JLG 1230ES Service Manual - Manual (Page 160)

SECTION 9 - GENERAL ELECTRICAL INFORMATION & SCHEMATICS
9-6 – JLG Lift – 3121222
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 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 nega-
tive side or grounded to the vehicle. What is actually being measured is the voltage drop across the switch. This could mis-
lead 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 volt-
age at both power terminals with respect to battery ground. The difference between those two measurements is the voltage
applied to the load.