I’m about to try installing a couple of decoders in two Athearn engines. hat’ste procedur t do so, n particular nsulating(isolating) the electric motor from the chassis. Having insulated the motor from the chassis how does the wiring work to retun power from the decoder to the motor and then to the chassis.
If your talking the Athearn BB type here…Once the motor is isolated (by removing the little tab on the bottom of the brush clip so that nothing can make contact with the metal chassis), just pick up power for the decoder from the right and left sides of the rail (one side from the trucks and the other from the chassis) then solder the motor leads from the decoder to the top and bottom brush clips.
The decoder has 2 wires that go to track pickups. You can use the frame for one of your power pickups, although its not the best way to do it. The decoder also has 2 wires that go to the motor, one to each terminal. What the decoder does is decide how much power to send to the motor depending on the commands you give it. So the motor is connected to the frame, but not directly. It has to go through the decoder first
Hope this answers your question.
If it’s an Athearn BB or one of the non DCC ready R-T-R models I take the easy way and purchase a Digitrax DH163AT or DH123AT decoder which includes a wiring harness for the Athearn mechanism so that no soldering is involved. As stated before, just make sure you isolate the motor from the frame.