Lesson 3/Learning Event 1
The ohmmeter is also valuable for testing switches. Suppose you have a small on-off switch that
you think is not working, even though the lever seems to click normally. The switch has two
electrical terminals. To test the switch with an ohmmeter, first zero the ohmmeter as explained
above, then connect one test lead to each terminal of the switch. With the switch in the OFF
position, the ohmmeter hand should not move. Then move the switch lever to the ON position.
The ohmmeter hand should move to zero.
If the ohmmeter hand remained on zero in both positions, ON and OFF, what would this tell you?
(Remember what makes the hand move to zero: a complete circuit through the ohmmeter and
switch.) This tells you the switch is not working properly; it does not break (open) the circuit. On
the other hand, if the ohmmeter indicated infinity in both ON and OFF positions, the circuit is not
being closed. This means the switch is not making contact inside.
You now see how the ohmmeter can be used to locate open circuits caused by a defective switch or
a broken wire.
You can also use the ohmmeter to check for shorts or an unwanted grounding effect. Let's go
back to that simple on-off switch and check it to see if it is grounded. You know that the vehicle
chassis acts as one wire or part of the circuit. If the switch is mounted on metal, the circuit
through the switch must be insulated from the metal or the circuit will be grounded out. If the
insulation fails, due to cracks, breaks, wear, and so forth, the contacts or terminals touch the metal
frame and ground out. Test the switch for grounding by connecting one ohmmeter test lead to the
metal frame of the switch and the other to one terminal. The ohmmeter hand should not move if
the switch is not grounded. Then, change the switch position and look at the meter. Test both
terminals in this manner. If the ohmmeter hand moves from infinity, the switch is grounded.
You have a complete circuit or continuity from a terminal through the switch metal frame. The
switch must be replaced.
So far, we have tested using infinity and zero, but how about the numbers in between these two?
The numbers are used when you want to measure resistance.