JLG 740A, Service And Maintenance Manual

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SECTION 7 - BASIC ELECTRICAL INFORMATION & SCHEMATICS
3121160 – JLG Lift – 7-1
SECTION 7. BASIC ELECTRICAL INFORMATION & SCHEMATICS
7.1 GENERAL
This section contains basic electrical information and
schematics to be used for locating and correcting most of
the operating problems which may develop. If a problem
should develop which is not presented in this section or
which is not corrected by listed corrective actions, techni-
cally qualified guidance should be obtained before pro-
ceeding with any maintenance.
7.2 MULTIMETER BASICS
A wide variety of multimeters or Volt Ohm Meters (VOM)
can be used for troubleshooting your equipment. This
section shows diagrams of a common, digital VOM con-
figured for several different circuit measurements. Instruc-
tions for your VOM may vary. Please consult the meter
operator’s manual for more information.
Grounding
"Grounding the meter" means to take the black lead
(which is connected to the COM (common) or negative
port) and touch it to a good path to the negative side of
the Voltage source.
Backprobing
To "backprobe" means to take the measurement by
accessing a connector’s contact on the same side as the
wires, the back of the connector. Readings can be done
while maintaining circuit continuity this way. If the connec-
tor is the sealed type, great care must be taken to avoid
damaging the seal around the wire. It is best to use
probes or probe tips specifically designed for this tech-
nique, especially on sealed connectors. Whenever possi-
ble insert probes into the side of the connector such that
the test also checks both terminals of the connection. It is
possible to inspect a connection within a closed connec-
tor by backprobing both sides of a connector terminal and
measuring resistance. Do this after giving each wire a
gentle pull to ensure the wires are still attached to the con-
tact and contacts are seated in the connector.
Min/Max
Use of the "Min/Max" recording feature of some meters
can help when taking measurements of intermittent condi-
tions while alone. For example, you can read the Voltage
applied to a solenoid when it is only operational while a
switch, far from the solenoid and meter, is held down.
Polarity
Getting a negative Voltage or current reading when
expecting a positive reading frequently means the leads
are reversed. Check what reading is expected, the loca-
tion of the signal and that the leads are connected to the
device under test correctly. Also check that the lead on the
"COM" port goes to the Ground or negative side of the sig-
nal and the lead on the other port goes to the positive side
of the signal.
Scale
M = Mega = 1,000,000 * (Displayed Number)
k = kilo = 1,000 * (Displayed Number)
m = milli = (Displayed Number) / 1,000
μ = micro = (Displayed Number) / 1,000,000
Example: 1.2 k

= 1200

Example: 50 mA = 0.05 A
Voltage Measurement
• If meter is not auto ranging, set it to the correct
range (See multimeter’s operation manual)
• Use firm contact with meter leads
Figure 7-1. Voltage Measurement (DC)