I am wondering, I have a dcc engine with a circuit board and a decoder but to convert an engine you only need a decoder ? Also how many wires come out of the decoder ?
It sounds like your wondering why there's a DC board along with a decoder, and the answer is that like most locomotives now come with simple circuit boards for lights and motor resistors, most decoders don't replace these boards bit instead are just pluged into them. There's usually 9 nmra color coded wires coming out of a decoder.
Often times, locomotives are sold as DCC ready and have an 8-pin socket into which you can place a plug that is attached to the wires coming from a decoder. Typically there are 8 wires that come from the decoder, but there may be more, depending on the number of extra functions they can control. The wires 1) connect the decoder to track power, 2) send power to run the motor, and 3) turn lights on and off. If you have an engine that is not DCC ready, you can still convert it, but the process is a little more complicated. You also need to understand that while most decoders sold today are dual-mode, which means that they will run on DC, there is no advantage to doing so. To take advantage of DCC's features, you need, in addition to a decoder equipped locomotive, a DCC throttle, command station, and booster. Hope this answers your question.
Rusty
The minimum number of wires that come out of a mobile decoder is normally 7. That is two wires to pick up power from the rails, two wires to run the motor, and 3 wires from the front and rear lights. There can be a lot more.
Jim
You're right, Jim. I keep wanting to give the backup light its own ground. :-\
Rusty
Actually, Rusty, you can give the backup light its own ground and sometimes this is a usefull way of reducing the number of wires between tender and locomotive. If you put the decoder in the locomotive, then the only wires you need between locomotive and tender are two for track pickup and one for the tender light. The return side of the light goes to both track pickup wires via diodes (the cathodes or bar ends to the light, one anode end to each pickup wire.) Of course this only applies to tenders with track pickups.