Motor problem? or Tach Reader problem?

I'd suspect a motor, drive train, or wiring issue.  If you have binding in the drive train, it can cause those symptoms.  Another thing to look for is a possibility of a short of one motor lead to the chassis.  Since the sounds are cutting out, it's obviously pulling down the voltage on the board a bunch, I'd sort that out before you smoke the board.

A useful exercise is when the board is on the tester, put some friction on the drive motor flywheel and cause it to draw more current and work harder.  See if that reacts properly.

test the motor draw?

Triplex is a case study in engineering that can boggle the mind,….lots going on under the shell of that monster,…..gear towers, multiple drive shafts, the works,….as John suggested above, best to rule out any and all mechanical issues…..remove the shells and rotate flywheel by hand feeling for any binding/tight spots, address as found,…..On large articulated locomotives, I like to test mechanisms by unsoldering motor leads and either use a DC power source, or a bridge rectifier to test mechanics with no fear of smoking precious boards,…..I have a oober long pair of test leads to “ hot wire” motor to track and test on rollers or a 4 foot long section of straight track,….

Pat

Well one sysmptom eleminates drive train bind and tach reader.  It can't be moving along and then speed up with high current.  That is an intermittent Motor lead ground to chassis.  It would have to stall first before speeding up to be binding.  Tach readers either fail or may mis read which leads to faster then normal speed.  Very rare one is erratic and that would mean the potential for ups and downs in speed.  You probably have a wire problem.  G

I agree that the wiring problem is very likely, just not willing to totally rule out drive train issues.

If he described the symptoms wrong you are correct, but a binding problem can't make an engine take off fast with high current.  Instead it would slow down from the bind and generate higher voltage to the motor to try and maintain speed, but since motor is binding act like a stall and generate higher current first.  Then when the bind releases the engine would take off from the high applied voltage but the current would go down since motor can spin freely and speed control would reduce motor voltage now that it has over speeded.

Really accurate and specific symptoms can help eliminate many problems.  Frankly those heavy current spike can only really come from PV (source for motor or heat element) getting shorted to ground.  Since no smoke units only motor PV left.  (Excluding couplers which would have smoked at 3 amps).   G

Like I said, without seeing it, I just enumerate the possibilities.  If I were looking at it, I'd be able to quickly eliminate some of them.   Eye witness accounts are notoriously inaccurate.   Not saying it's the case here, just covering the bases.

I am just answering his question since Bruce is a tech and he isn't sending it to you or me