JLG 4394RT Service Manual - Manual (Page 245)

SECTION 6 - GENERAL ELECTRICAL INFORMATION & SCHEMATICS
3121249 – JLG Lift – 6-13
Automatic Switches
If the switch is actuated automatically, by temperature or pressure for example, find a way to manually actuate the switch
to test it. Do this either by applying heat or pressure, for example, to the switch. These switches may need to be ener-
gized to actuate.
1. Connect instrumentation to monitor and/or control the parameter the switch is measuring.
2. Observe switch state in control system with the Analyzer. See vehicle or control system documentation on how to do
this.
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.