Need some troubleshooting help Railking PS/3 Diesel problem

If the tachometer system isn't working (example broken wire from flexing, bad solder joint, optical sensor moves out of alignment) and becomes open loop (no feedback)= runaway.

I would put it up on rollers, or use alligator clip leads for power, rotate the trucks and thus flex the internal wiring after first doing a solid internal inspection.

Also, edit the title, this is a PS3 engine and that does matter for details because the tach sensor is different, as is the wiring harness between PS2 and PS3. Also, PS3 tends to be much smaller finer wire and thus I have seen bad solder joints and wire breaks due to flexing - especially in this tachometer area.

30-20323-1https://mthtrains.com/30-20323-1

Again, these PS3 engines, a few have the thin flywheel, and the tachometer optical sensor can slide up and down in the bracket and could be no longer aligned reading the stripes.

The tiny little sensor on the flywheel side of the PCB has to perfectly align to the center of the flywheel- and on a thin flywheel, there is no room for error.

If it runs cold and then takes off after the electronics warms up, it's sounding like it might be the PS/3 board.

Thanks for the replies.

Vernon, I have changed the topic title to indicate PS/3.

Overnight I was wondering if a dead battery could be the cause.  If the engine goes over some dirty track it could lose power for a fraction of a second and switch to conventional. 

The engine is stored at the location where we are doing our display;  I'll be able to take a look at it later today.

PS-3 doesn't use batteries.  Supercap.  If it will run correctly for a period of time then go full speed.  Either tach reader, board, or some software glitch.  Reload flash code and sound file.  Replace tach reader.  After that it is the board.  Can try swapping into another diesel PS-3.  If symptom moves with board, board is defective.  G