JLG 41AM (3120752) Service Manual - Manual (Page 67)

SECTION 3 - TROUBLESHOOTING
3120752 – JLG Lift – 3-5
3.5 ELECTRICAL SWITCH TESTING
Basic Check
The following check determines if the switch is functioning
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 possi-
ble. If not possible, keep in mind it may affect read-
ings.
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, consult
the schematic or switch diagram to determine what
terminals will be connected. The test is similar to
testing a switch with two terminals.
a. Place one meter lead on the common contact
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 indicate
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 manually operating the sensing arm.
Another type of limit switch used by JLG is the inductive
proximity switch, also referred to as a "prox switch". Induc-
tive 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 1,
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 documenta-
tion 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 diagnostics
should change.
5. When reinstalling or replacing switch be sure to fol-
low mounting instructions and properly set the gap
between the switch and object sensed.
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 pres-
sure, 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.