JLG 19AMI (3120758) Service Manual - Manual (Page 50)

SECTION 3 - TROUBLESHOOTING
3-2 – JLG Lift – 3120758
Polarity
Finding 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 Ω = 1200 Ω
Example: 50 mA = 0.05 A
Continuity Measurement Over Long
Distances
When trying to determine continuity of a harness or wire,
longer than the reach of standard instrument leads, is
possible to perform the check without excessively long
leads. Using the other wires in the harness one can deter-
mine the condition of a particular wire in the harness.
Requirements:
• Harness with at least three separate wires including the
wire under test.
• These wires must be able to be isolated from other
wires, etc.
• Jumper or method to connect contacts on one side of
harness.
• Meter that can measure resistance or continuity.
Procedure
Test multimeter leads resistance. Subtract this value from
the measured resistance of the wires to get a more accu-
rate measurement.
Consult the circuit schematic to determine which wires to
use in addition to wire under test, here called wire #1 and
wire #2, and how to isolate these wires. These wires
should appear in the same connectors as the wire under
test or are within reach of the jumper.
1. Disconnect all connections associated with the wire
under test and the two additional wires. If harness is
not completely isolated disconnect battery terminals
also, as a precaution.
2. Measure continuity between all three wires, the wire
under test, wire #1 and wire #2. These should be
open. If not, repair the shorted wires or replace the
harness.
3. On one side, jumper from contact of wire #1 and
wire #2.
4. Measure continuity between wire #1 and wire #2. If
there is continuity, both wires are good and can be
used for this test. If there is not continuity, either wire
could be bad. Check connections and measurement
setup. Redo measurement. If still no continuity,
repair wires or consult schematic for other wires to
use for test.
5. Jumper from wire under test to wire #1.
6. Measure continuity. If there is continuity, the wire
under test is good. Resistance of a wire increases as
the length increases and as the diameter decreases.
One can find the continuity of two wires, here #1 and #2,
at once by following steps 1 through 4. If there is a prob-
lem the third wire is used to troubleshoot the other wires.
To find the problem, start at step 1 and use the entire pro-
cedure.