Cummins/Onan — Adjusting Undervoltage Sensor 306-0136
Adjustments
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Before you start
- Failure to disable the generator set and remove the disconnect plug presents a serious shock hazard from the control panel; other components have high voltages
Procedure
- Open the cabinet door; move the operation selector switch to STOP; remove the ground cable of the starting batteries.
- Remove the disconnect plug to de-energize the control panel.
- Use an accurate AC voltmeter and a Variac to raise and lower voltage; make connections as shown in Figure 2-13.
- Note the sensor is rated for a nominal 120 volts across L1, L2; for higher line voltages a transformer is used, so account for the transformer ratio.
- Determine the transformer ratio by measuring primary and secondary voltage; then disconnect the lead from either L1 or L2.
- Establish the desired drop-out and pick-up voltages; drop-out can be adjusted 5 to 17 percent lower than pick-up.
- Divide the established values by the transformer ratio to get the voltages to go across L1 and L2.
- Vary the voltage from 0 to 150 volts; the relay should click on pick-up and drop-out.
- Turn the pick-up control to its clockwise limit; turn the drop-out control clockwise until the relay drops out, then one-fourth turn counterclockwise.
- Turn the pick-up control counterclockwise slowly until you hear the relay pick up.
- Adjust the voltage with the Variac to the desired drop-out value; turn the drop-out control slowly clockwise until the relay drops out.
- Raise and lower the voltage and note the pick-up and drop-out values; make slight adjustments as needed.
- Scribe a reference line to indicate the final adjustment of each adjusting screw.
- Remove the Variac, voltmeter, and jumper; repeat Steps 5 through 16 for the remaining voltage sensors.
- Reconnect the disconnect plug, move the selector switch to RMT or AUTO, reconnect the battery ground cable, and close the cabinet door.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Sensor rated for a nominal 120 volts across L1, L2
- Drop-out can be adjusted 5 to 17 percent lower than pick-up
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.