The Alco 251 diesel engine was developed by the American Locomotive Company to replace the 244 and 539. The 251 Prime mover was Alco’s best selling prime mover. In 1954, the 251 went into production with the inline-6 at Auburn, New York. The next year production of the V-12 followed. Samples of the 12Cyl versions were found in: RS-11, RSD-12, RS-18, RS-36, FPA-4, FPB-4, DL500B, RS-32, C420, MLW M420TR, M420W, DH643 and HR412
Ultimately a refined and successful design, the 251 outlived its designer. For a time it was built in Canada by Montreal Locomotive Works. Today it is still available from Fairbanks-Morse.
| Key | Functions | Sound slots | Volume CVs | Volume values |
|---|
| F0 | Directional Headlights | | | |
| F1 | Bell | 4 | 283 | 99 |
| F2 | Playable Airhorn | 3 | 275 | 128 |
| F3 | Coupler Clank | 5 | 291 | 128 |
| F4 | Dynamic Brake | 6 | 299 | 128 |
| F5 | AUX3 output (pre-set to Mars Light) | | | |
| F6 | AUX1 + AUX2 (Ditch Light) | | | |
| F7 | Switching Mode | | | |
| F8 | Sound (On/Off) | 1 | 259 | 99 |
| F9 | Diesel Manual Notching (Up) | | | |
| F10 | Diesel Manual Notching (Down) | | | |
| F11 | Radiator (Fan) Sound | 8 | 315 | 128 |
| F12 | Dimmer (Headlights) | | | |
| F13 | AUX4 function output | | | |
| F14 | Detector Sound | 9 | 323 | 128 |
| F15 | Crossing Communication | 10 | 331 | 128 |
| F16 | Rail Clank (works only if F8 is On) | 11 | 339 | 128 |
| F17 | Brake Set / Brake Release | 12 | 347 | 128 |
| F18 | Sanding Valve | 13 | 355 | 128 |
| F19 | Short Air Let Off | 14 | 363 | 128 |
| F20 | Compressor | 7 | 307 | 100 |
| F21 | Pop Valve Sound | 15 | 371 | 128 |
Make sure that index CV31 is set to 16 and index CV32 is set to 1 before changing a volume CV.