Cummins/Onan — Undervoltage Sensor Calibration
Adjustments
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Before you start
- Rear of control accessory panel and transfer switch are energized. Do not touch due to serious shock hazard!
Procedure
- Open the cabinet door of the automatic transfer switch.
- Move the operation selector switch to STOP.
- Turn all the undervoltage sensor knobs to minimum voltage setting.
- Turn the PICK-UP VOLTAGE knob of the sensor to be calibrated to its clockwise limit.
- Turn the PICK-UP VOLTAGE knob very slowly counterclockwise until you hear relay K3 pick up.
- This reading times the multiplying factor should equal the line voltage. Check line voltage with a voltmeter, divide by the factor to see if the reading is correct.
- If setting is too low: turn PICK-UP VOLTAGE knob to clockwise limit, then counterclockwise to desired setting; insert small screwdriver through CALIBRATE hole and turn counterclockwise very slowly unt
- If setting is too high: turn PICK-UP VOLTAGE knob to clockwise limit; insert small screwdriver through CALIBRATE hole and turn to its clockwise limit; turn PICK-UP VOLTAGE knob to desired setting; tur
- Repeat Steps 3 through 6 for each undervoltage sensor.
- For more accurate calibration and calibration of the % DROP-OUT DIFFERENTIAL knob, use the Onan Multi-Tester or a variac.
- Remove the twist-lock disconnect plug and open the control accessory panel.
- Remove the transformer leads from T2-1 (COM) and T2-2 (120-V) from left side of terminal strip and connect them to output leads of a 120-volt variac.
- Connect a voltmeter to the output leads of the variac and connect the variac to a 120-volt AC source.
- Reconnect the disconnect plug, turn all undervoltage sensor knobs to minimum, and adjust the Multi-Tester or variac to give a 120-volt output.
- Adjust PICK-UP VOLTAGE and % DROP-OUT DIFFERENTIAL knobs to desired settings; after calibration, remove disconnect plug, reconnect wire leads, install plastic cover, close panel and reconnect disconne
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Sensor PICK-UP VOLTAGE knob range is 90 to 140 volts for a nominal 120-volt system.
- At 120 volts output, relay K3 should pick up at 120 volts.
- For % DROP-OUT DIFFERENTIAL calibration, with knob at maximum, lower output to 108 volts and adjust % DROP-OUT DIFFERENTIAL knob counterclockwise until relay K3 drops out; knob should read approximately 10 percent.
Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.