This posting came out of a question raised by a friend.

Be aware that there are multiple arrangements of the wiring schematic for the EMD SW-1 switcher locomotive due to changes that were incorporated in the product during its long production run. As was the case with EMD product improvements in the 1940s and 1950s, these tend to be sparsely documented. Thus when you get on any particular SW-1 the control wiring arrangements could be quite different from what is shown in your reference manual.

One such progression of improvements was done to the automatic transition system (of those units equipped with automatic transition, it was an extra cost option) around 1949. The manuals printed before 1949 show a wiring schematic for the SW-1 with automatic transition that does not use a VT relay. Manuals printed after 1949 show a very different schematic that incorporates the VT relay. As was EMD’s habit, the later manuals make no reference at all to the earlier arrangement, they just show what was being used in production at the time the manual was printed, and do not tell that other arrangements existed.

When EMD made a change like this, they may or may not have issued a Modification Recommendation to show how to do the alteration on earlier production locomotives. That became a more common practice by the 1960s.

The best way to deal with this problem is to find a manual that was printed in the time period close to the production of the locomotive you are working on, and to check the schematic with the locomotive to make sure they agree, before starting any repairs.

If you only plan on keeping one edition of a manual for a particular type of locomotive, then the best one to have may be the last one printed before the locomotive type went out of production, as it probably reflects most or all of the changes.

The manufacturer’s address is:
Progress Rail Locomotive Inc.
9301 West 55th Street
La Grange, IL 60525