Originally this info was posted on the Atlas Forum - I'm re-posting it here since the original photos are all gone there.
I've seen several questions posted on various forums relating to the new Bachmann tender's internal wiring. I decided to examine the circuit and here are my findings.
While this Bachmann tender does contain a complex-looking circuit board, it is not a DCC decoder nor it is very useful for hooking up a decoder when used with a non-Bachmann locomotive. That's because this board is designed specifically to connect with the "umbilical cable" which Bachmann uses as an interconnect between the loco and the tender. Other than the cable connector there are also several components for reducing Radio Frequency Interference (RFI) for European markets, and few diodes and resistors which will most likely be discarded with non-Bachmann decoder installs.
Here is a photo of the tender with the body removed.
Here, the circuit board has been detached from the tender floor to provide a bottom view of the circuitry. Both tender trucks and the draw bar connections are wired to the circuit board using a red and black wire. Per standard wiring practice red wire is the electric pickup from the right track while the black wire is from the left.
A closeup of the bottom of this circuit board.
This photo shows a top closeup of the circuit board. The 6-pin male connector shown on the left is for the "umbilical cable" which connects loco to the tender. That cable carries the positive (M+) and negative (M-) motor connections, positive (L+) and negative (L-) headlight connections and redundant positive (R) and negative (L) track connections. I say that those are redundant because those connections are already made through the tender's draw bar. This circuit board also has 8 gold plated solder pads numbered out of sequence 8 1 2 4 5 6 7. There are2 shunts installed, connecting pads 8-1-2 and 4-5-6. This is the default setup for a Bachmann loco which utilizes this tender and it is sold as a DC loco (no decoder installed). Those shunts connect the positive and negative motor pins (M+, M-) to the right and left rail pickups (L, R). They also connect (through 2 diodes and a resistor in series) the headlight pins (L+, L-) to the rail pickups.
When a decoder is to be installed (in a DC Bachmann loco), those shunts will have to be removed and the DCC decoder can be soldered to the numbered pads. Note that there is no rear headlight so there is no spot on the circuit board for the the yellow decoder wire (F0R). If someone wants to install a rear headlight, it will need to be wired directly to the decoder.
This photo shows a most likely way to hook up a decoder when this tender is used with a Bachmann loco and connected to it via the "umbilical cable" (with the shunts removed). The decoder wires would be soldered to the pads on the bottom right of this board using the color codes as shown in the picture. I also do not know the purpose of pad number 7. The way it is wired on the circuit board doesn't make sense to me.
I traced all the connection on the circuit board and I created a wiring diagram shown here. The chokes (coils) and capacitors are used for RFI noise reduction (for non-US markets). I'm not sure what is the purpose of those two 220K ohm or the 470 ohm resistors. The 1.5K ohm resistor is used as current limiting resistor for the headlight LED. Using 2 diodes in the headlight circuit also seems a bit redundant to me.
I really don't see any advantage in using this circuit board when the tender is used with a non-Bachmann loco. Since there is no "umbilical cable" which would connect to the connector on this board, this circuit will not really be helpful. Even if someone created their own "Umbilical cable", the extra components on the circuit board would just complicate the wiring. The easiest way to use this tender in DC mode with a non-Bachmann loco is to ignore the circuit board and just use the draw bar connection for tender pickup. For wiring a non-Bachmann loco using a DCC decoder the simplest way would be to discard the circuit board and place decoder in the tender. Then use the draw bar for the track power pickup from the locomotive and run decoder's orange, gray, blue and white wires straight to the loco. Those will supply power to the motor and headlight.
If the circuit board is to be kept then remove the 2 capacitors marked C2, and C3 behind the "umbilical cable" connector on top of the board. Those are the small beige components with metal ends and no markings. Do not place jumpers across the empty pads. C1 can be left installed - it is wired across the connections feeding the LED headlight and will not have any negative effect on the operation. Then place wire jumpers across the pads to which the chokes are soldered. The chokes can either be left in place or removed.
