Slave motor problem

What's the product number and vintage?  Unless I grossly misunderstand, the only thing being "transmitted" to the motors is DC voltage.

I'm not sure why the voltage to the two motors would be unequal; perhaps this is by design but the disparity shouldn't be great.  Are all of the modular connectors firmly seated?  I don't believe any version of PS2/PS3 monitors back-emf from the motors (that's how ERR boards work; that system uses back-emf in lieu of a tach sensor.) 

My guess is that the front motor is weak or has failed.  Maybe it overheated or has been operating under high load for some time due to an undiagnosed binding condition?  If tho motor is shot, your choices are to find an identical replacement, or remove the front motor and just live with one in that loco.  Unless you're into single-headed 30 car trains it should pull fine and might even run better with just one motor.  -Ted

Phil,

Your test really doesn't prove anything at all. Both motors are wired in parallel to the PS2 or PS3 board.

The motor with the flywheel will always increase voltage to both motors when the tach reader detects that the flywheel motor has slowed. This is how speed control works. There isn't, however, any way for the tach reader to know when the other motor has slowed so it does nothing in that case.

Classic problem with speed control for either Lionel or MTH.  If the motor with the tach loses traction, the speed control thinks it needs to reduce power to keep the RPM constant.  Of course, that reduces the power to the other motor as well.  The only way around this is to have independent control of each motor, I doubt that's happening any time soon.

Barry and John,

You're missing an important point in Phil's original post.  Presumably this loco was working fine and started exhibiting this problem all of a sudden.  Something changed and is definitely not "normal" anymore.

My guess is that there's a problem with the power supply to the front motor, or the motor itself has weakened to the point where it's out of spec.

Many dual-motored locos made by Lionel have "back-drivable" gears, so one motor can push the other along when it begins to fail.  Every MTH diesel I've seen has two self-locking worm gears.  If one motor fails or starts to fail, bad things like this will happen.

Phil if you perform the test you described on another "healthy" PS2 loco, do you get the same result?  -Ted

Ted,  His test still give the same results.  First, the terminology isn't exactly correct.

In side a 2 motor engine you have a Tach motor and no tach motor.  They are parallel wired.  The tach motor controls the motor driver.  So it you try to slow it down, it will order the driver to add power to compensate.

The no tach motor just does what the tach motor tells the motor driver to do.  If you slow it down, there is no feedback to tell the motor driver what is going on, UNLESS

the motors are coupled via the track.

So his test doesn't help diagnose the issue.

The question is why is the engine derailing?  Does it derail in both directions?  Is the truck motor mount loose?  Has the shell been off recently and the wire harness or some other item is preventing complete rotation (Swivel) of the truck.

If this runs smooth and fine on the straights, I don't think motor is the issue.  Something else is causing derailments.  G

Phil,

The odds are that this is not an electrical issue or a DCS issue at all. It's almost certainly an engine mechanical problem or a track issue.

Check the single motor mount screw under the truck.  I've seen these be loose and cause these symptoms.

Thanks to all of you, thus far.  Here I'll report 2 more related issues, and will soon perform some of the additional tests suggested.  BTW, the engine is the 'big blow' gas turbine, with 4 total motors. [UP 26]

1.  At the suggestion of MTH, I sprayed some electronic cleaner into the armature area, which might clean any dirt/oil.  No effect.

2.  At low speed [say  < 10 mph] on tangent track, the front truck of the A-unit sort of 'stutters'; jerks.  Also, the rear truck of the B-unit sometimes derails....

I'll also test the A-unit alone.

From what I read, his test of loading the 'master' truck should have produced a speed increase on the 'slave' truck if things are working properly.

  BTW, the engine is the 'big blow' gas turbine, with 4 total motors. [UP 26]

If I remember correctly the very early big blow  (PS-1 )  had to have a re-designed floor to prevent derailing. Nah!  this couldn't be one of those. . Is this new engine Phil?

The PS1 was UP 5 & 5B.

....AND NOW, FOR THE REST OF THE STORY!

After the A alone ran OK, I concluded that the B-unit was the problem....It was. So now to go back a few weeks....The B-unit stopped running in forward.  A call to Don Lockwood @ MTH diagnosed the problem:  The FET in the slave board, in forward mode,  had shorted out.  I got a new one from MTH & installed it successfully; I thought.  But the problem that began this thread started after only a few minutes of run time.  Turns out that my soldering job was less than perfect....cold solder joint.

          Re-soldered & retested.   ALL IS OK NOW!!!

At least I learned a bit more re: master/slave motors.  Thx again, guys.

BTW, Dale:  Yes, the slave did speed up.

There's always another detail to the story!

True, Now that we know it is a 4 engine unit.  Prior to that the description did not make sense.  G

It is important to know model.  The Veranda has the 4 motors in the Lead Unit and that can be tricky to figure out.  G

FOUR  motors in ONE unit?  Really?  My Lionel Veranda has but 2....

Yep.  That is why I said wire routing around motors can cause derailments on trucks.  It binds against shell and doesn't completely rotate.

Here are the pictures.  4th motor under cab interior.  Slave board stacked on top of PS-2 3V board.  G