Electrical System Troubleshooting; Introduction; Troubleshooting With A Digital Multimeter (Dmm) - JLG sky trak mmv Service Manual

Electrical System
10.7 ELECTRICAL SYSTEM
TROUBLESHOOTING
10.7.1

Introduction

The electrical system produces, stores and distributes
electricity in the operation of the vehicle.
An engine-driven, 24 volt, 70-amp alternator equipped
with an internal, solid-state voltage regulator produces
electricity. Current from the alternator charges the battery
and powers electrical system components.
The battery provides power for starting the engine and
supplements the output of the alternator during periods of
peak demand.
The vehicle is equipped with two separate, but connected
wiring harnesses, an engine harness and a cab harness.
A multiple or "bulkhead" connector at the end of each har-
ness joins the two together.
Each wire within a harness has color-coded insulation
and is marked with a number to aid in identifying and trac-
ing each circuit and in making proper connections. Fuses
and relays are included with the cab harness to help pro-
tect the electrical system and its components.
The main wiring diagrams in this section indicate electri-
cal circuits and components respective of their approxi-
mate location on the vehicle.
The information in this section has been developed from
the perspective that all wires are connected and routed
as they were when the vehicle left the factory. Take into
account any disconnected or re-routed wires before be-
ginning to diagnose a circuit. Refer to the wiring diagrams
to test circuits for continuity or for shorts using a digital
multimeter, analog multimeter, test light, ohmmeter, or
low-reading voltmeter.
The text discusses each circuit and component, as well
as component function and replacement. Many of the cir-
cuits have a separate diagram specific to that circuit's
function which can be used for troubleshooting purposes.
Those same circuits also have their own troubleshooting
flow chart. Keep in mind that the various systems on the
vehicle include both electrical and hydraulic components.
What at first may appear to be an electrical problem may
turn out to be a hydraulic problem.
Note: All of the main electrical schematics in this sec-
tion are shown in the OFF position. The individual circuit
diagrams are shown in the ON position.
10.16
10.7.2 Troubleshooting with a Digital
Multimeter (DMM)
IMPORTANT: Circuits which include any solid-state
control modules should only be tested with a ten
megohm or higher impedance digital multimeter, or dam-
age 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 re-
sistance exists between two points along a circuit. Low
resistance in a circuit means good continuity.
IMPORTANT: Disconnect the power feed from the sus-
pect 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 compo-
nent 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 con-
necting the DMM to the circuit to be tested:
• Backprobe 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.
• Disconnect the harness at both ends of the
suspected circuit where it connects either to a
component or to other harnesses.
Model MMV Rev. 12-04