Multiple Unit Connections On UP Locomotives

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This page was last updated on May 2, 2021.

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During the early years of dieselization, multiple unit control was achieved by the use of twin receptacles, one with 12 pins and the other with 21 pins. On later units with dynamic braking, the necessary field loop connector mounted atop the left side MU boxes. From the mid 1960s on, UP converted the left side twin receptacles to a single 27 pin connector, putting a blank plate over the extra location. After the conversion, the twin receptacles (21/12-pin) were painted red, and the single receptacle (27-pin) was painted blue.

UP's GP9s equipped with dynamic braking had a Field Loop connector on top of the left-hand MU boxes. "Until recently GM units with dynamic braking required an extra jumper for the field loop - a feature of GM dynamic braking not used by other systems - in addition to the normal electric jumpers between units. As a result, some non-GM units have been built with field loop conductors to make full dynamic braking possible in mixed-make power consists. GM is not using the field loop on its present models." (Trains magazine, December 1968, page 48)

The 21/12 configuration was initially used by American Locomotive Co. Locomotives with 12/21 M.U. connections also used either 6DS or 6RL air brake schedules, which also meant that they needed matching air hose connections.

When the later 27-pin connections became available, sometimes known as AAR 27-pin, the air brake schedule was updated to 24RL.

Rich Sievers wrote on on February 20, 2004:

Using the photo as a guide, on the engineer's side from the top down are the Field Loop plug, the 12-pin m.u. plug, and the 21-pin m.u. plug. On the fireman's side from the top down are the Field Loop plug and the 27-pin plug (with its cover painted blue).

(View a photo showing the arrangement)

UP ordered the 12-pin/21-pin arrangement in addition to the standard 27-pin plug until late 1966, so the last "straight" SD40, 3082, should've been the last new unit to have it. Thereafter, UP used just the standard 27-pin plug, and many unit that still had the 12-pin/21-pin plugs had those removed.

Field Loop refers to a type of dynamic braking control that was common at the time. Today, virtually all dynamic brake control functions are trainlined through the regular 27-pin m.u. cable.

Beginning in the mid to late 1960s, there was an on-going mod that was removing the 21-12 plugs (the red ones) and blanking off the openings. At the same time, the DB field loop plug was being deactivated, with the 27 pin plug taking over the function (this required a mod to the DB control circuits). On the Geeps, there had been an earlier program to convert one of the left side plugs from 21-12 to 27 pins. This was done to match the GP30s and GP35s/DD35s. (Don Strack, posted to on February 20, 2004)

(See also: Jerry Pinkepank's article "Lash 'em Up!" in December 1968 Trains magazine; see below.)

One of the unique features about UP's ten EMD SDP35 locomotives was their rear MU connections. In addition to the usual lower MU connections below the walkway, these locomotives had a second set of MU connections mounted up on the rear of the carbody to match the high MU connections on any following EMD E8 and E9 locomotives, which had their MU connections on their noses, adjacent to the headlight.

EMD Connection Jumpers

Based solely on a review of EMD parts catalogs...

On the subject of multiple-unit controls for EMD locomotives, there are four separate systems that crossed from one locomotive in a multiple-unit consist to the next, or from the locomotive consist to trailing passenger cars. All receptacle and plug systems were offered in either nose to nose, nose to back, nose to train, or back to train configurations. The difference in these jumper cables was length.

A standard E3 through E7, and all F-units back-to-back jumper was 5 feet long. A nose-to-nose, or nose-to-back jumper cable was 8 feet, 8 inches long. A standard jumper length for E8, E9 and all F-units nose-to-nose, or nose to back was 9 feet.

Power Plant Control Jumpers

On Union Pacific, the E3 to E7, and some E8 locomotives used EMD's 16-pin power plant control receptacle and plug system. The Union Pacific GP7, SD7, GP9, some E8 and all E9 lococomotives used EMD's 21-pin power plant control system. Then, later, the GP7, SD7, GP9, E8 and E9 lococomotives used EMD's 27-pin power plant control system.

Dynamic Brake Control Jumpers

Dynamic braking jumpers and receptacles were a heavy-blade, 3-pin system using large conductors because of the amperage being conducted. The plug was a rectangle design.

Steam Generator Control Jumpers

A review of the EMD parts catalog shows that steam generator control jumpers were a confusing mix of 5-pin, 6-pin, 12-pin and 14-pin configurations. I have not been able to figure out which configuration Union Pacific used, if any.

