Sensor Testing Procedures
To quickly check Sensors we recommend the following:
Measuring AC voltage produced by induction type sensors
Some sensors report rotational speed by generating AC voltage when two magnetic materials move toward and then away from each other. The pick-up coil in a distributor and many crank sensors and anti-lock brake sensors use this principle of operation.
Testing AC Voltage - Using a DMM
If a labscope is unavailable, induction type sensors may be tested for AC voltage output as follows:
1. Set DMM to read “AC” Volts.
2. Connect the black COM lead probe tip to a sensor lead.
3. Connect the red Volts lead probe tip to the other sensor lead (polarity will not matter).
4. OBSERVE SAFETY RULES - BE CERTAIN THAT WIRES AND LEADS, AS WELL AS YOUR CLOTHING AND HANDS, ARE CLEAR OF MOVING PARTS. WHEN CHECKING WHEEL SENSORS BE SURE WHEELS ARE FREE TO ROTATE AND NOT MOVE VEHICLE OR CAUSE DAMAGE.
5. Run the device being tested.
Observe the voltage reading and compare with factory specifications or known good readings.
Keep in mind that DMM’s average the voltage and will not reveal the details that a labscope will display.
Magnetic Type Distributor Pickup Testing - Using a Labscope
· Magnetic type sensors always have two leads.
· One is positive, the other negative.
VOLTS/DIV 2V/div AC
SLOPE Rising Edge
Trigger Level .2V
Notice that at 29Hz the cursor measurement shows peak-to-peak voltage of 6.1 volts.
· Voltage varies with RPM.
· More RPM = higher voltage.
· Less RPM = lower voltage.
As these sensors age and are exposed to heat and vibration, the intensity of the magnet can weaken resulting in lower peak-to-peak voltage. Eventually, the voltage may be too low to be recognized by the module or computer. The example in fig. 2 shows only 2.0 volts peak-to-peak voltage.
Other failure modes include:
· A chipped or broken sensor producing an irregular waveform.
· A bent shaft causing a varying waveform.
· Poor connections resulting in an intermittent open.
PICK-UP COIL, CRANK SENSOR, ANTI-LOCK BRAKE SENSOR
· AC voltage produced by an induction-type sensor will vary in proportion to engine/wheel speed.
· Devices that produce AC will usually have two wires (+ & -) carrying the AC signal.
· Many service manuals give specifications for resistance (ohms) testing of these sensors.
· Resistance testing alone is incomplete. A weak magnet, producing less voltage will not be revealed.
· Using the DMM to verify voltage during actual engine operation/wheel turning is a more reliable test.
· Because DMM voltage is averaged, the voltage can read as expected but not be the clean signal that is needed by the module or computer.
· The most reliable test of an induction-type pulse generator sensor is made using a labscope. We recommend the ADL7103 labscope for this type of testing.