Switches; Diodes - MTD 188 Professional Shop Manual

Switches

To test a switch:
1. Remove the switch from the circuit.
2. Set the DVOM to the ohms scale (
3. With the switch in the off position touch one
probe to one of the tabs and the other probe to
the other tab.
4. For a NC switch there should be zero resistance.
For a NO switch the meter should show an open
circuit.
5. With the probes still attached, turn the switch on.
The readings should reverse, NO should have
zero resistance and a NC switch should indicate
an open circuit.
6. Repeat for all the circuits in the switch
7. If the switch fails in any of the tests, replace the
switch.

Diodes

A diode acts like a one way valve. They allow current to
flow in only one direction. There will be a band on one
end of the diode. This indicates the negative side of the
diode. Most DVOMs have the ability to test a diode.
See Figure 2.8.
Silver band
(-)
electrons flow from the negative to the positive.
Figure 2.8
Testing a diode:
1.
2.
Ω).
3.
4.
(+)
7
Isolate the diode in the circuit.
Set the DVOM to
See Figure 2.9.
Diode scale
Figure 2.9
NOTE: A diode requires and uses a small
amount of voltage before it allows current to fol-
low through it. When testing a diode the Ohms
(Ω) scale can be used, but it will read resistance
in both directions. One side will be higher then
the other. the reading are hard to interpret with-
out the datasheets of the diode, therefore the
diode scale is the preferred method of testing.
Attach the negative lead of the DVOM to the side
of the diode with a band on it.
Place the positive lead on the other side of the
diode.
ELECTRICAL SYSTEM
the diode scale.