Changing the speed steps from 28 to 128 on a DCC on board GP40

[TrainmanDoug]

I have a GP40 with DCC on board.  I need to change the speed steps from 28 to 128.  I have never done it before.  Which CV do I go to?
I have a Digitrax  controller.

[Hunt]

Read Decoder Speed Step Settings section in the Digitrax manual.

Read about CV 29 in the decoder documentation   Click Here .
The decoder CV 29 default setting of 6 includes the use of 28/128 in your DCC ON BOARD Bachmann GP 40 locomotive. Thus the decoder automatically changes to accept 28 or 128 based on command from the DCC command station. However, unlike most other decoders; this decoder only operates in 28 speed step even when receiving a 128 speed step command.  You will have to replace decoder to if you want 128 speed step operation.



Programming the Bachmann decoder factory installed in the locomotive
A 1000 Ohm, ¼ Watt resistor laid across the programming track is required for CV read and programming  a non-sound Bachmann (Lenz based) decoder used with a Digitrax Zephyr (DCS50) or Zephyr Xtra (DCS51).