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TM 9-2320-304-14&P
4-10
Organizational Maintenance Instructions (Cont)
4-18. TROUBLESHOOTING INSTRUCTIONS (CONT).
(2) Continuity checks.
(a) Place the function/range switch in any Ohm range.
NOTE
Some meters show “1+m”, or simply “1” when function/range switch is in any Ohm position.
(b) Connect the red test lead to the Volt-Ohm connector and black lead to COM input
connector on the meter. When the test leads are separated or measuring an out-of-range
resistance, the digital display will indicate “OL” (Over Limit).
(c) Put one test probe at one end of the wire or circuit to be tested. Use the other test lead to
trace the circuit. When continuity is established, an Ohm symbol will appear in the upper
left corner of the digital display. If contact in the wire is maintained long enough (about
1/4 of a second), the OL will disappear and the resistance value of the wire or circuit will
appear next to the symbol.
(d) If your multimeter does not work in this manner, learn how it operates before performing
troubleshooting.
(3) Voltage measurements. The M1120 is equipped with 24-Vdc circuits. Troubleshooting
procedures will reference 24-Vdc measurement, however the value can vary. When the batteries
are fully charged, 25.2-Vdc can be measured on an open 24-volt circuit and 29-Vdc can be
measured when the engine is running at 1000 rpm.
(a) Connect the red test lead to the Volt-Ohm input connector and the black lead to the COM
input on the meter. If a DC-AC switch is present, make sure it is set to the DC position.
(b) Set the function/range switch to the desired volts position. If the magnitude of the voltage
is not known, set the switch to a range which will be able to read most voltages seen on the
truck (typically, a 200V range will do). Then reduce the range until a satisfactory reading
is obtained.
b. General Wire Test Procedures. M1120 troubleshooting isolates problems down to the components
that could cause a specific failure. When all of the components in a circuit are tested without isolating
a fault, the wires are the only other components that could be suspected of being damaged. Each wire
that must be tested may pass through two or more connectors. The following procedures provide
general instructions for testing electrical wires. These procedures will either attempt to measure a
voltage at the working end of a circuit or continuity from the power end of a specific wire to the
working end. Before either of these tests are performed, all connectors in the circuit must be checked
for looseness.
CAUTION
Use properly sized test leads and ensure care is used when checking for resistance,
continuity or voltage at connectors or damage to equipment may result.
(1) Wire voltage drop test.
(a) Disconnect connector from the component (light, relay, motor, etc.) at the working end of
the circuit.
(b) Check connector terminal(s) for damage; repair or replace connector as necessary.
(c) Set up truck conditions that will create voltage at the working end of the wire.
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