SECTION 7 - GENERAL ELECTRICAL INFORMATION & SCHEMATICS
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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 stop-
ping current flow. This voltage can be seen if the mea-
surement 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 operat-
ing. 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 bat-
teryground. The difference between those two mea-
surements is the voltage applied to the load.
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