Chapter 24 Sensor, Actuator, Computer Service 475
electrical and mechanical problems. Like sensors, if an
actuator fails to function properly, the computer cannot
control the vehicle system.
Idle Speed Motor Service
A bad idle speed motor may not be able to maintain
the correct engine idle speed. Engine idle speed may be too
low or too high for conditions. The servo motor could have
shorted windings, opened windings, bad internal bearings,
or other problems that upsets its operation.
Figure 24-27 shows a common way to check the oper-
ation of the idle speed motor. Battery voltage is jumped to
specifi c terminals on the servo motor. This should make
the idle speed motor plunger retract and extend as the
connections are reversed. A faulty motor will usually not
function.
If the idle speed motor works when jumped to battery
voltage, check the wiring leading to the motor. The wiring
harness could have an open or short. A computer or relay
problem could also prevent motor operation.
Idle Air Control Valve Service
A bad idle air control valve will upset engine idle
speed like a bad idle speed motor. It uses solenoid action to
open and close an air passage bypassing the throttle plates.
In this way, it can increase or decrease engine idle rpm. If
failed, engine rpm will be constant and may not increase
with a cold engine or decrease as the engine warms to
operating temperature.
To check the idle air control valve, jump battery volt-
age to the windings. This should trigger the solenoid and
change engine speed. If the engine speed does not change,
check the output voltage to the idle air control valve before
removing the unit. An ohmmeter may also be recommended
to check the windings.
Air Vent Solenoid Service
A bad air vent solenoid can affect the operation of a
computer-controlled emission system and other systems
depending on its function. It usually controls venting of
the charcoal canister. Again, voltage can be jumped to
an air vent solenoid, or any solenoid, to check operation,
Figure 24-28.
Glow Plug Service
A bad glow plug will not heat up and the diesel engine
will be diffi cult to start when cold. Usually, the glow plug
will burn open and will not be able to conduct electricity.
Use an ohmmeter to measure glow plug resistance. This
can be done with the glow plug still in the cylinder head or
with it removed, Figure 24-29. Replace the glow plug if its
resistance is not within specs.
Figure 24-27. This actuator is an idle speed motor. To test it,
remove the wires to the unit and jump a battery to the recom-
mended terminals. Motor should run and reverse direction as
the jumpers are reversed.
4-Terminal
Harness
Connector
Idle Speed
Motor or
Actuator
End View of
Connector
Jumpers
Reversed
Battery
Reversed
*Never Connect
Voltage Source across
Terminals “A” and “B”
Jumpers
Plunger Out
(Extend)
Plunger In
(Retract)
Battery
A*
B*
C
D
A*
B*
C
D
Figure 24-28. Solenoid can also be tested by jumping volt-
age to the windings. This should activate the solenoid plunger
movement.
Figure 24-29 A glow plug is simply a resistive heating element.
Use an ohmmeter to measure resistance with the plug in or out
of the engine. A bad glow plug will usually be open and have
infi nite ohms. You can also use an ammeter to measure current
fl ow into each glow plug. An inductive ammeter can be slipped
over the wires and nothing has to be disconnected for testing.
No current fl ow usually means a bad glow plug.
Ohmmeter
Test Lead on
Plug Tip or Body
Glow
Plug
Test Lead
on Terminal
~V
OFF
V
Ω
mA
mA
A
~
∞