SECTION 8 - GENERAL ELECTRICAL INFORMATION & SCHEMATICS
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8.7
SWITCHES
Basic check
The following check determines if the switch is func-
tioning properly, not the circuit in which the switch is
placed. A switch is functioning properly when there is
continuity between the correct terminals or contacts
only when selected.
1.
De-energize the circuit.
2.
Isolate the switch from the rest of the circuit if
possible. If not possible, keep in mind it may
affect readings.
3.
Access the terminals to the switch.
4.
If the switch has two terminals:
a.
Measure resistance across the terminals.
b.
Change the switch position.
c.
Measure resistance again with the leads in
the same positions. If the meter was reading
short, it should read an open. If the meter
was reading open it should read short.
5.
If the switch has more than two terminals, con-
sult the schematic or switch diagram to deter-
mine what terminals will be connected. The test
is similar to testing a switch with two terminals.
a.
Place one meter lead on the common con-
tact and the other on a different contact in
the same circuit.
b.
Cycle through all positions of the switch.
The meter should read short only when the
switch connects the two terminals and open
otherwise.
c.
If the switch has more than one common
contact repeat the process for that circuit.
Limit Switches
Limit switches are used to control movement or indi-
cate position. Mechanical limit switches are just like
manually operated switches except that the moving
object operates the switch. These switches can be
tested the same way as a standard switch by manu-
ally operating the sensing arm.
Another type of limit switch used by JLG is the induc-
tive proximity switch, also referred to as a "prox
switch". Inductive proximity switches are actuated
only by ferrous metal (metal that contains Iron, such
as steel) near the switch. They do not require contact,
and must be energized to actuate. These types of
switches can be used to detect boom or platform
position, for example. These switches have a sensing
face where the switch can detect ferrous metal close
to it. To find the sensing face, take note how the
switch is mounted and how the mechanisms meet
the switch. Test this type of switch as follows:
1.
Remove prox switch from its mount.
2.
Reconnect harness if it was disconnected for
step a, and turn on machine.
3.
Hold switch away from metal and observe
switch state in the control system diagnostics
using the Analyzer. See vehicle or control system
documentation on how to do this.
4.
Place sensing face of switch on the object to be
sensed by the switch. If that is not available, use
a piece of ferrous metal physically similar to it.
The switch state in the control system diagnos-
tics should change.
5.
When reinstalling or replacing switch be sure to
follow mounting instructions and properly set
the gap between the switch and object sensed.
Automatic Switches
If the switch is actuated automatically, by tempera-
ture or pressure for example, find a way to manually
actuate the switch to test it. Do this either by apply-
ing heat or pressure, for example, to the switch.
These switches may need to be energized to actuate.
1.
Connect instrumentation to monitor and/or con-
trol the parameter the switch is measuring.
2.
Observe switch state in control system with the
Analyzer. See vehicle or control system docu-
mentation 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 mea-
sured 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 termi-
nals. Also, the technician can measure 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.
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