I have several Digitrax SDH164D and they seem to run real hot. Even if the locmotive has just been sitting and just idling for a while.
I also have 2 of the same decoders that blow there horn a lot on there own.
Have to checked the track voltage? It may be too high.
I had a pair of SDH164K1Cs that I put in a cpouple of Kato F40PH and they did the same thing, every time I powered-on the layout they each went into the full grade crossing sequence, TWICE! I could never get them to shut up! Every time there was a slight power interruption they would go through the tooting all over again, twice.
They were the first and last Digitrax sound decoders I ever owned.
Regards, Ed
Track volatge is 18V AC. I have several other locomotives that run just fine.
Ed,
I’m ready to jump the Digitrax ship as well. I have an SW1500 with a Soundtraxx, Tsunami and love that thing. I was just hoping to fix what was arready on hand. Thanks for the help. 
WHat speaker are you using with it? One thing that can get a sound decoder hot is to use a speaker with the wrong impedence. It makes the amplifier run very hot. The motor portion is the same as other Digitrax HO decoders so should be nowhere near the limit with that loco, unless it has a problem with the motor.
Also what sound project is loaded? The default Digitrax ones aren;t all that good, but there is a whole series in the Sound Depot that others have properly designed to work with the decoders.
–Randy
18VAC is somewhat on the high side for an HO-scale decoder.
I forget what the exact NMRA spec is, about 22 or 24V, so you’re not over, but you’re definitely towards the upper end of the range. HO is usually about 14-15V, and some folks even run HO on their command station’s N-scale setting (12V).
So between that, the possible speaker impedence mismatch Randy mentioned, and other factors such as that particular decoder’s specs, bulbs vs LED’s for lighting, that loco’s heat-sinking and airflow characteristics, etc, I could see how you might have a situation where a particular model of decoder runs hotter than others.
First thing I’d do is drop that voltage down to a more suitable level for HO-scale decoders. Then I’d start looking at why those two loco/decoder combinations run hotter than others.
Oh yeah, didn’t see that. 18V is high for HO. The decoder is good to 20 or 22, or should be, but 18V is already 3V higher than the Digitrax HO voltage of 15V.
–Randy