Re: Converting older steam loco to dcc a scale
I don't know that there are any decoders specifically made for S scale. I did a quick check for you on the Soundtraxx website. Here is a link for you and their recommendations:
I highly recommend going with a decoder that has sound capability. Sound brings model trains to life. Also check out TCS's WOW! sound decoders and ESU's LokSound. These are all popular with modelers.
You can't wrong using a decoder for O-scale. If your motors are high efficiency, you might be able to use HO scale decoders. The main thing is that their stall current doesn't exceed the capability of the decoder. Put your loco on a test track using "old-fashioned" DC and briefly hold the loco so the wheels don't turn. Measure the current it draws. If this is within the current limit of the decoder, you can use a smaller HO locomotive.
Wiring for DCC