DCC Loco Speed "Hunting"

[mrnimble]

I have installed a Digitrax DZ143 decoder in a HO Kato SD45 loco and all checks out well, HOWEVER - when running at about half speed the locomotive seems to be speed "hunting" i.e. - surging and then backing off, or pulsing, if you will, as it traverses my layout on the level.  Low speed is smooth and wide open is smooth.  The only thing that seems to moderate this action is when the loco runs up or down a grade.  I've used DecoderPro to look at the various speed affecting CVs and don't see anything there.  I have other HO locos running with DZ143s without any problems.  Any insight?  Is this a decoder issue or a Kato motor issue?  Also, at wide open throttle the loco only runs a scale 58 mph forward and 72 mph in reverse.  Thanks, Geoff

[rrinker]

 Try a reset first, CV8=8, but it sounds more like you have some binding in the mechanism somewhere, especially with a wildly different forward and reverse speed. Did you try it on DC first?  You might also have to fiddle with the BEMF CVs and turn it down a bit.

                 --Randy

[The_Ghan]

You seem to have several problems here.  I suggest the following:

1. Test the forward / reverse speeds on a DC layout.  If there's a noticeable difference you probably need to do some maintenance on the loco itself.  At the very least, you should be familiar with the operating characteristics of each loco before you DCC it;

2. Some motors have filter capacitors installed.  If the "hunting" problem is rhythmic then look for a capacitor and remove it;

3. Reset the decoder and try running with factory defaults.  If the problem persists swap the decoder with one in a loco that has never had the problem.  If the problem moves to the other loco then send the decoder back to Digitrax.  If the problem stays with the loco then there is likely to be a cicuitry problem somewhere.

Cheers

The_Ghan

BTW, Digitrax do recommend that decoder for the loco you're running.  If you have further problems contact Digitrax directly.

[mrnimble]

Per Randy's tip I took a look at the BEMF or Speed Stabilization CVs (55,56,57) and they were somewhat out of whack in relation to recommended defaults.  I don't know what the decoder was used on before - it came out of the junk box as I was in a hurry.  Meanwhile, bringing the dynamic comp CV56 back in line and then tweaking it along with CV57 got everything back to running smoothly.  Indeed, now this is about the smoothest running loco on the layout at the moment.  Thanks for everyone's consideration.   Geoff