problem with mth ho berkshire steam engine

Could be, but didn't the new motor come with a Flywheel?  I have not worked on an HO berk.  Those symptoms do sound like a tach issue.  If not tach, may have to swap out boiler board.   G

I did install a new tach board which i bought along with the motor. 

could my problem be the flywheel?

the motor did not come with a new flywheel.  Would you have any to sell?

Not a flywheel no.  What model # is this?  G

80-3172-1

Ignoring the jerking, does the engine generally respond to changes in speed command?  The complete absence of chugging sounds suggests the tender electronics is not getting the tach feedback telling it the motor is turning.

There still may be an issue with the tach/flywheel circuit in the engine electronics, but if there is some semblance of speed control, then it could be a loose or shorted connection in the drawbar which sends the tach feedback to the tender.  Worth a look on both sides of the drawbar.

I have checked the tether connection and I don;t see them shorting out  any other suggestions would be appreciated

The parts diagram is listed on the web, so if the face of the flywheel is clean so the tach read can read it, how is the gap since you replaced it?  To large a gap can cause issues.  The boiler board controls motor features and a bad motor can take out a diode on the board so I would inspect the boiler board.   G

the motor has been replaced and the space between the tach board and flywheel is between .5mm and 1.5mm.  I also  replaced the ps3 boiler board.  I haven't replaced the main pcb.

I don't know but when replacing the boiler board does the chain file need to be re-loaded?

What is the chain file.  How do I reload it?

Not needed, the tender board flashes the Boiler board in HO.  Maybe the new tach was bad.

I would disconnect boiler board and run motor on DC current to see how it performs.  Smooth operation and no high current?  This would remove mechanical as issue.  A tech could take your boards and test them in a bench set.  At that point eliminate boards.  That would leave you with wiring, tach reader, flywheel.

I assume that you have control over the boiler features.  Headlight, smoke unit?  If so, the draw bar connection is fine.  Certainly sounds like tach issue.  What does the face of the flywheel look like?  Do you have clean stripes?  G

the motor runs smooth on dc with out the boards.  What are these clean stripes.  where do i get them?

the flywheel looks worn, some portions of it are a little faded.  could this cause problems?

did you answer the question of whether the engine responds to changes in speed command?  If you command 10 MPH is it roughly twice as fast as if you command 5 MPH?  A properly spaced flywheel that is worn/faded typically presents as a speed error but responds to changes in speed commands.  A broken tach of completely mis-aligned flywheel will typically just take off at full speed irrespective of the speed command.  This is because the electronics uses the number of black/white reflections from the flywheel to determine how fast the engine is going for the purposes of adjusting the motor drive.  These reflections are also used to trigger the chugging sound (as well as the smoke puffing if you have that turned on).  Identifying which functions work and which don't help break down where the problem lies.

The engine does respond to changes in speed command.  so is the problem the flywheel.  would you know of who sells these, orther than going to mth directly?

What does it actually do?  Will it move at 5 smph without jerking?  Will smoke puff?  Just not chuff?   I would think you would need to get a flywheel from MTH.  This is not a normal wear item, and I doubt anyone would stock them.  I still would recommend testing before you just replace everything.  How about wheel and pickup cleanliness?  Reload sound file.

No chuffing and puffing is usually tach related, but jerky motion can be many things.   G

Doesn't HO loco's use a inverted flywheel to save room? What I mean is it's sort of like some lionel installs where the "tach" reader is mounted "inside" the flywheel? I seen something like that in a MTH HO scale SD70 that a friend had apart.  

yes it does have a flywheel with the tach reader inside.  The engine jerks at even 5mph.

I made a mistake,  The tach is mounted to the motor  it is not inside the flywheel.  sorry for the mistake