Voltage Drop Test - Mercury MerCruiser Service Manual

Starting System

Voltage Drop Test

The voltage drop test is used to identify areas of high resistance within the battery circuit. High resistance can result from loose
connections, undersized cables, corrosion, or other imperfections in the starting circuit. Each area of high resistance acts as a
small load on the starting circuit, reducing the voltage intended for energizing the starter motor. The net result is a sluggish
starter.
As you perform this test, remember that voltage drops are additive. Identify and correct the worst areas, seeking to reduce the
cumulative voltage drop to the point where 9.5 volts (or more) are available to the starter motor.
1.
Test the battery positive (+) cable first:
a. Connect the voltmeter positive (+) lead directly to the positive (+) post of the battery. Do not use the battery cable
terminal. Connect the voltmeter negative (–) lead directly to the large, threaded starter motor positive (+) terminal to
which the battery positive (+) cable is connected.
b. Crank the engine over while observing the voltmeter. The voltage displayed on the meter is the voltage drop. It will
normally be 0.3 volts or less; if it is higher, the battery cable might be undersized. If the drop is excessive, test the
connections at the ends of the cable:
• To find the point where the resistance is highest (as indicated by the highest voltage drop), leave the voltmeter
positive (+) lead on the battery post and move the voltmeter negative (–) lead to the battery positive (+) cable ring
terminal, which is on the threaded starter terminal. Crank the engine while observing the voltmeter.
• Move the voltmeter negative (–) lead to the battery cable itself, which is inside the crimped battery cable ring
terminal. Crank the engine while observing the voltmeter.
• Continue to test each battery cable connection back to the battery positive (+) post.
c. If a battery switch is used, check between the battery cable ring terminal and the switch's terminal.
2.
Check for voltage drop across the solenoid contacts:
a. Connect the positive (+) voltmeter lead to the large, threaded starter motor positive (+) terminal to which the battery
positive (+) cable is connected. Connect the negative (–) meter lead to the lower stud on the solenoid.
b. Crank the engine over while observing the voltmeter. The voltage drop should be 0.2 volts or less; if it is higher, the
contacts are likely dirty or damaged.
3.
Check for voltage drop from the starter mounting bolt to the engine ground stud:
a. Connect the positive (+) voltmeter lead to a starter mounting bolt. Connect the negative (–) meter lead to the engine
ground stud.
b. Crank the engine over while observing the voltmeter. The voltage drop should be 0.2 volts or less; if it is higher,
remove the starter and clean the mating surface. Reinstall the starter and retest.
4.
Check for voltage drop on the battery negative (–) cable:
a. Connect the voltmeter negative (–) lead directly to the battery negative (–) post, not the battery cable ring terminal.
Connect the voltmeter positive (+) lead to an unpainted surface of the starter housing.
b. Crank the engine over while observing the voltmeter.
IMPORTANT: The maximum allowed drop is 0.3 volts.
c. To find the point where the resistance is highest, leave the voltmeter negative (–) lead on the battery negative (–) post
and move the voltmeter positive (+) lead to the ground stud where the battery negative (–) cable is connected.
d. Move the voltmeter negative (–) lead to the battery negative (–) cable ring terminal that is on the ground stud.
e. Move the voltmeter positive (+) lead to the battery cable itself, which is inside the crimped battery cable ring terminal.
f. Test each battery cable connection in this manner back to the battery post.
Page 4A-12 © 2016 Mercury Marine 90-8M0099748 eng DECEMBER 2015