SECTION 6 - GENERAL ELECTRICAL INFORMATION & SCHEMATICS
3121642 – JLG Lift – 6-11
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
energized 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 Ana-
lyzer. 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 tempera-
ture, for example. 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 measure the voltage at both power ter-
minals with respect to battery ground. The difference
between those two measurements is the voltage applied
to the load.
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