Section VII. MAGNETO

Connect an ammeter to the shunt and connect a dc

(1) Using a circuit test lamp, test between each

voltmeter between the starting motor terminal and starting

brush holder and the commutator end frame. If the lamp

motor field frame. With the voltage adjust to 23.4 volts,

lights on one of these tests, the brush holder is grounded.

the average current should be 36 amperes at 6,300 rpm.

(2) Test between adjacent brush holders. If the

The direction of rotation should be clockwise as viewed

lamp lights, the two brush holder groups are shorted to

from the drive end. If the current is high and the speed is

each other.

low, inspect the bearing and armature for binding and

incorrect alinement.

(3) Test between opposite brush holders. If the

lamp fails to light on one of these tests, remove the brush

(2) Stall Torque Test.

With the starter

holders and clean the mounting surfaces and hardware.

connected as in (1) above, fasten a torque arm and a

spring scale to the armature at the drive end. Adjust the

rheostat to give 6.6 volts. The correct readings are 225

b. Armature and Field Frame Testing. Test the

amperes maximum and a stall torque of 15 foot-pounds

armature and field frame of the starting motor as

minimum. The stall torque is the product of the spring

described in TM 5-764.

scale reading in pounds, multiplied by the length of the

torque arm in feet. If the current and torque are both low,

c. Bench Testing.

inspect for high resistance in internal connections and for

improper brush contact. High current and low torque may

be caused by a defective armature or field coil winding.

(1) No-load test. Connect a 24-volt battery in

series with a load rheostat and an ammeter shunt of a

capacity greater than 200 amperes, connect this group to

the starting motor terminal and starting motor field frame.

Section VII. MAGNETO

7-24. Disassembly and Reassembly

7-22. General

Disassemble the magneto as shown in figure 7-6, in the

The magneto is a heavy-duty, radio-shielded unit, located

sequence indicated by the key numbers. Reassemble in

on the left side of the engine and driven by the camshaft

opposite sequence.

gear. The entire magneto is enclosed in a grounded

metal frame. The magneto revolves clockwise. When

7-25. Cleaning, Inspection and Repair

the magneto breaker points are forced apart, the primary

electrical circuit is interrupted, building up high voltage in

a. Clean all metal parts in an approved solvent and

the secondary electrical circuit. This causes the high-

blow dry with compressed air.

tension electrical current to jump the air-gap of the spark

b. Inspect the distributor gear brush for wear.

plug electrodes, completing the secondary electrical

Replace a brush worn to less than 0.031 inch from the

circuit. The impulse coupling retards the timing of the

end of the distributor gear shaft. The rounded end of the

ignition during the starting operation and at the same time

new brush should extend 0.062 inch beyond the end of

produces an extremely high voltage which would

the distributor gear shaft. Adjust the clearance between

otherwise be impossible at low engine cranking speeds.

the distributor gear electrode and the distributor block

The impulse coupling disengages when the engine

terminals to 0.010 inch.

reaches operating speed.

c. Adjust the contact points from 0.008 inch to 0.012

7-23. Removal and Installation

inch.

a. Time the ignition as directed in paragraph 3-49.

d. The stop pin plate is adjustable from 5 to 35

b. Remove and install magneto and drive gear as

depending on the lag angle of the engine. Set the stop

shown in figure 3-24.

pin plate at 35by alining the arrow on the stop pin plate

with the "0" in the magneto frame.

e. Replace all defective parts.

7-11