@Tom Tee posted:This professionally installed PS-2 up graded 2 rail SS 4-8-4 has only been used conventionally with a MTH Z-1000.
Never used with DC track power.
The throttle, horn and bell functions were all normal prior to the lead truck derailment and shorting. Estimated total hours of use well under 20.
So is the plan to send the engine in for repair? I suppose you can wait a bit to see if anyone else comes forward with a proverbial "Get out of Jail" card to restore your engine's functionality...but I suspect you may be waiting a long time. 
My diagnosis is a failure in the voltmeter circuit in the engine. The PS2 electronics has what amounts to a built-in voltmeter that can measure both AC and DC voltages. It is implemented by a combination of some basic circuitry to scale down the "high" AC or DC track voltages to lower voltages that can be processed by a low-voltage (3 Volt) processor chip using an A/D (analog-to-digital) converter. Then there's a bunch of software to convert the voltage measurements to engine commands for sound and motion. I'm hoping that the fault is in the basic scale down circuit because these are inexpensive parts that a skilled tech should be able to diagnose/replace. If the fault is in the processor-side of the voltmeter circuit then it's game-over since the processor chip is effectively irreplaceable.
As for the discussion about conventional DC operation, I see it this way:
Again, everything goes through the processor chip. The motor receives no voltage unless the processor says so! The relay (that flips motor voltage polarity) only changes if the processor says so!
In DC operation, the MTH PS2/3 engines have a speed-vs-voltage relationship roughly as shown above. There is a small voltage window where the engine will indeed just sit there with idle sounds. IIRC this small window is about 2-3V. Go above this window and the engine starts moving. Go below this window and there is not enough voltage to run the electronics so the engine starts playing the shutdown sound and turning off. There is symmetrical behavior with negative DC voltages except the processor flips the direction relay.
I believe the voltmeter circuit is broken. On powerup, the engine decides if it is to be a conventional AC engine or a conventional DC engine - how it decides to be a DCC or DCS command-control engine is not relevant for this discussion. The broken voltmeter is somehow interpreting the AC track voltage to be a DC track voltage and going into DC mode. Then, when looking for the DC voltage to set the speed, it is somehow "exaggerating" the modest DC offsets from Horn and Bell buttons on the controller thinking the DC voltage is above the threshold to move the engine. All speculation.
