Automatic Switches; Switch Wiring - Low Side, High Side - Oshkosh JLG 1930ES Service Maintenance Manual

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 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
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 terminals with respect to battery ground. The difference between
those two measurements is the voltage applied to the load.
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SECTION 7 - GENERAL ELECTRICAL INFORMATION & SCHEMATICS
7-13