If you look at sensors on a vehicle, the majority use a 5-volt reference. It doesn’t matter if the sensor detects pressure, temperature or movement.
The basics of the circuit are that the sensor is typically supplied with 5 volts from a module. The ground used by the circuit might be a signal ground that goes through the module, but some use the chassis as a ground. Both the power and ground are “clean” current with very little noise, changes in voltage or stray AC voltage. The third wire coming from the sensor is the signal or return line.
Power is important for turning electrical motors, lights and other loads. But, power is also important for “solid state” or “binary” communications and sensing between modules and sensors. If the voltage levels are not within specifications, it can cause a lot more issues than a slow crank.
Serial networks switch power on and off at specific intervals to transmit information to other modules on a network. Some networks like a LIN bus can use voltages between 9 to 15 volts. Other networks like a CAN bus might use a range of 1 to 6 volts. If the power is lower than set levels, the networks can no longer communicate. Also, many sensors operate on a 5- to 7-volt reference voltage. If the reference voltage drops, so does the accuracy of the sensor.
This is a CAN bus waveform. Information is shared by pulsing power on and off at set intervals to transmit information.
The sensor can then change the resistance of the circuit to change the 5-volt reference voltage, thanks to Ohm’s law. In the case of a position sensor, the passing of a metal tooth or magnet on an encoder ring will cause the reference voltage to turn on and off.
Since the module generates the reference voltage, it can properly evaluate the return or signal voltage.
But it is a two-way street. If the circuit has no integrity for the reference voltage, ground or return/signal circuits, the sensor will not work accurately.
Testing
In most cases, the first step is identifying the circuit you are dealing with using the wiring diagram. In the case of a temperature sensor, these are usually a negative temperature coefficient thermistor. The electrical resistance decreases when the temperature increases.
An engine coolant temperature sensor is connected to the main computer (powertrain control module or PCM). The PCM supplies a reference voltage (typically 5 volts) and constantly monitors the ECT sensor signal. Some of these sensors use a two-wire connector with one wire being a reference and the other is signal return. The ground for the sensor is through the threads and body.
Corrosion in the connector might not change a 12-volt power feed, but corrosion can prevent a sensor using a 5-volt reference from accurately communicating.
Some PIDs seen on a scan tool can show reference and return voltages. This is how the ECM sees the values of the reference and signal voltage on the circuit board. At the sensor, the reference voltage might be lower if there is an issue with the wiring or connector. If the voltage is higher than 5 volts, it might be a short to power. The same is true for the return or signal voltage.
Many circuits can go into a self-preservation mode if an open or short is detected. With the sensor disconnected, the ECM might cut power to the circuit to prevent internal damage to the module. The great thing is that the system will set a code for the circuit to help you focus your diagnostics.
Temperature and Pressure
Since many sensors use changes in resistance to show pressure and temperature, you can test the resistance in the circuit at the sensor or the connector on the ECM. The service information might provide you with an Ohm reading for the sensor. However, in some cases, the data PID might give you a better indication of whether the sensor is operating as it should.
A 5-volt reference is used to generate the signal of the crankshaft position sensor.
Position
Position sensors use a 5-volt reference. The magnetoresistive sensors have a small transistor circuit embedded in the sensor that detects the passing of a tooth or window. The faster a crankshaft turns the more “switches” from 0 to 5 volts there are for a given period.
You can’t check the resistance of the sensor. But you can check the power and ground at the sensor. If the sensor is receiving 5 volts, you back-probe the circuit connecting to the signal and ground and observe the waveform.
Passive or inductive sensors have a permanent magnet with a coil around it. The magnetic field strength changes when a magnetism-sensitive object passes through the magnetic field of the magnet. This changing of the magnetic field induces a voltage in the coil. The polarity of the induced voltage depends on the direction of the moving object, moving away or toward the sensor. This sensor doesn’t need a 5-volt reference.
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