"Jerky" low speed operation

Maybe check the spacing  between the reader & timing tape. (should be the thickness of a dime)  Is the tape clean?

try turning the fly-wheels by hand for any binding.  (do a full circle of the wheels)

Thanks Gregg.......The tape is clean I used denatured alcohol on it (it wasn't dirty to begin with)  The motor turns freely, checked it when not connected to the wheels.  Checked wheels for free turning, when not connected to the motor.  Motor and wheels turn freely.  Is there an adjustment for the sensor that reads the "tape" on the flywheel?  I didn't see anything that would allow any adjustment. 

  Is there an adjustment for the sensor that reads the "tape" on the flywheel?  I didn't see anything that would allow any adjustment. 

No. How's the spacing?  I've resorted to using a  plastic wire  tie to pull the reader down or  sticking something under the reader bracket to raise it.   (the teck  guys are shaking their heads) but I would avoid trying  to solder in a new spacer. (if needed)

How does it run without the other units?

You can take the flywheel/tach/speed-control out of the equation by turning OFF speed-control using the Whistle-Bell-Bell sequence (and getting the double-toot confirmation).  For the same track voltage, the engine will go much faster.  But on a section of level and straight track, for a fixed track voltage does it still jerk at the appoximate same speed.  If so, then you could have an issue in the gearing or mechanical couplings - such as a loose screw holding the motor to the truck.

I like operating the loco on its side with leads attached to a center pickup and touching one wheel (MTH locos don't have a good chassis ground).  Cycle the power to go in forward direction and note if both wheelsets operate smoothly.  If one is jerky, troubleshoot the offending one.

With a track voltage like that, I presume you're operating in "conventional" mode (variable track voltage.)  Have you ever tested it in full DCS mode with 18-19VAC on the track?  The higher voltage in command mode gives the on-board circuitry more to play with, and high-voltage pulses have more inductive kick. 

It may also be operating normally. For  many years MTH has stated that their locos will run smoothly at 2-5 scale mph (the claims have varied, but in some cases it's as high as 5 mph.)  My personal opinion is that almost all vertically-motored diesels are geared too tall ("toy train" speeds in excess of 100 MPH.)  Also, since the worm gears are self-locking, the motors can't "help" one another keep moving, as they could on a Lionel 2383 or FM Trainmaster.

Good luck in your quest and please let us know if you find the cause.

"My personal opinion is that almost all vertically-motored diesels are geared too tall ("toy train" speeds in excess of 100 MPH.)"

I certainly am a card-carrying "low-gearing" guy; I've complained about "zoom-zoom" here more than once - but - his loco has PS2, which has a cruise control. A very good one. I have vertical can motor diesels/electrics that run at decent low speeds without cruise (some older ones) and at a sub-5 smph creep with PS2/ERR/Odyssey. The vertical motors are not the issue with fast speeds. Some are geared so low that they won't break 60 smph.

But - I've had consistent bad luck with single-motor-driveshaft units - 100% failure rate, actually (one 3rd Rail Dash-9; one Lionel Alco switcher). Neither works; mechanical failure, the Lionel even after I bought new parts. So - I'm prejudiced against them.

There may be a software/board issue with the subject loco.

I found the cause of the jerky low voltage operation, but not settled on a solution yet.  I took the shell off, placed the engine on  it's side, and connected power to the engine.  Both motors work......however, the motor which has the sensor and the black/white tape and white flywheel, runs slower than the motor with the black flywheel.  I disconnected the white flywheel motor from the wheel assembly.  The wheels move very freely, no binding here.  The white flywheel motor is free, and I can easily turn it by hand, but it seems to have a small amount of resistance to it, almost like there is something magnetic keeping the motor from spinning freely....if that makes any sense.  Aside from replacing the motor....any ideas? 

Also make sure the tach reader is vertically not too high or too low, you want it to read the entire stripe if possible.

You mention you think its the lead A unit that is the problem but have you tried running it alone? I only have one engine with a slave unit and its the slave unit that causes the less than smooth movement. 

Pete

AMF: That "something magnetic keeping the motor from spinning freely" is normal in DC can motors. There actually are permanent magnets in them that gives you that 'cogging" effect as the armature turns within the magnetic field those magnets provide. This happens even with no power applied to them.

You may be thinking of the older Lionel AC "Pullmor" motors-their "magnetic field" is not present until you provide power to the field coil when you increase the track voltage.

Thanks D&H 65......the "cogging effect", as you mention, is what I am feeling when I turn the motor by hand.  Not enough to bind, but enough to make it different than the other motor in the engine.  I believe that this is what is causing the "jerky" starting at low voltage.  It is interesting though that the other motor runs smooth, without the "cogging effect".  Is there a cure, without having to replace the motor?  (Of course the other option is to run it the way it is!  Just apply more voltage during the start!......problem solved!) 

And to everyone: thank you for your inputs....it's an adventure in "problem solving", and experts will say that this type of "thought processing" will stave off dementia!.......

To summarize:  The tach reader is centered on the black/white band and seems to be spaced properly based on the description provided here.  The gears are completely free when the motor is disconnected.  I agree, models don't need 100 plus mph capability, (unless it is a bullet train) but low speed smooth operation is desirable.  I operate in conventional mode only.  Hope everyone had, or is still having a great Thanksgiving!

If you want to dive deeper, try turning OFF the speed control as I suggested earlier.  This means the tach is taken out of the control loop with track voltage directly driving the motor(s) old-school style.  On a flat section of track, adjust track voltage to whatever speed has been giving you the jerky behavior.  When you place the engine on its side with the wheels/trucks spinning freely "in-the-air" (as opposed to on a track with the weight of the engine), you don't get the same friction/load on the motor and transmission/gears.  The point being you need to load it as it is used.

Another troubleshooting technique is to run the engine on just one motor.  As you saw, if you remove the motor from a truck, the truck wheels will spin freely.  So you carefully place the unused motor on the chassis protecting against wire shorts or interference, you can try one motor or the other (with speed control turned OFF) to see if each motor/truck behaves the same UNDER LOAD. 

Way down the troubleshooting path there have been issues with certain diesel trucks with gear meshing.  This gets fairly involved and there have been some threads about it.  Typically it is solved by inserting a shim (a high-tech (LOL) piece of cardboard) in the truck.