If the output of the decoder is producing 15 v dc power (i can measure it with dc meter)
as the dc engine is running at full speed does that tell us anything about the decoder.
?If i did short the output and damaged it would it still produce the dc power.?
A shorted output is highly likely to damage the transistors that drive the motor, and in that condition their failure mode is likely to be short circuit, so yes, it sounds like this is what has happened.
The other likely failure mode is the transistors act like a fuse and go totally open circuit, in which case the motor never drives, but it sounds like you have the short circuit case.
