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Basic instructions
HOW TO MEASURE VOLTAGES IN THE ELECTRONICS SYSTEM
When asked to measure a voltage, the voltage being measured is always at one point with respect to (relative to) the
voltage at another point.
Example: To measure the voltage at point A with respect to point B, place one meter probe (typically red in color, and
connected to the meter connector labeled "V"). Place the other meter probe (typically black in color, and connected
to the meter connector labeled "COM").
If the units of voltage are specified as "volts dc", be sure your meter is set to "dc". If the units of voltage are specified
as "volts ac", be sure your meter is set to "ac".
North American automotive electrical systems often use the chassis (metal frame) of the automobile as the return path
(often referred to as ground) for electrical current. The electronics system does not use the chassis for a return path,
and no voltage measurements should be made with respect to the chassis. All components in the electronics system
should be considered to be electrically isolated from the chassis, although at the tractor the electronics system return
is connected to the battery negative terminal which is in turn connected to the tractor chassis.
NOTE: On VR air carts with a battery and a hydraulic motor driven alternator, the air cart battery negative terminal is
electrically isolated from the air cart chassis.
ELECTRICAL ISOLATION
Two points are electrically isolated when the resistance between them is "infinite" (very large, greater than
10,000,000 ohms). To verify two points are electrically isolated.
1. Set your meter to measure resistance (usually labeled with the ohm symbol).
2. Hold the two probes apart from each other in the air. The meter must indicate infinite resistance (usually indicated
by the infinity symbol or on digital multimeters, "++++" or "0L" for overload).
3. Hold the two probes together. The meter must indicate a very low resistance, less than 1.0 ohms. The resistance
measured will vary depending on what scale the meter is set to.
4. Place one probe on one point and the other probe on the other point. It does not matter which probe is placed on
which point when measuring resistance. The meter must indicate infinite resistance as it did in 2 above for the two
points to be electrically isolated.
ELECTRICAL CONTINUITY
Two points have electrical continuity when the resistance between them is very small, less than 0.1 ohms . To
verify two points have electrical continuity
1. Set your meter to measure resistance (usually labeled with the ohm symbol).
2. Since we are expecting to measure a resistance of 0 ohms, set the scale to the lowest available.
3. Hold the two probes apart from each other in the air. The meter must indicate infinite resistance (usually indicated
by the infinity symbol or on digital multimeters, "++++" or "0L" for overload).
4. Hold the two probes together. The meter must indicate a very low resistance, less than 1.0 ohms. Record or
memorize this resistance. This is the probe resistance.
5. Place one probe on one point and the other probe on the other point. It does not matter which probe is placed
on which point when measuring resistance. Subtract the probe resistance measured in 4 above from the meter
reading. If the meter reading minus the probe resistance is less than 0.1 ohms, the two points have electrical
continuity.
RESISTANCE
To measure the resistance between two points.
1. Set your meter to measure resistance (usually labeled with the ohm symbol).
2. Hold the two probes apart from each other in the air. The meter must indicate infinite resistance (usually indicated
by the infinity symbol or on digital multimeters, "++++" or "0L" for overload).
INTRODUCTION
84425026 11/04/2011
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