JLG 1250AJP Troubleshooting Manual page 319

6.1 INTRODUCTION
A wide variety of multimeters or Volt Ohm Meters (VOM) can be used for troubleshooting your equipment. A digital meter
with reasonable accuracy (within 7%) is recommended for the measurements in these procedures. This section shows dia-
grams of a common, digital VOM configured for several different circuit measurements. Instructions 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 connector 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 technique, especially on sealed connectors. Whenever possible insert probes into the side of the connec-
tor such that the test also checks both terminals of the connection. It is possible to inspect a connection within a closed con-
nector 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 contact 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 conditions 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 location of the signal and that the leads are connected to the device under test cor-
rectly. Also check that the lead on the "COM" port goes to the ground or negative side of the signal 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 kΩ = 1200 Ω
Example: 50 mA = 0.05 A
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SECTION 6. MULTIMETER BASICS
1250AJP
SECTION 6 - MULTIMETER BASICS
6-319