Post on 03-Jun-2018
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Computer Input
(Sensor and Actuator)
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Sensor
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Thermistor
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A thermistoris made of semiconductor material whose resistance varies inversely
with temperature. For example, at -40C a typical coolant sensor has a resistance of
100,000 ohms. The resistance decreases to about 70,000 ohms at 130C.
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Using the circuit shown in Figure above, if the
value of the fixed resistor is 10K ohms and the
value of the thermistor is also 10K ohms, the
voltage sensing circuit will read a voltage value of
2.5 volts. If the thermistor is a NTC, as the
ambient temperature increases its resistance
decreases. If the resistance of the NTC is now 8K
ohms, the voltage reading by the voltage sensing
circuit will now be 2.22 volts. As ambienttemperature increases and the NTC value
continues to decrease, the voltage sensing circuit
will measure a voltage decrease (Figure 10-3). If
the thermistor was a PTC, the opposite would be
true and the voltage sensing circuit would
measure an increase in voltage as the ambient
temperature increases.
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Temperature Sensor TestingThe temperature sensor can be tested while they are installed in the system by backprobing
the terminals to connect a digital voltmeter or lab scope to the sensor terminals. The sensor
should provide the specified voltage drop at any temperature. The scope trace of an NTCthermistor should indicate a smooth transition from a high voltage (about 4.0 volts) to a low
voltage as the sensor warms.
Consider Figure above as an example for circuit diagnosis. Simply disconnect the
sensor and observe the voltage on the scan tool. The displayed voltage should
equal that used by the control module circuit. If the voltage displayed by the
scan tool is 0, the problem is either the signal circuit is shorted to chassis ground
or to the sensor ground circuit, or a faulty control module.
If the specified voltage is present with the sensor unplugged, use a jumper wire
to connect the two terminals at the sensor harness connector. The scan tool
should display 0 volts.
If the voltage is still 5 volts, connect the signal circuit wire to chassis ground. If
the reading is now 0 volt, the sensor ground circuit is open. While doing these
steps if the voltage is above 0 volts (but less than 5 volts), this indicatesresistance in the circuit.
0 V
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Intermittent faults may be located by
using a lab scope or the data-recording
function of the scan tool. Since these
instruments indicate a change in
voltage over time, any change that
occurs on the trace that cannot
actually happen in the time frameindicates a problem with the circuit.
For example, engine temperature
cannot increase 80 degrees in 2
seconds. Since the ECT sensor is used
by some body systems (such as air
conditioning), a fault with this sensor
may result in many different customer
concerns being expressed. Figure 10-7
is a data recording of a normally
operating ECT and Figure 10-8 is a data
recording of an intermittent fault.
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Diagnosting PTC Circuit Protection
Device
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Pressure Sensor
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Pressure SwitchPressure switches will usually use a diaphragm that works against a calibrated spring or other form of
tension (Figure 10-6). When pressure is applied to the diaphragm that is of a sufficient value to
overcome the spring tension a switch is closed. Current that is supplied to the switch now has a
completed path to ground. In a very simple warning light circuit, the closed pressure switch completed
the circuit for the bulb and alerts the driver to an unacceptable condition. For example a simple oil
pressure warning lamp circuit will use a pressure switch.
Computer monitored pressure switch circuits use the change in voltage as an indication of pressure.
When the pressure change (either from low to high or high to low) changes the state of the switch, the
voltage change is interpreted by the computer (Figure 10-7). Pressure switches are used to monitor the
presence of pressure that is above or below a setpoint; they do not indicate the exact amount of
pressure being applied.
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Piezoresistive Device
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Wheatstone Bridge
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Position and Motion Detection Sensor
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PotentiometerA common position sensor used to monitor linear or rotary motion is the
potentiometer. A potentiometer is a voltage divider that provides a variable DC
voltage reading to the computer. These sensors are typically used to determine the
position of a valve, air conditioning unit door, seat track, and so on.
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Magnetic Pulse Generator
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Hall-Effect Sensors
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Hall-Effect Sensors
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Hall-Effect Sensors
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Hall-Effect Sensors
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Hall-Effect Sensors