JLG 260MRT Service Manual - Manual (Page 79)

SECTION 3 - GENERAL ELECTRICAL INFORMATION & SCHEMATICS
3121801 – JLG Lift – 3-13
3. Operate system such that the switch actuates. This could be going over a certain pressure or temperature, for exam-
ple. The state indicated in the control system should change.
Switch Wiring - Low Side, High Side
When controlling a load, a switch can be wired between the positive side of the power source and the load. This switch is
called a "high side" switch. The switch supplies the power to the load. When a switch is wired between the negative side
of the power source and the load, it is a "low side" switch. The switch provides the ground to the load.
A low side switch will allow voltage to be present on the load. No power is applied because the switch is stopping current
flow. This voltage can be seen if the measurement is taken with one test lead on the load and the other on the battery
negative side or grounded to the vehicle. What is actually being measured is the voltage drop across the switch. This
could mislead a technician into thinking the load is receiving power but not operating. To produce an accurate picture of
power or voltage applied to the load, measure voltage across the load’s power terminals. Also, the technician can mea-
sure the voltage at both power terminals with respect to battery ground. The difference between those two measure-
ments is the voltage applied to the load.
3.6 TROUBLESHOOTING
General
This section contains troubleshooting information to be used for locating and correcting most of the operating problems
which may develop in the aerial platform. If a problem should develop which is not presented in this section or which is not
corrected by listed corrective actions, technically qualified guidance should be obtained before proceeding with any mainte-
nance.
Troubleshooting Information
The troubleshooting procedures applicable to the aerial platform are listed and defined in following troubleshooting tables.
Each malfunction within an individual group or system is followed by a listing of probable causes which will enable determi-
nation of the applicable remedial action. The probable causes and the remedial action should, where possible, be checked
in the order listed in the tables.
It should be noted that there is no substitute for a thorough knowledge of the equipment and related systems.
It should be recognized that the majority of the problems arising in the machine will be centered in the hydraulic and electri-
cal systems. For this reason, every effort has been made to ensure that all likely problems in these areas are given the fullest
possible treatment. In the remaining machine groups, only those problems which are symptomatic of greater problems
which have more than one probable cause and remedy are included. This means that problems for which the probable
cause and remedy may be immediately obvious are not listed in this section.
The first rule for troubleshooting any circuit that is hydraulically operated and electrically controlled is to determine if the cir-
cuit is lacking hydraulic oil and electrical control power. This can be ascertained by overriding the bypass valve (mechani-
cally or electrically) so that oil is available to the function valve, then overriding the function valve mechanically. If the function
performs satisfactorily, the problem exists with the control circuit.
Hydraulic Circuit Checks
The first reference for improper function of a hydraulic system, where the cause is not immediately apparent, should be the
Troubleshooting Chart. The best place to begin the problem analysis is at the power source (pump). Once it is determined
that the pump is serviceable, then a systematic check of the circuit components, beginning with the control, would follow.
For aid in troubleshooting, refer to the Illustrated Parts Manual for hydraulic diagrams of the various circuits