TM 9-2320-273-20
5-5. INTRODUCTION (Continued).
(2) Genera/ Approach. First, isolate to the subsystem level (in cases where more than
one subsystem IS involved); next, isolate the problem to a single circuit within the subsystem; then
isolate the problem to the faulty component. An example of-an exception to this rule would be in
cases where a lamp is out; the first step would be to check the bulb.
(3) Wiring. Frayed, broken, loose, or corroded wiring is a common source of problems
in any electrical circuit. Always make a visual inspection before starting detail troubleshooting.
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Observe in particular, contacts to ground. Components with case grounds are especially troublesome.
(4) Circuit Breakers.
Never jumper a cycling circuit breaker unless specifically instructed to do
so in the troubleshooting procedure. The overload could cause serious
damage to equipment and result in a fire.
The circuit breakers in these circuits are the automatic recycling type. If an overload exists, the
circuit breaker will open and shut off all circuits being fed by that particular circuit breaker. After
cooling, the circuit breaker will close. If the problem still exists the circuit breaker will again open.
This cycle will continue until the problem is corrected. When a circuit breaker is cycling, it will
feel warm to the touch and you can hear a clicking sound.
(5) Re/ays. A relay is basically a two-element device – a coil and a set of contacts in a
common housing. When voltage is applied to the coil, the contacts close (normally open relay) or
open (normally closed relay). The relay provides a path of supply voltage to a circuit or component
which draws a heavy current load. In troubleshooting, first be sure the coil is functioning. Connect
a multi meter between frame ground and the hot side of the coil and check for 12 volts dc nominal.
If voltage is present, disconnect the multi meter and measure voltage across any set of contacts with
the coil voltage present. The meter should indicate either 12 or 24 volts dc nominal depending on
the circuit involved. Sometimes it is advantageous to simply jumper across the relay contacts and
if the inoperative circuit being checked works, a bad relay or voltage supply wiring is indicated.
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