Service Manual

Part Numbers
78-6990, 113997, 92-6885, 93-3160, 115629,116277, 119930
Purpose
This clutch electrically controls the engagement and disengagement of the
PTO pulley.
How It Works
The PTO clutch is composed of three major components: the field, the
clutch plate, and the friction plate. The clutch plate always turns with the
engine. The field is a coil of wire on an iron core which becomes an
electro-magnet when power is applied. The friction plate is the only piece
that can slide up and down on the crankshaft axis. It is normally spring
loaded so that it is not in contact with the clutch plate and is pressed
against the brake material opposite the clutch. When power is applied, the
friction plate is drawn toward the clutch plate and the two rotate as one.
Testing
1. Disconnect the PTO clutch from the harness.
2. Connect a charged 12 volt battery to the clutch (polarity makes no
difference.) You should hear a click, the clutch should engage. If not,
check the wiring and connectors leading into the field. If no problems
can be found, replace the field.
3. Resistance Check: The field should have a resistance as listed in the
table on the left.
Mechanical Adjustment: The gap between the clutch and friction plates
should be set at .012-.015 inches. Adjust by rotating the three nuts
attached to the field.
Part Numbers
71-6000
Purpose
The coil boosts voltage from 12 volts to in excess of the 20,000 volts
necessary to fire spark plugs.
How It Works
The coil is actually two separate coils of wire wrapped around a common
iron core. One coil, the primary, has fewer windings and thicker wire. The
other coil, the secondary, has many windings, and uses a much finer wire.
With this arrangement, whenever a small voltage change is made to the pri-
mary, a large voltage change results in the secondary. There is a catch,
though. The coil can only produce large “spikes” in the secondary when
there is a rapid change in the primary.
Therefore, the coil relies on the trigger module to produce this change by
quickly opening the primary circuit at the precise moment the plugs are
supposed to fire.
Testing
1. Test the resistance of the primary winding by connecting an ohmmeter
across the “+” and “-” terminals on the coil. The resistance should
read between 2.90 and 3.60 ohms.
2. Check the resistance of the secondary coil by connecting one VOM
lead in each of the two spark plug wire cable holes. The resistance
should read between 14,500 and 19,800 ohms.
3. If either of the above readings were out of specification, replace the
coil.
Clutch
Electric
Coil
Ignition
Clutch # Resistance
78-6990 2.5W ± 20%
115629 2.5W ± 20%
113997 2.2W ± 20%
116277 2.2W ± 20%
119930 2.2W ± 20%
Glossary
GLOSSARY
3-6 Demystification Guide