Types of Sensors
A description of the types of sensors follows.
A thermistor is a type of sensor that changes its electrical resistance with temperature to which it is being exposed. The electrical resistance of the thermistor decreases as temperature increases and increases as temperature decreases. The sensor, in conjunction with a current limiting resistor in the ECM, forms a voltage divider network that provides a voltage signal that will be matched with a particular temperature value.
The top half of the voltage divider is the current limiting resistor internal to the ECM. A thermistor sensor has two electrical connections, signal return and ground. The output of a thermistor sensor is non linear analog signal. Examples are:
- EOT, Engine Oil Temperature
- ECT, Engine Coolant Temperature Sensor
- IAT, Intake Temperature Sensor
A potentiometer is a variable voltage divider used to sense the position of a mechanical component. A reference voltage is applied to one end of the potentiometer. Mechanical motion connected to the wiper causes it to move along the resistance material in a rotary fashion. The voltage on the wiper changes at each point along the resistive material. This voltage is proportional to the amount of mechanical movement. An example is:
- APS, Accelerator Position Sensor
Variable Capacitance Sensor
Variable capacitance sensors are used to measure pressure. The pressure that is to be measured is applied to a ceramic material. The pressure forces the ceramic material closer to a thin metal disk. This action changes the capacitance of the sensor.
These sensors are connected to the ECM by three wires: VREF, signal return and signal ground.
The sensor receives the VREF and returns an analog signal voltage to the ECM. The ECM compares the voltage with pre-programmed values to determine the pressure.
The operational range of the sensor is linked to the thickness of the ceramic disk. The thicker the ceramic disk, the more pressure the sensor can measure. Examples are:
- EOP, Engine Oil Pressure Sensor
- ICP, Injection Control Pressure Sensor
- MAP, Manifold Absolute Pressure Sensor
Hall Effect Sensor
The Hall Effect Sensor is an electronic device that generates voltage. The shape and frequency of this signal depend on how the magnetic field of this sensor is disturbed.
The camshaft position sensor is an example of a hall effect sensor. It contains a permanent magnet, transducer, signal conditioner, and a switching transistor. The permanent magnet of the sensor generates a magnetic field around the transducer and the timing sensor disk mounted to the camshaft gear. As the magnetic field is disturbed by the rotation of the timing disk (vanes and windows), the transducer generates a signal that is filtered and conditioned by the internal CMP sensor signal conditioner.
Once the signal has been conditioned, it is applied to the base of the switching transistor that causes the transistor to switch on and ground the 5 volt line from the ECM. At this time, the ECM no longer senses the 5 volt reference signal.
Each time a vane passes, no signal is generated by the transducer. This action causes the transistor to shut off and allows the ECM to see its 5 volt reference signal.
This switching action allows the ECM to determine crankshaft position and engine speed that is required by the ECM to control engine operating parameters such as injector timing, duration (injection pulse width) and injection control pressure.
This sensor is connected to the ECM by three wires: a 5 volt signal supply, a 5 volt power feed for the signal conditioner VDREF and signal ground.
A magnetic pickup is a sensor used to generate an alternating frequency that indicates speed. Magnetic pickups normally have a two-wire connection for signal and ground. This sensor is constructed with a permanent magnetic core surrounded by a wire coil. The signal frequency is generated by the rotation of gear teeth that disturbs the magnetic field created by the magnet. For example: See Figure
“Magnetic Pickup (Vehicle Speed Sensor)”
- VSS, Vehicle Speed Sensor
1. Transmission Case
4. Permanent Magnet Field
2. Output Signal
5. 16-teeth Speedometer Gear
3. Magnetic Pickup Sensor
Figure 1. Magnetic Pickup (Vehicle Speed Sensor)
Switch sensors are used to indicate positions. They operate either open or closed, allowing or preventing the flow of current. A switch sensor can be either a voltage input type switch or a grounding type switch. A voltage input type switch will supply the ECM with a voltage when it is closed, while a grounding type switch will ground the circuit closed, causing a zero voltage signal. Grounding type switches are usually installed in series with a current limiting resistor.
This sensor is connected to the ECM by two wires: a 12 volt signal and a signal ground. These sensors are considered to be a low speed digital input. Examples are:
- IVS, Idle Validation Switch
- BNO, Brake Normally Open
- CLS, Coolant Level Switch
- DDS, Driveline Disengagement Switch
|Series 40E Troubleshooting Manual – 6SE241|
|Generated on 10-13-2008|