Multiple Unit Connections On UP Locomotives
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This page was last updated on June 15, 2024.
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Overview
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 was mounted above 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.
Early E and F units (and other cab units) were not built with MU capability on the front end. As railroads desired more flexibility in their use of locomotives, front end MU capability was added. Some railroads made the modifications based on their own in-house designs, while others purchased a retrofit kit from EMD. These kits from EMD included a standard 27 pin MU receptacle and the spring-loaded curved door which matched the cab unit nose curvature when installed in accordance with the EMD Maintenance Instructions.
Dynamic Brake connector boxes needed much bigger cable and were not mounted high on a cab nose. When used they were found in a lower location.
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 term "Field Loop" refers to a type of dynamic braking control that was common during the 1950s and early 1960s. From then onward, the dynamic brake control function was trainlined through the regular 27-pin MU cable.
The 21/12 configuration was initially used by American Locomotive Co. Locomotives with 21/12 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.
Looking at a photo of a UP GP9, the MU connection on the engineer's side, from the top down, are the Field Loop plug, the 21-pin MU plug, and the 12-pin MU plug, all three painted red. On the fireman's side, from the top down, are the Field Loop plug (painted red) and the 27-pin plug (painted blue).
(View a photo showing the arrangement)
All MU receptacles were red, except the right side (as you face either end), which was blue. This stems from the modification UP began doing with the delivery of the SD45s in 1968. Up until that time, all MU connections were 12-pin (upper receptacle) and 21-pin (receptacle). In about 1968-1969, UP began a modification that combined the right side connections into a single 27-pin receptacle. The lower plug was removed and it's opening was covered over. The remaining upper receptacle's cover was painted blue to denote the change. The SD45s were delivered with a single receptacle on the right side, but it was the lower one. Some of the older units have had their left-side MU stands removed, with just the right side, single receptacle stand remaining. On older units, the small square receptacle on top the MU stand is for the dynamic brake field loop. By the time of the delivery of the SD40s in 1966, new units did not have the square dynamic brake receptacle.
UP ordered the 21/12-pin combination of MU connections to the standard 27-pin plug until late 1966, so the last SD40, UP 3082, delivered in 1966, was likely the last new unit to have it as delivered.
The SD40 demonstrators (UP 3040-3047) were delivered with both red and blue receptacles. They had double receptacles (red) on the left side and single receptacle (blue) on the right side.
A UP company photo of UP 1400 new at Omaha shows the SDP35s with four MU receptacles, as yet unpainted either red or blue. The photo also shows the dynamic brake field loop connection on only the right side.
Union Pacific's use of red and blue MU connection covers may date from the delivery of the SD24s in 1959, which was also when UP began using its "Dependable Transportation" slogan.
Beginning in the mid to late 1960s, there was an on-going modification program that was removing the 21/12 plugs (the red ones) and blanking off the openings. At the same time, the dynamic brake field loop plug was being deactivated, with the 27 pin plug taking over the function (this required a mod to the dynamic brake control circuits). On the GP units, 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 delivered in 1964 and 1965.
Research suggests that nose-mounted MU connections were not included on units delivered new in the 1940s to the 1960s. The variations are surprising. The later modifications seem to be undocumented, with the best source being dated photographs. The variations continue through to retirement. Some units have the factory-applied doors for their nose-mounted MU connections, and some have the large attachments, with matching 12- and 21-pinned plugs, and later 27-pinned connections. Some observers have suggested that these later large connection boxes were part of the requirements for compatibility while in service nationwide as units either sold to or leased to Amtrak after 1971.
Other observers have noted that the nose-mounted MU connections were in response to the 1968 combining of UP's passenger trains across Wyoming. Jack Wolff wrote in the Spring 2021 issue of The Streamliner, published by the Union Pacific Historical Society.
Beginning in August 1968, six E-units were modified with nose mounted MU receptacles, those being the 904, 905, 907, 926, 930, and 939. It was my understanding that this was done when trains 9 and 10 were combined with the City of Los Angeles, trains 103 and 104, between Cheyenne and Ogden. This was apparently done to minimize locomotive switching moves in Ogden. When 9 and 103 arrived in Ogden, the lead unit would be simply removed at the time the San Francisco cars were dropped. With fewer cars 9 and 103 could proceed to Los Angeles with the remaining power. The process was reversed for the eastbound counterparts, with a unit added in Ogden and then dropped in Cheyenne.
I have never seen any discussion of that practice in any books or articles about UP passenger train operations and this is based on my photographic evidence. It appears that practice did not last much past November 1968 and rarely seen, if ever, after that date and never seen with the creation of the City of Everywhere in September 1969.
After the creation of Amtrak in May 1971, UP's remaining E-units, 928, 951, 954, and 960 all eventually received nose mounted MU receptacles.
Included are two views of trains 10/104 eastbound out of Laramie on October 10, 1968, with E-units 947 and 930, and trains 9/103 out of Cheyenne on October 20, 1968, with E-units 959 and 905 operating with nose-mounted MU receptacles. (Each train with sets of five E units, with two cab units facing forward.)
Research also suggests that the flush-mounted nose doors were installed on 900-class E-units as early as 1965 when the 1400-class SDP35s were delivered. 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.
(See also: Jerry Pinkepank's article "Lash 'em Up!" in December 1968 Trains magazine) (PDF; 8 pages; 2.8MB)
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 locomotives used EMD's 21-pin power plant control system. Then, later, the GP7, SD7, GP9, E8 and E9 locomotives 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, and was known as the Field Loop connector.
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. Research has not yet found if Union Pacific used these connectors.
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.
Photos
MU Connections -- A photo album showing the multiple-unit (MU) connections on a few UP locomotives, focusing on the E8 and E9 locomotives. These UP passenger units were originally delivered without front end MU connections, but received them later as part of a modification program.
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 for 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.
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