JLG 450A_AJ Series II Service Manual - Manual (Page 417)

SECTION 7 - BASIC ELECTRICAL INFORMATION & SCHEMATICS
3121180 – JLG Lift – 7-1
SECTION 7. BASIC ELECTRICAL INFORMATION & SCHEMATICS
7.1 GENERAL
This section contains basic electrical information and
schematics for locating and correcting most operating
problems. If a problem develops which is not presented in
this section or not corrected by listed corrective actions,
obtain technically qualified guidance before proceeding
with any maintenance.
7.1 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 meter
operator’s manual for more information.
Grounding
"Grounding the meter" means to take the black lead
(connected to COM (common) or negative port) and
touch it to a good path on negative side of voltage source.
Backprobing
"Backprobe" means to take the measurement by access-
ing a connector’s contact on the same side as the wires,
on back of the connector. Readings can be done while
maintaining circuit continuity. If connector is the sealed
type, great care must be taken to avoid damaging seal
around the wire. It is best to use probes or probe tips spe-
cifically designed for this technique, especially on sealed
connectors. Whenever possible, insert probes into the
side of the connector so the test checks both terminals of
the connection. It is possible to inspect a connection
within a closed connector by backprobing both sides of a
connector terminal and measuring resistance. Do this
after giving each wire a gentle pull to ensure wires are still
attached to the contact and contacts are seated in the
connector.
Min/Max
Use of "Min/Max" recording feature of some meters can
help when taking measurements of intermittent conditions
while alone. For example, you can read 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 leads are
reversed. Check what reading is expected, location of sig-
nal, and leads are connected to device under test cor-
rectly. Check lead on "COM" port goes to ground or
negative side of signal and lead on other port goes to pos-
itive side of 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 correct range
(See multimeter’s operation manual).
• Use firm contact with meter leads.
Figure 7-1. Voltage Measurement (DC)