MTH PS3 steam engine tach board speed control problem

Probably the first thing to check is the spacing between the optical sensor and the flywheel, it should be around 1mm.  If that's good, I'd consider replacing the sensor board, they're available at MTH Parts.

For PS-3 if you have control over the engine function but no speed control the problem will be in the engine.  Not drawbar or tender since the boiler board and tach control this.

As long as reader is over stripes it should be fine.  Since it takes off you have no tach input.  So, tach reader bad, maybe intermittent wire break, or spacing as mentioned though you don't have much option on a PS-3 as the bracket and sensor are fixed.  Lastly could be a boiler board issue.  G

Thank you both for the informative, helpful and timely responses. I really appreciate it!

With this excellent guidance, it's back to the train room repair bench with added confidence!

Best regards,

Walter

I've seen this issue on two PS3 GS4/GS6s. For some reason the bracket holding the tach gets bent down slightly and puts the tach just close enough to the flywheel that it can't read the stripes correctly. The fix I've found is to remove the tach mounting bracket, get it warm with a heat gun, and slightly bend it back to get the specified 1mm gap from the flywheel. That fixed the issue on both locomotives.

Hi everyone,

Turns out the tach sensor spacing was 2+ millimeters. I used Lou's heat gun approach to bend the plastic bracket down a bit to get it closer, and that fixed the problem. I still have some startup runaway problem under a certain power up condition, but the tach otherwise works fine. Thanks to all who contributed to solving my problem.

This plastic bracket issue does seem like a missapplication of somewhat flimsy plastic material in a millimeter level sensitivity. Seems like either a poor design or manufacturing implementation. Thankfully this user forum provides relief from such oversights.

Best regards,

Walter

For sure 2mm is clearly a problem, at that spacing your symptoms fit perfectly.

I have an 0-8-0 late model PS2 that I posted about a while back. It will smooth out at 8 smph but below that it is very unsteady. Really atypical for a PS2 or 3 steam engine. The worst symptom is rolling the DCS speed wheel up to 5 smph. The engine sits for a second then lurches forward and moves in a jerky fashion that improves with speed. I had the shell off and didn't feel any resistance when turning the flywheel by hand. Also the tach reader looks to be positioned correctly. Any advice since we're on the subject?

Otherwise it's off to George and whatever it takes to get smooth low speed performance I LOVE this engine but really hate the low speed stagger.

Thanks

@Scott J posted:

I have an 0-8-0 late model PS2 that I posted about a while back. It will smooth out at 8 smph but below that it is very unsteady. Really atypical for a PS2 or 3 steam engine. The worst symptom is rolling the DCS speed wheel up to 5 smph. The engine sits for a second then lurches forward and moves in a jerky fashion that improves with speed. I had the shell off and didn't feel any resistance when turning the flywheel by hand. Also the tach reader looks to be positioned correctly. Any advice since we're on the subject?

Otherwise it's off to George and whatever it takes to get smooth low speed performance I LOVE this engine but really hate the low speed stagger.

Thanks

This could be a mechanical issue, if unsure how to go about diagnosing the issue, best to send it off to George for that diag……that could be any number of things creating a drag, and making the locomotive lug through those lower speed steps…..

Pat

@harmonyards posted:

This could be a mechanical issue, if unsure how to go about diagnosing the issue, best to send it off to George for that diag……that could be any number of things creating a drag, and making the locomotive lug through those lower speed steps…..

Pat

Thanks Pat. I'm going to take the shell off one more time and then it's most likely going to G^3