I know someone who is considering buying a former SP TEBU slug to run with their high horsepower GP. The question is how the TEBUs were wired to their SP mothers, and what it would take to convert a new mother.

The SP mothers were GP40-2 SP #7940-7959 bought new in 1980, and GP40R SSW #7960-7966 rebuilt by SP to Dash-2 standards in 1981-1982.

I happen to have a partial wiring diagram for a TEBU, and it shows them as wired in series-parallel, and capable of being powered by a single mother at either end, or by two mothers. To the best of my knowledge, all GPs with AR traction alternators are wired in full parallel (4P), which means at minimum continuous speed, the traction motors and traction alternator are at their maximum continuous amperage. Adding a slug in parallel to this circuit would overload the AR alternator.

Did the SP mothers have series-parallel contactors? I am guessing that without a TEBU they ran as 4P like any other GP-2, but when powering a TEBU they ran in series-parallel as 2S3P (or 2S4P if single ended). At about 20-25 mph, they would cut out the TEBU and transition back to 4P. Does anyone have access to an SP or EMD print to confirm this? (The SDs with ARs of the era were 2S3P to 6P, except SD38-2s did not have transition to 6P.)

The TEBU units used a GE reverser, a GE axle drive wheel slip system, and had EMD style extended range dynamics with DP and DG cards. What I don’t have, is how dynamic braking was controlled. I assume the mother unit supplied the field current for dynamic braking through the slug traction connections.

It also appears that for using a TEBU single ended with a one mother, all four traction cables were connected between slug and mother. For double-ended use with a mother at each end, only one pair of cables connected at each end?

I am also curious to what SP rated these with one or two mothers for continuous tractive effort. I don't think a slug doubled the tractive effort, but maybe added 50% comparable to an SD. With two mothers, you basically got a third locomotive. High hp EMDs of the era typically derated below 15 mph to 500 hp per axle (even the mighty SD45 dropped to 3000 hp at 10 mph). More modern locos have adhesion systems that allow them to use more power at low speeds, but once a DC motor heats up it will still derate to the continuous thermal rating. You might take a good run at the bottom, but if the hill is too long and your amps are too high you may not make it before it derates.

Thanks