TT 97 20 01 SS1

Technical Tip
97-20-01TT Voltage Drop Testing Procedures
Transaction No:
2059380/2
Release date:
4/30/2020
Condition
ATTENTION:
THIS IS A TECH TIP, NOT A TECHNICAL BULLETIN.
TECH TIPS ARE NOT ASSOCIATED WITH WARRANTY CLAIMING.
Applicable Vehicles
Model(s)
Year
Eng. Code
Trans. Code
VIN Range From
VIN Range To
All
2010 -2021
All
All
All
All
Revision Table
Instance Number
Published Date
Version Number
Reason For Update
2059380/2
4/30/20
97-20-01TT
Update pictures for clarity.
2059380/1
4/24/20
97-20-01TT
Original publication.
Technical Background
Technical direction for testing voltage drop in a circuit.
Service
The purpose of voltage in a circuit is to provide the required electrical energy to operate a load. Resistance and
voltage drop across a load (such as a light bulb) are required for circuits to work correctly. But, in the wrong place
(such as corrosion in a connector) resistance and voltage drop may cause electrical issues, such as dim bulbs, slow
motors, heated wires, etc. A load can be defined as anything that causes resistance. This includes the wires and
devices like switches, diodes, bulbs, or motors, etc.
Resistance can also be created by partial connectivity caused by loose terminal pins, pitted relay contacts, loose
connections, or even corrosion. If a circuit has excessive resistance, it prevents the wire or component from carrying
sufficient current under high load conditions. In a normally operating circuit, normal resistance is small enough that it
doesn’t keep the load from operating properly. When measuring voltage in a circuit, you’ll find that it is lower after the
load (resistance) than it was before the load. The “voltage drop” or the amount that voltage lowers as it goes through a
load is an indication or measure of how much electrical energy was used when it was converted into another form of
energy (light, heat, or electromagnetic movement). The reason for using a voltage drop test rather than just an ohm
meter to measure resistance, is that sometimes a resistance is not evident (not measureable) unless the circuit is
placed under load. To measure voltage drop in a circuit, the circuit must be activated and under load.
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Technical Tip
97-20-01TT Voltage Drop Testing Procedures
Transaction No:
2059380/2
Release date:
4/30/2020
Note: Before starting a test always check the wiring diagram. Some vehicle circuits may have a resistor
intentionally installed to reduce voltage and current. Examples include the rheostat that dims the instrument panel
lights, ballast resistors in some fuel injector circuits, and motor resistors used to limit blower fan and electric fuel pump
speeds. Be sure you know your circuit and identify any "intentional" resistance that can give high ohm reading or high
voltage drop readings. Also, the wiring diagram often has specifications for electrical testing.
When performing a voltage drop test as shown in this example; the volt meter’s positive lead should be connected to
the circuit, in the direction of the power source and negative lead toward the ground.
Operate or turn ON the circuit. Voltage will always follow the path of least resistance. So, if there is excessive
resistance in the circuit, your meter becomes the path of least resistance for some voltage and will give a voltage
reading.
Meter #1
12 volts at the battery.
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Technical Tip
97-20-01TT Voltage Drop Testing Procedures
Transaction No:
2059380/2
Release date:
4/30/2020
Meter #2
0 volt reading between the battery positive post and the switch. The wire has no resistance so all of the voltage goes
through the wire and none goes through the meter, giving a 0 volt reading - no voltage drop.
Meter #3
0.1 volt drop across the switch. The switch has a small resistance. 11.9 volts goes through the switch and the
remaining voltage (0.1 volts) goes through the meter, giving a voltage drop reading of 0.1 volts.
Meter #4
4.0 volt drop across the damaged wire. The wire has high resistance at the fault point, allowing only 7.9 volts through
the wire. The remaining 4 volts takes the path of least resistance through the meter giving a voltage drop reading of 4
volts. A voltage drop this high will likely cause problems, such as a dim bulb.
Voltage drop is the amount of voltage lost as it passes through a component or resistance in a circuit. In order to
perform a voltage drop test, voltage must be present in a circuit. It’s the difference between voltage at the source or
battery and the voltage at the component. The meters leads are connected in parallel with the circuit being tested.
To test voltage drop in the starter circuit, the starter must be cranking the engine with the volt meter connected in
parallel to the starter cables.
To test the positive starter cable, connect the red lead from the DVOM to the positive battery terminal and connect the
black lead to the cable connection on the starter. The voltage drop should not exceed 0.5 volts.
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Technical Tip
97-20-01TT Voltage Drop Testing Procedures
Transaction No:
2059380/2
Release date:
4/30/2020
Test voltage drop on the ground side by placing the black lead of the DVOM to the negative battery terminal and place
the red lead to the housing of the starter and activate the starter. If the voltage drop exceeds 0.5 volts, move the red
lead from the starter housing to the ground cable connection on the engine block or transmission housing. At this point
we are testing the voltage drop between the negative battery terminal and the ground cable connection. If the voltage
drop exceeds 0.5 volts, replace the ground cable or clean the ground connection.
Additional Information
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the latest information available at the time of printing and is subject to the copyright and other intellectual property
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