Manual

Electrical System
189
Voltage Drop Test
To review:
• Ohm’s law states that it takes voltage to push
current through a resistance.
• Kirchhoff’s voltage law states that the sum of all
the voltage drops equals the source voltage.
• Combining those two laws, we see that any
restriction in a circuit (e.g.: loose connector
damaged wire, or corroded terminal) will use up
some voltage as the current is pushed through.
• A voltage drop test is a way of looking for that
voltage.
• Because electricity needs to complete a full cir-
cle (circuit), voltage drop tests are useful on
both the positive or the negative side of the sys-
tem.
• This text will address the negative side to begin
with. Bad grounds are responsible for as many
electrical failures as the positive side of the sys-
tem, yet the ground side is frequently neglected
by technicians. See Figure 7.60.
NOTE: Ultimately, all current will find its way back to the negative post of the battery.
To check ground-side voltage drop: set the DMM to measure 12V DC.
1. Make a good electrical connection between the black (-) probe and the negative post on the battery.
2. Make a good electrical connection between the red (+) probe and the suspect point of ground.
3. Power-up the circuit in question.
4. The voltage indicated on the meter is the voltage that is being used to pass current through a resistance in the
circuit.
5. Voltage drop on a good circuit should be less than 0.1 volts. A voltage drop reading on the meter of greater
than 0.2 volts indicates a fairly substantial problem that demands attention.
• As an example, if the starter solenoid does not
engage properly, check for voltage drop between
the ground point for the starter solenoid and the
negative post on the battery.
See Figure 7.61.
• With the starter engaged, this machine exhibited a
voltage-drop reading of 0.308 volts, indicating a
poor ground connection.
Figure 7.60
Figure 7.61