Voltage...
That is weird, since that means they do not meed NMRA standard.
I did look up the manual, and indeed Keith is on to something, the decoder is an OEM by Soundtraxx and the large scale Tsunamis all fail to support the NMRA DCC voltage per the standard.
I have used and tested quite a few, actually from the first days they were available (years ago). The Tsunami family will go into overheat or overvoltage mode at 19-21 volts DCC. Operation I have seen is erratic, or the damn thing just stops with lights blinking.
What I would do is first, if you have an accurate way to measure DCC track voltage, it would be good to confirm that, but again I think Keith is right on the nose.
I would not turn down the track voltage (why punish other locos that meed NMRA standards) but put a "voltage dropper" in the loco one one if the "track pickup" sides.
This is a simple and inexpensive circuit to reduce the DCC voltage without interfering with the signal quality.
here is one assembled in a manner to make the wiring clear, it can be made much more compact:
This particular circuit drops six times 0.7 volts, or 4.2 volts.... this would drop a system that put the maximum NRMA standard of 24 volts to 19.8 volts, but very few systems put the full 24 volts DCC on the rails.
Every diode you "add" the the "chain" reduces dcc voltage 0.7 volts....
This has the added benefit that no matter whose system you run this loco on, it will not go into shutdown.
Greg Elmassian web site on large scale trains and garden railroads, cigars, and computers
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Greg