Current Measurement; Checking Switches; Basic Check; Limit Switches - JLG E450A Service And Maintenance Manual

Current Measurement

Figure 7-4. Current Measurement (DC)
• Set up the meter for the expected current range.
• Be sure to connect the meter leads to the correct jacks
for the current range you have selected.
• If meter is not auto ranging, set it to the correct range
(See multi meter's operation manual).
• Use firm contact with meter leads.
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SECTION 7 - BASIC ELECTRICAL INFORMATION & ELECTRICAL SCHEMATICS
7.3

CHECKING SWITCHES

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 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, consult the
schematic or switch diagram to determine what termi-
nals 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 posi-
tion. 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 prox-
imity switch, also referred to as a "prox switch". Inductive prox-
imity 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:
7-3