I consider the Digitrax DZ121 decoder as ideal for these locos. It is very easy to install with no modifications required to the locomotive.
These locos are notorious for power pickup problems, especially after being worked on during decoder installation, and they have often nearly driven me (and many other owners) up the wall. So while you have the locomotive already disassembled, plenty of offcut decoder wires handy after installing the decoder, and your tools ready, you might as well do an easy modification that will turn this loco into one of the best performers in your fleet.
This loco's shell has to be disassembled to remove it. Since you're going to be poking around the insides with a soldering iron or solder gun, you might as well strip the shell off completely. That way there will be no chance of accidental heat damage to the shell.
First, unclip the handrails from the cab and the cab sill by slipping a thin tool like a pocketknife blade between the handrails and the body and carefully twisting the blade to unclip them.
Then remove the cab. It is held in place on the sill by four plastic clips on the four corners. Again, carefully insert a pocket knife blade between the cab and the sill, in the middle below the cab side, and twist it to unclip. Unclip both sides before trying to remove the cab.
Then lift off the cab, holding it lightly. (I once held one too tight while lifting it off and broke it in two!)
Next, remove the short and long hoods. The hoods have clips on the sides and ends to hold them in position, but they normally unclip easily.
Then remove the left and right cab sills. First unclip the handrails from the cab sills and then lift out the sills. They do not clip into the shell's base, they have dowels at the bottom and are held in position by the cab, handrails and hoods.
Last, remove the walkway with the handrails still attached. Pry the sides of the walkway open to unclip it from the plastic chassis and lift it off carefully.
The loco has no rear light installed, although the rear lenses are in place, so if you want you can install a rear lamp while you have the loco disassembled. For a lamp you'll need a lamp similar to the front one. (I've found that the little 12v lamps that are used to light up radio/VCR/TV controls work well, but it's probably best to get the proper lamp from your hobby shop or from LifeLike.) You'll also need a 20 to 40 ohm resistor for each lamp.
Remove the rear ballast weight to make room for the decoder and, if you want to install it, the rear light. In case you're worried about the weight loss, I've found that the removal of the rear ballast weight has no negative effect on the loco's traction. The front weight remains in place and the fuel tank is also a ballast weight, and these two are more than sufficient.
Unsolder the two headlight wires from the chassis. Be careful, since you're unsoldering from copper contact strips that are mounted on thin plastic. I found it best to hold the wire taut and then to just briefly touch the solder spot with a soldering iron that's already hot.
Also unsolder the wire to the motor's top brush from the brush cap's copper strip and from the chassis, and discard the piece of wire. You can't get to the bottom brush cap yet, so just unsolder the end of its wire from the chassis. Be very careful with the bottom one, since there's not a lot of room to stick the soldering iron into, and there's plenty of plastic that can get melted. Again let the iron heat up fully first, then only a split-second touch to the solder blob will suffice to let the wire come loose before the surrounding plastic can get damaged. Leave all the solder remains on the chassis, since you're going to solder to some of them again.
Lift out the motor from it's seat in the plastic chassis. As you lift it, the rubber tube drive shafts will pull out of their recesses in the flywheels. Cut off the copper strips on the motor brushes, leaving just enough to be able to solder wires to them. You're now ready to begin decoder installation.
Do the grey motor wire first. It goes to the bottom brush cap on the motor. The decoder will rest on top of the long hood end's truck tower/gearbox, with the end of the decoder hanging over the end of the gearbox about 3 mm (one eighth inch). The grey wire has to be threaded through a hole below the flywheel into the motor cavity, so thread it through there with the decoder in position on top of the gearbox and snip the wire off at a length that will be sufficient to reach the bottom brush along that path. Keep in mind that there must be enough room left for the rear flywheel.
Then pull the grey wire through into the motor cavity as far as the decoder will allow it to go, and solder it to the bottom brush cap. (The bottom of the motor is the side with the hole in. If the loco is supposed to run long hood forward, you can either install the motor upside down, or just swap the orange and gray wires - your choice.)
After soldering the grey wire to the motor, reinstall the motor in its seat, making sure that the two drive shaft tubes go into position in the flywheel holes. After doing this, it's a good idea to temporarily tape the decoder in place above the gearbox, because a decoder with six more loose wires flapping around while you're working can become tiresome. It will also make it much easier to determine the correct lengths at which to cut the remaining decoder wires.
Next, connect the orange wire to the top motor brush. (The camera decided that the orange would look better in yellow in the following picture. And don't be too concerned yet with the red and blue wires coming out of the truck tower - it's part of the modification that I mentioned at the outset.)
Take a good look at the brass strips on the underside of the plastic chassis, and at the wipers on top of the trucks that touch them, so you'll know which one picks up power from which track. This is important, since both strips are anchored to both sides of the chassis, so that some of the tabs on the engineer's side actually are powered by the rail on the fireman's side, and vice versa!
Connect the black wire to the left (fireman's) side of the chassis, and the red wire to the right (engineer's) side of the chassis. Here's where the solder remains come in handy that were left on the chassis when you unsoldered the original wires. After cutting the wires to the proper lengths, tin their ends, hold the tinned ends in position on the solder remains and just touch them briefly with a hot soldering iron, and you're all set without any damage to the plastic chassis.
In case the picture confuses you: The black goes to the tab furthest away as you look at the picture, and the red wire that's also visible to the rear of the gear tower is part of the modification mentioned earlier. The red decoder wire, marked in the picture, goes to the forward of the two tabs between the gear tower and the fuel tank.
Avoid making wire joints directly above the motor, since the shell is a tight fit on top of the motor with just enough space to allow decoder wires to lie on top of the motor. Rather make the joints above the flywheels.
Connect the yellow wire to one of the rear light wires. (If you turned the motor around or swapped the gray and orange wires to make the loco run long hood forward, remember here to also swap the white and yellow light wires.) Fold the blue wire backwards over the decoder and, at a spot in line with the decoder's end, strip a little insulation off. Connect the bared piece to one end of a 20 to 40 ohm ballast resistor, and the other end of the resistor to the second rear lamp wire.
I let the rear lamp hang loose behind the rear gearbox, but with the wires bent double so that the light bulb "stands" on its wires to get the light more or less in line with the rear lenses.
With the front lamp in position on the weight in the short hood end of the chassis, solder the white wire to one of the headlight wires. Make sure all connections are properly insulated, preferably by using heat shrink tubing. (Again, if you turned the motor around or swapped the gray and orange wires to make the loco run long hood forward, remember to also swap the white and yellow light wires.)
The rest of the blue wire is strung to the front of the loco and connected to the second wire of the front lamp, with another 20 to 40 ohm ballast resistor added in the circuit as shown in the picture. And you're all done.
Remember to triple check for shorts before taking the loco to the programming track. There must be no direct contact between the track and the motor. Make sure that all wires are clear of any moving parts of the loco and that all wire solder joints are insulated. Also remember to remove the tape, if you taped down the decoder and the front weight.
Do all your programming and testing before replacing the loco shell, since it's a big nuisance to have to take it off again! While reassembling the shell on the loco (piece by piece, in reverse order of disassembly), make sure each component is properly clipped in place before fitting the next, and also make very sure that no wires go into places where they don't belong, like touching a flywheel or a drive tube.
If anything about the procedure is perhaps still unclear, or if you used this guide and it worked for you, please drop me a note.
Good luck, and keep them on track!
The following pages contain decoder installation guides for some other N scale diesel locomotives:
And the following pages contain guides to servicing or performance enhancement of some N Scale diesel locomotives:
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Created in February 2001. Last updated on 11 September 2003.