Lull 6K Service Manual - Manual (Page 564)

Transmission
8-16 Service Manual — Models 644B, 6K, 844C, 8K, 1044C, 10K
Shunt Diodes
Description
Fig. 8-7: Typical Shunt Diode Application
Fig. 8-7 represents a typical shunt diode application. When the switch is
opened, the magnetic field surrounding the coil collapses and generates
reverse polarity voltage. If enough voltage is present, it will jump (arc) the
contacts in the switch and may cause damage.
The diode protects the switch contacts from this reverse voltage by
providing a path of lesser resistance for the electricity.
The shunt diode can fail in two ways:
1. If the diode has failed and allows forward voltage or the voltage
intended to energize the coil to pass through to ground, the result will
be a blown fuse.
2. If the diode has failed but will not allow the reverse polarity voltage to
pass to ground, the reverse spike will seek the next path of lesser
resistance and arc the switch contacts of the DW-3 shifter. This may
also occur if the ground connection for the shunt diodes is not securely
fastened to the ground post.
Diode Test The diode should be tested with both forward and reverse voltage applied.
Use a DMM, with diode testing ability, already set for diode testing.
Fig. 8-8: Shunt Diode Test
1. Remove the diode from the circuit.
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