I have a DC model railroad (using MRC 9950 power packs) which I have been constructing over the last 6 months.  Today I encountered an interesting issue with a pair of Intermountain SD40-2s that I purchased last year.  Up until a month ago I was using an old MRC 501 and an MRC 2400 (from the 1970s and 1980s) to power the two tracks (it's a double-track mainline).  The pair of SD40-2s (no sound) ran just fine with both of those power packs, although the throttle setting response of the locomotives was different for the two power packs.

Last month I hooked up the new MRC 9950s, which have volts and amps readouts on an LCD display.  The pair of SD40-2s would not even power up until approx. 14 V, and then rapidly accelerated to a high speed.  When I barely dialed back the throttle they shut down.  So what I had were two SD40-2s that didn't power up until the throttle was nearly fully open, and had no sensitivity to the throttle adjustment (because there wasn't much more left to adjust to!)  It's like having two racehorses on the track.

So, I pulled out the old power packs and the multimeter to delve into the Intermountains' behavior.  Using the 1970s MRC 501, the locomotives responded across the entire throttle range-but when I checked the multimeter, the "range" was from approx. 12 V on up--that old MRC 501 had essentially no low voltage capability--it just jumped right up to 12 V and increased from there, which the Intermountain SD40-2s could handle just fine.

Then I tried the 1980s MRC 2400.  The locomotives first responded at about 7 V, which was about a third of the way between stop and max on the throttle.  The response of the locomotives was good from that point on up.

There is something different about the MRC 9950s electronics that is causing these two Intermountain SD40-2s to behave strangely.  I had BB Athearns, new Bowser U25Bs, and even a new BLI 4-8-4 that all behave quite normally with the MRC 9950s (good response across the entire throttle range; the BLI has a high starting voltage even though the sound is off), but the Intermountains have something different with their electronics design that seems to make them incompatible with the MRC 9950.  It's strange, and unfortunate as I really like these two SD40-2s (MILW Hiawatha scheme).  But having two locomotives that either don't move, or move at their highest speed, is not acceptable, obviously.

Does anyone understand what is going on here, and if it's something that can be corrected?  Remember that this is a DC layout, no sound, so there are no CVs to play with.  It's old school stuff.

Thanks,

Brian