Yesterday, I noticed that my Athearn RTR GP35 was surging in random spots on my layout even though the wheels and track were clean and the motor was properly lubed. I replaced the Digitrax DH123d decoder inside it with a newer DH126 that I removed from another unit (for a TCS). I didn't see much difference with it. How can I fix it so it won't surge anymore? I noticed after I installed 2 1/4 oz. lead weights since it was too light. I'm not sure if that would have anything to do with the issue since I ran it without the shell (where I put the weights) yesterday and it still surged.
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Just a guess but you might check to see if you still have it DC enabled. Sometimes that can cause spurios responses. Try disabling DC. I don't remember what CV that is but it should tell you in the decoder instructions.
Marty C
Especially if it's a steamer(I'm thinking might be the case since you added weight), be sure and check the running gear. This area is particularly vulnerable when working on steamers with things like bending rods accidentally. Be sure and lube the rods too. Make sure all elements are straight and not binding. Sometimes a screw can get loose,etc. Also check the electrical pick up and make sure a wire hasn't come partially loose.
Richard
If the GP-35 is a "blue box" engine, it just might be because those engines don't have great motors and do often "surge" like that. Replacing the motor usually improves the engine greatly. Some newer Athearn Blue Box engines are designed to be able to accept Athearn Genesis motors, which are much better.
If that's not the issue, check the CV for Back EMF too. If that's too high, it can cause problems, especially running in a consist with another engine. Try turning BEMF off and see if the engine runs better.
The old blue box Athearn engines also had sintered iron wheels that weren't all that great for making contact in DC ! DCC is much more sensitive to positive pick up.
I have some old Athearn blue box GP38-2's that run considerably better once I replaced the factory wheels with after market nickel silver ones.
Mark.
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You could try tweaking the BEMF (Scalable Speed Stabilazation, as Digitrax calls it).
Instructions "To set up a loco with scalable speed stabilization" are located about 2/3 of the way down this page: http://www.digitrax.com/tsd/KB902/scaleable-speed-stabilization-back-emf-in-digitrax/
Another thing to consider - has adding weight exacerbated a binding issue? The Binding might have the BEMF confused and hunting for a sweet spot, if you will. This site http://girr.org/girr/tips/tips5/dcc_tips.html discusses in the "Binding" section.
Would the engine happen to be going downgrade when the surging occurs? If that is the case, then that is backlash in the driveline, a very common issue especially with older engines.