I first tested the decoder as per Digitrax's instructions and followed all of their installation procedures.

NOTE: THIS CONVERSION MAY VOID SOME, OR ALL OF THE ARISTOCRAFT WARRANTY

Testing of Loads
 

Disassembly

• Remove handrails if already installed.
• Remove access cover between horns and cab. (Just pry's off).
• Remove one screw under cover.
• Turn unit over.
• Remove 4 screws holding on fuel tank.
• Remove 1 screw in center in front of air tank.
• Remove 2 screws at rear of unit
• Remove 1 screw in the center of the weight, but not the ones holding the weight on.
• Remove long rear hood, unplug plugs first.
• Remove two screws at front to remove short hood.
• Remove short hood.
• Remove 2 smaller screws under front hood to remove cab.
• Remove cab, unplug plug first.
• Remove cab floor. Just pulls out. WATCH HOW THE WINDOWS COME OUT AS YOU WILL NEED TO PUT THEM BACK IN THE SAME WAY. Do not take out the two screws as they just hold the seats in. The driver may have fallen off the seat by the time you get the floor out. Goop was used to glue the driver back in. On reassembly I just remove the left, or both legs which is not noticable when together and makes for easier assembly.
• I disconnected and removed the rear circuit board with the switches on it in the rear nose of the unit. It could be left in but since it was not being used  and I needed the extra room I removed it. The whole nose just slides down and the board is easily removed. (The nose could be tight).

You will need to remove some existing wires and you will need to drill some new holes, but when you put the two small boards back in at the front and back the above wiring codes should be correct. As an added precaution you may want to disconnect the block plugs and ensure all four circuits are independent of each other,  before you soldier in the decoder. For the Dallee sound system you will also need to add 1 extra wire to each of the four circuits near where you will be mounting the sound unit

Above is the small rewired board in the front of the unit, bolted in and tested ready to go. You may notice a red circle around a bolt hole on the right side. I usually mark any bolt mounting holes that have a boss coming down to them so I do not mount anything in the way.

The above small board was made up to handle the rear headlight and the smoke unit. NOTICE The diode across the positive of the smoke connection on the right. The decoder was wired into this using the large blue wire for this board as it was supporting the smoke unit. Brown for the smoke itself as it handles 1 amp on F3 and yellow to the rear light.
 

The above shows the new small board mounted on the left, the Dallee sound unit velcroed to the hump and the original board in the upper corner that was previously rewired. CAUTION Be sure to keep the sound board far enough to the front so it does not interfere with the rear hood.

The above shows an overall view on the rear of the installation. The small plug  that was for the sound unit now is connected to the speaker connection on the Dallee board and reinstalled in the original spot. The speaker that fits into the fuel tank will plug into this when reinstalled. The DG580L it bolted to the front weight bolt and a heat sink from the surplus store has been added. The rest gives an overall view of previously described components.

CAB
I removed the headlight diode and ran a 3 wire plug to; Blue Common, White head light and a Green/White to the cab light. The female part of the  plug went to the decoder leads utilizing the small blue lead for the common.

The bell and Horn were connected to F1 and F2 on the decoder.

This is the third U-25 I have added DCC to. All installations are slightly different. I have found that the small air tube running from the smoke blower to the smoke unit is always kinked and I believe this reduced the effectiveness of the smoke unit. To reduce the kink I shortened the bower mounting pins by 3/4 inched and used some thin flexible black tubing I got from Home Depot to replace the clear existing one. So far this has made a great improvement in the smokability.
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