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Quick-Check
Sensor Testing Procedures
To quickly
check Sensors we recommend the following: |
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Sensor Testing
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.
PICK-UP COIL, CRANK SENSOR,
ANTI-LOCK BRAKE SENSOR
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AC voltage produced
by an induction-type sensor will vary in proportion to
engine/wheel speed.
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Devices that
produce AC will usually have two wires (+ & -)
carrying the AC signal.
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Many service
manuals give specifications for resistance (ohms) testing of these
sensors.
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Resistance testing
alone is incomplete. A weak magnet, producing less voltage will
not be revealed.
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Using the DMM to
verify voltage during actual engine operation/wheel turning is a
more reliable test.
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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.
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The most
reliable test of an induction-type pulse generator sensor is
made using a labscope. We recommend the ADL7100 or ADL7000
labscope for this type of testing.
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Magnetic Type Distributor Pickup Testing
Using a Labscope
HOOKUP:
- Magnetic type sensors
always have two leads.
- One is positive, the
other negative.
SCOPE SETTINGS:
| VOLTS/DIV |
2V/div
AC |
| TIME/DIV |
20mS/div |
| SLOPE |
Rising
Edge |
| Trigger
Level |
.2V |
Notice in fig. 1 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:
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A chipped or broken sensor
producing an irregular waveform.
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A bent shaft causing a varying
waveform.
- Poor connections resulting in an intermittent open.
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Fig. 1
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Fig. 2 |
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Testing AC Voltage
Using a DMM
If a labscope is
unavailable, induction type sensors may be tested for AC voltage
output as follows:
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Set DMM to read “AC” Volts.
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Connect the black COM lead probe tip to a
sensor lead.
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Connect the red Volts lead probe tip to the
other sensor lead (polarity will not matter).
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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.
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Run the device being tested.
Observe the voltage reading and compare with
factory specifications or known good readings. |
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