Voltage drop is the reduction
in voltage in an electrical circuit between the source and load. Another way to
look at it is the difference in voltage potential between two points.
A voltage drop check is a
more accurate indication of the condition of a circuit than a resistance check.
Imagine a 12-gauge wire with all but one strand of wire cut. The circuit would
have good continuity if checked with a Digital Multimeter (DMM), but once
current was forced through the circuit, it would undoubtedly fail.
There will always be a
certain amount of voltage drop in any electrical circuit. This is because
voltage is lost as it passes through a conductor due to the resistance of the
conductor. However, excessive voltage drop in a circuit on either the power or
ground side will result in poor operation of electrical components as less
voltage is available to operate the load. In fact, electrical motors may be
damaged by excessive voltage drop.
When performing voltage drop
checks, the circuit being tested must be operational. With the positive lead of
the DMM connected to one point of the circuit and the negative lead connected
to the other point, operate the circuit to get the difference in voltage
between the two points. (Fig. 14)
When checking the integrity
of a ground circuit, use the best ground on the vehicle -- the battery's
negative post. Likewise, when checking the circuit's power side, use the
battery's positive post as the starting point. Generally, anything over a 0.5
volt drop on either the power or ground side is considered excessive drop for
most circuits, but always refer to published Service Information for exact
specifications.
- Thanks to Mike DeSander
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