Electro-Pneumatic Air Brake Jumpers

The 16-pin to 16-pin air brake control receptacle and plug system was used on all EMD locomotives delivered after January 1947.

On Union Pacific, there was both a 12-pin to 12-pin, and a 16-pin to 16-pin Electro-Pneumatic Air Brake jumper for locomotive nose to train connection, and for locomotive rear to train connection. Union Pacific did not use EMD's 16-pin to 6-pin nose to train, or EMD's 16-pin to 12-pin nose to train or 16-pin to 12-pin back to train jumpers.

Lash 'em Up by Jerry Pinkepank

(excerpt from an article in the December 1968 issue of Trains magazine)

Let's examine the M.U. jumper cable —the device which carries all of these electrical signals between units.

By far the most prevalent jumper is the 27-wire type first standardized by GM in the 1940's. The 27-point receptacle succeeded the 17-point jumper originally used for FT units and the 16-point jumper still used by some roads for E units. It is now the Association of American Railroads recommended standard, but since locomotives are not interchange equipment, the AAR standard is not binding.

The most notable dissenter from the 27-point receptacle is Union Pacific, which uses the old Alco dual jumper with 12 and 21 points. Furthermore, the fact that a unit is equipped with a 27-point receptacle is no assurance it is compatible with another unit so equipped. Although there are few variations on the pin numbers used for the A, B, C, and D engine-speed control wires and the forward and reverse control, many other pins of the 27-point receptacle have been used for a variety of conflicting functions.

When EMD and non-EMD units are mated, pins used for transition will cause trouble unless they meet on blind "spare" pins. On some units various alarm circuits may or may not be trainlined, or may be trainlined on different pins, with the result that the hot-water alarm on a trailing unit may sound the low oil-pressure alarm on the leading one. One case is documented in which the trainlined ground lightswitch of one class of Alco RS-3's could operate — as the unintentional result of modification in the roads own shop — the dynamic brake control on another class of RS-3's.

Since most of these supplementary functions are not really necessary to M.U. operation, on many roads a quick cure has been simply to cut the wires leading to the conflicting pins and to tape them to prevent grounding. A 27-point receptacle may get along on about 10 functioning pins. This follows the philosophy of those who think trainlining of alarms is a waste of effort, since most alarm functions announce themselves by shutting down the engine. When a locomotive leaves the line, the effect is instantly apparent to the head-end crew; if the alarm circuit rings on only one unit, the trouble is localized more rapidly than if the alarm rings in all units in the consist. One alarm which does not shut down the engine is the hot-water warning, and for this reason it is often trainlined. Obviously, lead-unit control of the headlights, ground lights, and indicators of the trailing units is not vital to operations. If the cut-and-tape method is unappealing, though, it is relatively simple for a road to publish a standard 27-point receptacle chart and have its shops move the wires around the pins as needed. Usually the only work required is on the receptacle itself, not inside the unit.

Another cure for nonmatching 27-point receptacles is a special jumper in which a wire connected to pin 7, for example, on the "A" end is connected to pin 5 on the "B" end. Such jumpers must be carefully labeled and segregated from regular jumpers, but still the day will come when someone sticks the green end in the blue receptacle or uses the jumper on some incompatible third-party unit, with the result that ground relay action on a trailing unit will turn out the headlight on the leading one, or perform some other mischief. Special jumpers of this type are sometimes used for pool arrangements in which power of two different railroads with differing 27-point receptacles must multiple. Union Pacific and Burlington have cooked up a special V-shape jumper that connects UP's 12- and 21-point twin receptacles to CB&Q's 27-pointers for the roads' long-standing pool arrangements.

The AAR has tried for the past 12 years to settle on a standard 27-point receptacle for adoption by all railroads, but the recommended standard has been changed so often that any road which attempted to keep up with it would have spent a fortune by now in electrician man-hours.

Until recently GM units with dynamic braking required an extra jumper for the field loop —a feature of GM dynamic braking not used by other systems — in addition to the normal electric jumpers between units. As a result, some non-GM units have been built with field loop conductors to make full dynamic braking possible in mixed-make power consists. GM is not using the field loop on its present models. In place of the 27-point receptacles, a few other varieties can be found on units not required to mix with other types. The old E-unit 16-point jumper is often paired with another 16-point jumper foi' carrying additional functions. The old Alco 12 and 21 pairs survive in a few enclaves other than UP. The special Baldwin, Alco switcher, and Fairbanks-Morse receptacles occasionally are seen.