354 Section 3 Fundamentals of Electricity and Electronics
(two conditions). You may have to remove the unit
so you can control its temperature using a hot plate
or a heat gun. You can also use a thermometer to
measure the temperature of the switch while it is
installed in the engine. If the resistance values are
not within specifi cations for specifi c temperatures,
the temperature switch has failed.
A quick way of checking a temperature switch
and its circuit is to bypass it. Use a jumper to con-
nect the two leads that go to the temperature switch.
This should make the temperature light glow or
temperature gauge read fully hot or cold, depend-
ing on gauge design.
CAUTION
Never bypass a sensor switch unless
you are sure the switch is designed
to have almost zero resistance when
closed. If a normally operating sensor is designed
to have some resistance, even when closed, you
could short out the circuit and cause circuit damage
when testing.
Testing Relays
Relay problems usually result from worn, burned,
or sticking contact points that prevent normal cir-
cuit operation. The small coil windings in a relay can
also break, preventing the points from closing.
Many circuits are controlled by relays. Always
look for relays in wiring diagrams and check them
if power is not reaching a circuit on the output side
of a relay.
Relays can be located almost anywhere on a vehi-
cle—in the engine compartment, under the dash, on
a fuse box, or under the rear seat cushion. The ser-
vice manual will give relay locations for the vehicle
being repaired.
To test a relay, fi rst make sure a voltage signal is
being fed to the relay windings. You can check this
Figure 22-27. These are some examples of resistance
measurements. A—A wire conductor should have low
resistance. B—A coil is basically a conductor wound in a spiral.
It should have very low resistance. C—The ohmmeter should
show a high resistance when connected in one direction
across a diode. It should show low resistance in the other
direction. This shows that the diode is functioning as a one-way
electric check valve.
Figure 22-28. This ohmmeter is being used to measure the
internal resistance of a resistor pack. The leads must be
disconnected so that other components do not affect the
readings. Compare this to Figure 22-26.
Broken wire
no continuity
Wire has
continuity
A Checking Wire Continuity
Diode forward bias Diode reverse bias
C Checking Diode
Low resistance
shows continuity
Infinite resistance
indicates open
B Checking Coil
~V
OFF
V
Ω
mA
mA
A
~
~V
OFF
V
Ω
mA
mA
A
~
∞
0.00Ω
~V
OFF
V
Ω
mA
mA
A
~
0.01Ω
~V
OFF
V
Ω
mA
mA
A
~
∞
~V
OFF
V
Ω
mA
mA
A
~
~V
OFF
V
Ω
mA
mA
A
~
0.01Ω
Ω
LO
OFF
MED
HI
1 = 4 Ω
2 = 2 Ω
3 = 0 Ω
Blower switch Resistor pack Blower
motor
1
2
3
~V
OFF
V
Ω
mA
mA
A
~
4.00
Ω
Disconnect from
blower motor
Disconnect from
switch to isolate
from power