The old Atlas/Roco GP40 is a lot different internally than the Atlas/Roco FP7, so I'm not sure the FP7 instructions will be very representative of a GP40 install.However, I've converted an Atlas/Roco GP38 (same internally as the GP40) to DCC using a Digitrax DH163A0. Although the DH163A0 is designed as a drop-in replacement for other locos, that isn't the case here.What I did was tape it to the two plastic supports so it sat over the motor. Then I just connected the pickup wires from the front truck to the two outside front pads on the decoder, and repeated with the rear pickup wires.The front light was wired to the F0F tabs with the required 22 ohm resistor for the high-current bulb (this install was done as a sort of experiment so no LED's, etc.).Finally, the motor leads were soldered to the motor tabs and away she went.
(Although you may have to reverse those motor leads if 'away she goes' in reverse!)It wasn't a difficult conversion at all. You could certainly use other drop-in boards, or even a wired decoder in similar fashion.
Roco 4178B Install of a DCCconcepts Decoder. BR 103 Fleischmann DCC Decoder Install with Tail Lighting. The TCS T1 decoder is only a two-function decoder, on previous installs I have used a 4 function decoder to power the headlight at each end and the centre part of the locomotive, by installing two diodes on the circuit board both the.
Here's a link to the DH163A0. Even if you use a different decoder, looking at the board layout on page 6 and reading this description of my conversion may help:Good luck!Steve.
The motor leads need to be isolated from the frame. From the sound of the wiring, this is already the case.
(Two wires coming from each truck would make this likely.) Still, it doesn't take much to put a meter on the thing and make sure there is no electrical path from either side of the motor wires to the engine frame. Some older engines used the frame as 'ground' and only ran wires from one side of the trucks.If you find the motor and frame are together electrically, then you must remove the motor, put in an insulating material like electrical tape, and replace the motor using plastic screws. After that, just wire up the decoder according to its directions and try it out on the programming track. The programming track, by the way, usually won't blow out the decoder if the engine is mis-wired. If you can't program the address (and read it back, if your DCC system supports that) then you should re-check your wiring before putting it on the main line.