Atlas F3 poor performance at low speed steps

The easiest test would be to check that the sensor is reading the flywheel tape correctly. With power on turn the flywheel slowly. Make sure the red LED blinks at every pass over the black stripe. If it skips one, the Sensor may need realignment or the tape may need cleaning or replaced. This has worked for me with TAS products.  

If it isn’t what Rod said about the sensor the next thing I would check is the motors themselves. I once had an Atlas GP35 that was very jerky at slow speeds. I sent it back since it was under warranty and it turned out to have a bad motor. Good luck. 

Thanks, guys.  I'll check the flywheel tape and call Atlas if it checks out OK.

George

Personally I would plan on replacing that EOB with ERR.  It's just a more modern, more reliable product.  No tach sensor, easier to install and no down-side except the cost.

If you don't mind while you have it apart, can you please put a small mark on the wheel tread and turn the flywheel by hand?  Please count how many turns of the flywheel it takes for one revolution of the drive wheels.  I was always curious what gear ratio Atlas used in these, and how it compares to Lionel and MTH.

Both Lionel and MTH are around 10:1, which is really Toy Train territory.  Consequently the speed control has to work very hard to keep the motor running at single-digit speeds.  After its excellent SW switcher, Atlas was roundly criticized by the 2-rail O scale community for the use of a "toy train" motor and gear train in its road diesels.  Supposedly they lowered their gear ratios in response, or at least discussed lowering the gear ratio on the Atlas forum.

I could never get a straight answer out of Atlas, but inquiring minds want to know