Taylor-Dunn B0-248-TT, Operation, T Roubleshooting And Replacement Parts Manual, Page: 125 / 204

Maintenance, Service, and Repair
Electrical Troubleshooting
Page 15
ISO
Remove the wires and the resistor from the ISO contactor and perform the following tests:
Test continuity across the power contacts.
•
This should be an open circuit. If it is not an open circuit then the contactor should be
replaced.
Measure the resistance of the resistor that was removed.
•
Should be 250 Ohms (+/-10%). If it is not 250 ohms (+/-10%), then replace the resistor.
1. Make sure the key-switch is in the “OFF” position, then remove the key.
2. Place the forward-reverse switch in the center “OFF” position.
3. Set the park brake.
4. Place blocks under the front wheels to prevent vehicle movement.
5. Disconnect the main positive and negative cables at the batteries.
The rear drive wheels may rotate during some of the following tests. Block the
front wheels, raise the rear drive wheels off the ground, and support the vehicle
with jack stands. Failure to do so may cause the vehicle to move and cause
severe bodily injury and/or property damage.
Rotating rear drive wheels are a potential hazard. Keep hands, arms, legs and
loose clothing away from the rear drive wheels while conducting tests. Failure
to do so may cause serious bodily injury.
Contactors
There have been a few isolated instances of a specific failure of a contactor that results in both the
NO and NC contacts of a contactor being closed at the same time. This condition results in a shorted
motor field winding circuit. In most cases of this fault, it occurs in only one direction due to a fault in
one contactor. Although unlikely, it is possible for both contactors to fail at the same time resulting in
these symptoms occurring in both directions.
Continued operation of a vehicle with this fault will result in a burnt motor armature, shorted speed
control or both.
A simple test for this fault is to disconnect one of the field wires (S1 or S2) from the motor and attempt
to operate the vehicle. Since during operation, the field and armature is connected in series, there
should be no motor current due to the open connection. If there is still current flowing through the
armature, then the field circuit is shorted in the vehicle either in one of the contactors, or in the power
wiring.
If you find the vehicle has armature current with the field disconnected, remove the contactors and
perform a bench test to confirm the fault.