Cummins/Onan — Adjustment of Mechanical Overspeed Switch
Adjustments
From a Cummins/Onan owner's manual covering: KT38, KTA19, Kt19, KtA38, KtA50, N-855, NH-672, NH-743, NH-855, NT-855, NTA-855, NTC-270…. Check it applies to your exact model before use.
Applies to: Cummins/Onan Kt19, Cummins/Onan KTA38, Cummins/Onan KTA50
Before you start
- Do not break or remove the lockwire.
- Do not use the ST-1224 if stop crank adjustment is required.
Procedure
- Remove the tachometer drive adapter.
- Install the ST-1224 in place of the standard drive adapter; connect the tachometer and overspeed stop switch to the ST-1224.
- Start the engine and warm to operating temperature.
- Set the engine speed to the appropriate overspeed switch set speed in Table 1-3 as indicated by the tachometer.
- Remove the round head dust cover screw marked 2 from the top of the switch.
- Insert a 1/16 inch hex allen wrench into the adjusting screw located just below the surface of the cover.
- Turn the adjusting screw counterclockwise to lower the engine shut off speed; clockwise to raise it.
- Replace the dust cover screw.
- Reset the switch by pushing the reset button on top of the switch.
- Replace the ST-1224 with the original drive adapter and reconnect the cables.
Tools required
- ST-1224 Tachometer drive adapter (2:1 ratio) Amazon
- tachometer Amazon
- 1/16 inch hex allen wrench Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Overspeed switch must be set to shut off the engine at a speed 20 percent above the required speed of the fire pump
- Do not turn the adjusting screw more than three (3) revolutions in either direction from the factory setting
Full service-parts list: Cummins/Onan Kt19 →, Cummins/Onan KTA38 →, Cummins/Onan KTA50 →
Source: Cummins/Onan owner's 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.