Lull 944E-42 Service Manual - Manual (Page 208)

Electrical System
9-50 944E-42
9.13.3 Troubleshooting with a Digital
Multimeter (DMM)
Note: Circuits which include any solid-state control
modules should only be tested with a ten megohm or
higher impedance digital multimeter (DMM), or damage
could occur to the module.
A DMM should be used instead of a test lamp in order to
test for voltage in high impedance circuits. While a test
lamp shows whether voltage is present, a DMM indicates
how much voltage is present.
The ohmmeter function on a DMM shows how much
resistance exists between two points along a circuit. Low
resistance in a circuit means good continuity.
Note: Disconnect the power feed from the suspect
circuit when measuring resistance with a DMM. This
prevents incorrect readings. DMM’s apply such a small
voltage to measure resistance that the presence of
voltages can upset a resistance reading.
Diodes and solid-state components in a circuit can cause
a DMM to display a false reading. To find out if a
component is affecting a measurement take a reading
once, then reverse the leads and take a second reading.
If the readings differ, the solid-state component is
affecting the measurement.
Following are examples of the various methods of
connecting the DMM to the circuit to be tested:
• Back probe both ends of the connector and either
hold the leads in place while manipulating the
connector or tape the leads to the harness for
continuous monitoring while you perform other
operations or test driving.
Note: DO NOT back probe a sealed connector or a flat
wire connector.
• Disconnect the harness at both ends of the
suspected circuit where it connects either to a
component or to other harnesses.
9.13.4 Probing Electrical Connectors
a. Front Probe
Disconnect the connector and probe the terminals from
the mating side (front) of the connector.
Note: DO NOT insert test equipment probes into any
connector or fuse block terminal. The diameter of the
test probes will deform most terminals. A deformed
terminal can cause a poor connection, which can result
in system failures. Always use a connector test adapter
kit or a flat wire probe adapter kit (usually supplied with a
DMM) when front probing terminals. DO NOT use paper
clips or other substitutes as they can damage terminals
and cause incorrect measurements.
b. Back Probe
Note: DO NOT back probe a sealed connector or a flat
wire connector.
DO NOT disconnect the connector and probe the
terminals from the harness side (back) of the connector.
• Back probe connector terminals only when
specifically required in diagnostic procedures.
• Back probing can be a source of damage to
connector terminals. Use care in order to avoid
deforming the terminal, either by forcing the test
probe too far into the cavity or by using too large of a
test probe.
• After back probing any connector, inspect for
terminal damage. If terminal damage is suspected,
test for proper terminal contact.
c. Measuring Voltage
Testing for Voltage Drop
This test checks for voltage being lost along a wire, or
through a connection or switch.
1. Connect the positive lead of a DMM to the end of the
wire (or to one side of the connection or switch)
which is closer to the battery.
2. Connect the negative lead to the other end of the
wire (or the other side of the connection or switch).
3. Operate the circuit.
4. Voltage at the wire should be displayed.