Figure 2.2 Volt drop testing 14

14
Advanced automotive fault diagnosis
Normal faultfinding or testing techniques can
be applied to the sensors and actuators. For exam-
ple, if an ABS system uses four inductive type
wheel speed sensors, then an easy test is to meas-
ure their resistance. Even if the correct value were
not known, it would be very unlikely for all four to
be wrong at the same time so a comparison can be
made. If the same resistance reading is obtained
on the end of the sensor wires at the ECU then
almost all of the ‘inputs’ have been tested with just
a few ohmmeter readings.
The same technique will often work with ‘out-
puts’. If the resistance of all the operating wind-
ings in say a hydraulic modulator were the same,
then it would be reasonable to assume the figure
was correct.
Sometimes, however, it is almost an advantage
not
to know the manufacturer’s recommended
readings. If the ‘book’ says the value should be
between 800 and 900 ⍀, what do you do when
your ohmmeter reads 905 ⍀? Answers on a post-
card please … (or see Section 2.3.3).
Finally, don’t forget that no matter how 
complex the electronics in an ECU, they will 
not work without a good power supply and an
earth!
2.5.8 Sensor to ECU 
method
This technique is simple but very useful. Figure 2.4
shows a resistance test being carried out on a
component. ⍀
1
is a direct measure of its resist-
ance whereas ⍀
2
includes the condition of the
circuit. If the second reading is the same as the
first then the circuit must be in good order.
Warning
The circuit supply must always be off when car-
rying out ohmmeter tests.

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