Q: I have a late 70's or early 80's Athearn EMD DD40. An article on Wiring For DCC suggested a DH83FX - 2amp decoder for Athearn "Covered Wagons." The pictures look like the inside of my loco except the DD40 has twin motors with joined drive shafts. In researching your site, I realize that the DH83FX is no longer manufactured, but I was led to the DH163AT as the current replacement. Will this work? I have some other Athearn SD40 units, same vintage, but single motors. Is this the correct decoder for them?
A: The "AT" in the decoder name refers to "Athearn", and decoders with this extension in their model number have a 9 pin harness with no-solder connections for Athearn Standard Locomotives.
The DH163AT is powerful enough for the SD-40. As far as the power consumption of the DD-40 is concerned, there seem to be two approaches which you can try.
1. Install a single DH163AT; it is rated at 1.5 Amps, with a 2.0 Amp peak load. Wire the second motor in parallel with the first to the decoder, with the two motor leads going to both motor connection pairs. It is important that this be done correctly so that both motors operate in the same direction. The DHAT is the basic Athearn harness that could be used for the second motor connections. In trying to make do with one decoder, you do risk the possibility of burning out the decoder.
2. The safer approach is to install two decoders, one for each motor. There's plenty of extra room under the DD-40 hoods. When you program the double motored DD-40, both decoders will pick up the programmed information and settings. When a command is sent to this locomotive, both motors will operate in tandem because both decoders have the same address. In this case, the first decoder would be used for headlight and other function control, while the second would be only for motor control. Again, getting the motors operating at the same time in the same direction will be the central issue. If you have two motors that are not well matched, you may need to use the decoder lock feature and program each one separately to match the motors.
Incidentally, when first manufactured, the Athearn DD-40 was offered in both single and double motor versions. Initially, each motor drove one truck on the giant DD-40, but the two motors were eventually linked together after it was discovered that running one locomotive with the two independent motors caused problems when one motor wanted to run faster than the other, even when both were drawing the same voltage. This is a common problem with all units operating "MU"; in this case, the multiple unit was inside one locomotive.