ENGINE AR-COMPLETE 2171224 - Caterpillar

following procedure tests the varistor that is used with rectifier assembly in the generator. If any meter reading does not fall within the given range, replace the varistor.
Separate the varistor from the diode bridge in order to measure the resistance. The resistance should be a minimum of 15000 ohms.
Place the test leads from the megohmmeter on the terminals to the varistor. The varistor is not sensitive to polarity.
If the megohmmeter is not capable of reading DC voltages, then connect a DC voltmeter across the terminals to the varistor.
Select the 1000 VDC range on the megohmmeter.
Apply 1000 VDC across the terminals to the varistor.Note: Do not apply voltage for more than three seconds.
The megohmmeter should indicate between 735 VDC and 910 VDC.
If the reading is greater than 910 VDC, the varistor is "OPEN". The varistor has failed. Replace the heat sink assembly. Refer to the "Removal / Installation Instructions" section.
If the megohmmeter reading is less than 735 VDC, then the varistor is "SHORTED" or "DEGRADED". The varistor is failing. Replace the varistor. .Note: If the varistor has failed, the main stator may have received a voltage transient., A failure of the varistor is typically caused by a voltage transient. The following is a list of possible causes for a failed varistor:
Reverse VAR or reverse power event
Ambient temperature that is too high
Large sudden increase or decrease in the load
Incorrect paralleling or loss of synchronization