The oxygen sensor has a special ceramic core made of zirconium dioxide. The surface of the ceramic core is coated with platinum. The coated ceramic core has the ability to produce a voltage output when exposed to heat and a difference in oxygen levels on each side of the ceramic element. The ceramic voltage-producing device is enclosed inside a metal housing. Terminals are provided for connecting the sensor to the computer wiring harness.
Oxygen sensor is placed in exhaust system either at exhaust manifold or in pipe leading from exhaust manifold, as shown here. (Renault and AMC)
Cutaway of exhaust sensor. Note how it operates. Graph at right shows how oxygen content changes voltage of sensor's signal to computer. (Fiat)
Oxygen Sensor Operation
When the sensor is cold, it produces no voltage. The system then operates on data preprogrammed into the computer. When the oxygen sensor is heated above about 300 °F (149°C), it begins to produce a voltage signal.
When the fuel mixture is too rich, there is a small amount of oxygen in the engine exhaust gases. This produces a large difference in the oxygen levels on each side of the ceramic sensing device. Negative oxygen ions flow through the ceramic device and a voltage output is produced for the computer, Fig.3.41A. About a ONE VOLT signal is fed to the computer. The computer can then shorten injector pulse width to lean the air-fuel mixture slightly.
When the engine's fuel mixture is too lean, there is an excess amount of oxygen in the engine exhaust. This reduces the difference in the oxygen levels on each side of the sensor's ceramic element. Very few oxygen ions flow through the sensor and the sensor's voltage output drops to a fraction of a volt. this signals the computer to increase injector pulse width. This helps maintain an almost perfect air-fuel mixture.
A -Small amount of oxygen causes high voltage signal from sensor. B - Large amount of oxygen reduces signal strength. (Volkswagen)
Idle Speed Control
Most Motronics control idle rpm by a combination of the idle-speed stabilizer and ignition timing. The stabilizer is described in the section on LH-Jetronic. Inputs include rpm, closed-throttle signal, and engine temperature. The control unit sends on-off or digital signals to the idle-speed stabilizer. Early Motronics use the auxiliary air valve to increase air flow during warm-up, also described in the L-Jetronic section. In these, cold-engine idle rpm is increased according to temperature; it is an open-loop system.
Idle-speed control by idle air bypass. Idle-speed stabilizer handles coarse rpm corrections.
Timing Signals: RPM, TDC
For the most accurate measure of engine timing and speed, Multi – pint EFI systems read the position of the crankshaft directly, instead of from the ignition system as in L-Jetronic. Special sensors, shown in Fig. below, pick up signals from the flywheel teeth. Taking RPM and TDC timing signals from the crankshaft avoids inaccuracies from gear-lash or belt-drive such as when rpm and timing are determined in a camshaft-driven distributor, causing "spark scatter".
The rpm sensor (also called the engine-speed sensor) is an inductive-pulse sender that picks up pulses from a toothed wheel, usually the flywheel. The rpm signal can be displayed on a scope just as it is sent to the control unit, one blip or spike for each tooth as shown in Fig. below. It is so accurate it can sense an rpm change while the crankshaft turns only a few degrees.
RPM sensor (1) sends engine-speed signals from flywheel teeth; TDC sensor (2) sends one pulse per passing of set screw (arrow) each crankshaft revolution.
The TDC, or reference-mark sensor (reference from cylinder 1 TDC), is triggered by a set screw on the flywheel. Each time the screw passes the TDC sensor, the sensor signals one blip for each crankshaft revolution as shown in Fig. below. Both sensors are magnetic, with a soft iron core that stores the magnetic field. When a tooth in the flywheel or the reference pin moves through the magnetic field, the change induces an electrical voltage in the winding. This voltage is the input signal to the control unit. The sensor is known as a passive diffusion-field sensor because it does not require a current supply.
RPM sensor scope pattern shows one pulse per flywheel tooth
TDC sensor scope pattern shows one pulse per crankshaft revolution
One of these flywheel sensors provides input of rpm to the control unit. The other sensor provides input of TDC reference. The air flow sensor provides input of engine load. From the control unit ROM, an output signal to the coil primary sets timing advance and dwell for the next spark firing. Some engines operate with only one sensor that combines both functions, using a toothed timing wheel instead of the flywheel. Two missing teeth signal TDC, as shown in Fig. below.
In some Motronics, special timing wheel replaces starter gear ring or is mounted on front of crankshaft. Single sensor picks up rpm from teeth, and picks up TDC from gap in teeth.
Other Sensors
Other sensors besides those just covered can be used in a computer control system. Some of them include sensors checking the operation of the transmission, air conditioning system, brake system, and emission control systems. When information is required on any of these sensors, refer to a service manual. It will give the exact details of the specific system. It will explain its operation and how the sensor should be tested or serviced.
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