Speed CV Programming

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My apologies in advance for the long post. I'm including as much detail as possible to try to get the most accurate help possible.

I'm trying to match my HO Athearn Dash 9 (Part # 79885) and my Athearn SD38 (Part # 88569) for MU. Both units are equipped with Digitrax DH155A0 decoders without sound. I'm using a Digitrax Zephyr for control and programming.

My Dash 9 is significantly faster than my SD38, so I'm looking to slow down the Dash 9 (unless it's possible to speed up the SD38, but I kinda doubt it). I've read and re-read the manuals for both the DH165A0 and the Zephyr, but still cannot figure it out. The MU function is not where my issue is at, I have no problems getting the locos to run with the same throttle input, just getting them to run at the same speeds.

As far as I can tell, I'm supposed to:
1. set the loco to be programmed on the program track
2. press the program button until I see PAGE
3. press STEP until Pxxx is displayed (xxx representing the CV#, such as P029 for CV29)
4. press 3 to program CV03 (Acceleration)
5. press STEP until dxxx is displayed (xxx representing the data value for the CV)
6. press CV-RD to determine current value
7. press xxx (xxx representing the desired value)
8. press CV-WR to write the desired value to decoder
9. repeat steps 3-8 for CV04 (deceleration) and CV05 (V-Max or Max voltage)

These steps are as I understand them from the manuals (though a bit more broken down and consolidated than the manuals).

As for the "d" values for each CV:

000=faster acceleration

Haven't gotten to this one yet, I'm still having trouble getting them to accelerate at the same rate and obtain the same max speed.

000=lower max voltage=slower max speed (but may be reversed in some decoders)
I didn't notice a difference after I changed this value.

This isn't a huge issue for my home layout, as these locos will most likely be running on separate lines. However, at the club, I'll most likely be running them in consist and I'd like to be able to get their decoders set up so they're running the same to make the consisting at the club go a bit more smoothly. As an addition, it would be nice to have them able to run the same speeds in opposite directions on different lines at home (even though the Dash 9 would be a more powerful locomotive prototypicly).

I'm sorry if this is a bit of a green question. I'm totally new to DCC, and the Digitrax manuals leave much to be desired in the realm of helping new users figure out all of the features and how to program them.

Am I going about this the wrong way? If I'm going about it the right way, is there a source of reference to how the values affect the CVs? If I'm going about it the wrong way, what am I doing wrong and what should I be doing to get it right?

Thanks in advance,


Ok, run them both at max speed at the same time. If they both have the same max speed, then crank up CV 2 on the SD38 until it matches the Dash 9 on speed step 1. Once that is done, they should both be running at the same speed due to them having the same gearing and the same decoder speed curve (assuming that the speed curve has not been changed from factory specs)
Thank you diburning.

Since the max speeds were quite a bit different (I had to set the SD to 254, while the Dash 9 is only about 126 on CV05 after speed matching), I had to change the max speeds to get them together, then work from there with CV02 and CV06. It took me about an hour of changing a value, then testing, then changing again to get it all right.

Good new is, though, I got it now, or at least close enough. I may have to continue fine tuning them, but they're in the ballpark.

Thanks again,


Hmm that is odd. They should have the same motor, same drive, and same gear ratio so a speed limit that great either has to do with the decoder or the motor.
The SD38 has always been one of my slowest locomotives. It's not the decoder, because the DH165A0 is a board replacement decoder. It was still painfully slow with the factory light board. When running it in DC with my GP38, I would have to put the GP in the back because even with Kadee couplers, the GP would break away from the SD. It's actually faster with the decoder installed.

I'm going to the club today for our 3rd Saturday open house, so I'll see how the MU works on a large layout without elevation (not to mention better laid track) and running a bit more power than my home layout.


Active Member
Sounds like you figured it out. The only thing to add is turn off BEMF if the decoder has it. Otherwise the loco's will bounce on you.

The loco's only have to be close to the same speed. Just place the one that's a little faster in front and you'll be all set.
Sounds like you earned a good running session with all that work ;)
BEMF? What is that and what does it do? How would I go about finding out if it has it, and if so, how do I turn it off?

Believe me, I'll be getting in an operating session today. I'll be putting a bit of a workout on these two engines by pulling the Juice Train, our club's 23 Tropicana Orange Juice Reefer cars. I know, it's certainly not the longest train that these locos could pull (I imagine that just the Dash 9 should be able to pull nearly double that on it's own, depending on the layout), but it's about the longest I'll be able to run at the club.

Thanks again,
Though I didn't program anything with the BEMF, I did get a nice running session in this afternoon at the club. I even made a video of it.

Here's the link to it:


With all the frustrations I've had and all the lessons I've learned throughout my transition to DCC, I was able to help a club member this evening get his MTH SD70 locomotives programmed and set up for MUing. He mostly uses DCS for his stuff at home (both G and HO), but was having some difficulty getting his HO locos set up for running on the club's Digitrax layout (I had a couple of issues transitioning from the DCS50 to a DT400R, but was able to quickly resolve them).

On another note from the club, I set a new club record on the O gauge layout for the longest train pulled. Most of our O gauge members run steamers and E/F unit diesels at the club. I came today with a dual motor SD70ace. The guys there usually don't run more than a dozen or so cars. With some rolling stock rearrangment help from the guys, I managed to get it to pull 32 cars plus a caboose. The locomotive still had power and traction to spare.

All in all, a good day at the club.


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