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Starter Motor Testing
Before assuming an electric starter requires
service, check the engine for freedom of rotation
by removing the spark plug and turning the
crankshaft over by hand. Any belt, clutch, or
other parasitic load will affect the starter cranking
performance, so ensure their effects are
minimized.
The following list is given to aid in diagnosing
problems for the 12 Volt starting system.
1. Engine Cranks Slowly
• Parasitic load affecting performance
• Discharged, defective, or incorrect
battery
• Faulty electrical connection (battery
circuit)
• Dirty or worn starter motor commutator,
bearing, weak magnets, etc.
• Wrong engine oil viscosity for ambient
temperatures
• Defective starter clutch
• Flywheel brake misadjusted
• Battery leads too long or wire diameter
too small
2. Engine Will Not Crank
• Improperly operating interlock system
• Discharged battery
• Loose or corroded electrical connections
• Open circuit in starter motor switch
• Open circuit in starter motor
• Brushes sticking
• Inoperative starter solenoid
• Blown fuse in equipment wiring harness
3. Starter Motor Spins But Does Not Crank
Engine
• Sticking pinion gear
• Damaged pinion or ring gear
• Starter motor clutch slipping
• Incorrect rotation due to reversed motor
polarity
Check Starter Motor Drive and Clutch
When the starter switch is activated, the pinion
gear should rise, engage the flywheel ring gear,
and crank the engine. The pinion gear must
rotate counterclockwise, as viewed from the
gear. If the starter motor drive does not react
properly, check the helix and pinion gear for free
operation. If the pinion gear sticks, clean the
helix and gear with a mild solvent and
compressed air. If the pinion continues to stick,
replace the entire starter drive assembly.
NOTE: Do not oil the pinion gear or clutch helix.
The starter motor clutch is designed to prevent
damage from shock loads such as engine
backfire. If the clutch slips while cranking,
replace the entire starter drive assembly.
Test Starter Motor
1. Clamp starter motor test fixture in vise
(Figure2-8).
2. Remove starter motor from engine and
mount to fixture.
NOTICE:
Do not clamp the motor housing in a
vise or strike with a hammer. Starter motors
contain ceramic magnets that can be damaged if
the motor housing is hit, deformed, or dented.
3. Set meter to DC Amps position.
4. Connect the optional starter switch
(A, Figure 2-8), starter motor, battery (F),
black lead (C), red lead (D), shunt (E), and
tachometer (B).
Figure 2 - 8
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