ps2 3 volt engine speeding up/ acting eradicly when smoke unit is on.

No, unlikely to be "the board" going bad.

"when I turn the smoke unit off the engine behaves as it should and runs without issue. turn it back on and issue comes back up immediately."

I'm thinking either PV related and thus the diodes might be suspect, or could be the smoke unit somehow shorting to frame, another known factor is a failing smoke fan affects 5V and could impact the tach sensor.

https://ogrforum.ogaugerr.com/...5#118977936478807965

Again, you said tied to smoke operation. Fan motors do fail and draw higher than normal current.

https://ogrforum.ogaugerr.com/...4#161349519415496504

I would definitely again check for possible smoke unit to frame short first as that can happen and could be bad.

Again, on MTH PS2 3V steam engines, the board is in the tender and connects via the tether.

Inside the engine, at the back is the circuit board with the socket that the tether plugs into. Some have 2 diodes on that board, other engines kind of laying right beside the motor, there can be 2 diodes under heatshrink tubing to the main motor terminals. They form a "Y" with typically a purple wire coming out of the "Y".  That is how voltage "PV" is developed in the engine from the yellow and white motor wires. PV is used for both smoke heater and headlight.

Example this one has no diodes, so then they must be in the harness around the motor

Or if on that board

From this topic about why they did it that way to save extra wires in the tether.

https://ogrforum.ogaugerr.com/...2#153463017418897932

@gunrunnerjohn posted:

The motor leads are diode-or'ed to provide the PV voltage, that's the 20-22 volt unregulated DC voltage that is used for motors, lighting, couplers, etc.  I suppose you could consider it a "return", just a positive one, it's positive compared to DC common on the PS/2 boards.  The only reason they did that was to save a wire, one of the motor leads are always at PV through the drive relay, so it was a cheap way to get the PV to the locomotive without using an extra wire.  Even in neutral one side of the motor is sitting at PV, it's the other side that is controlled by the FET that determines the power actually supplied to the motor.  The FET provides the PWM pulses to DC ground to determine the average current fed to the motor, obviously in neutral that current is zero.

Again, given the symptom of,

@Lionelzwl2012 posted:

guys, I have a mth 20-3144-1 ps2 3 volt hudson engine with bcr installed.

it was running fine for at least 2 hours. then all of a sudden it started jerking and taking off fast. then it would slow down and then again jerk and take off fast and so on. all the lights would flash off and on all of a sudden also.

when I turn the smoke unit off the engine behaves as it should and runs without issue. turn it back on and issue comes back up immediately.

I check the smoke unit and wires to and around it. they are all in order. the smoke units resitors are good and the batting is new with plenty of fluid.

whats going on here? is my ps2 3 volt board going bad? help please.

Since both the lights are going off, and then taking off fast kinda points to either a short in the smoke unit, or something with a PV diode possibly failing? This is just a path where frame ground AC can make it's way into the DC of the motor and with diodes and other details, possibly inject some voltage across the motor causing the speed runaway and also light failure.

Oh man, just realized I forgot another possible area of failure.  This is a 4-6-4 Railking chassis.

I don't know if that only applies to PS3 engines, or if this problem went into PS2 engines too? From what I've seen in PS3, this affected a bunch of those chassis.

https://ogrforum.ogaugerr.com/...8#160364097356136878

Front pickup roller interfering with front 4 wheel truck yoke.

thanks vernon for the diagnosis. I will check for what you described.

Pull the smoke fan plug and turn smoke on for a temporary test.  If it runs fine, the smoke fan motor going bad and noisey on the 5V DC line causing issues with the tach.  Replace smoke fan motor.  G