Fitting Decoders to Split Chassis Locos


Mention the phrase 'split chassis loco' to many DCC users and one is usually greeted with expressions of shock and horror and exclamations of difficulty!
However, fitting decoders to split chassis locos is nowhere near as difficult as is often portrayed. This article attempts to remove the mystery of fitting decoders to split chassis locos.


Background

There are essentially two different types of split chassis locos which have been available on the Uk market, these being from Palitoy/Mainline and latterly, Bachmann.
This article deals with the two types separately, but the principals involved are exactly the same as for any loco, whether split chassis or not.


Palitoy/Mainline Split Chassis (no longer available)

With these locos, the two halves of the chassis are directly connected to the wheels and carry electrical current of opposite polarity. The principal issue with these locos it to isolate the motor from the chassis.
The motor has two metal strips attached to the bottom two retaining screws (covered in red sealant). One strip touches one half of the chassis and the other touches the other half of the chassis. Both also retain the motor brushes. In the first picture below, the strips have actually been cut off at the screw head. Doing this, isolates the motor from the chassis.

The decoder fitted here is a TCS M1 and it can be seen lying on the top of the chassis block at the front of the loco (second picture below). Halfway along the top of the chassis block, it can be seen that the red and black wires have been soldered to each chassis block half.
Soldering does not always work with this chassis block material and it is often necessary to screw self-tapping screws into the block and solder to these.
The orange and grey wires can be seen connected to the motor.


Some of the really early Mainline locos such as the 22xx and the 04 shunter had a method of construction where the motor casing was an integral part of the split chassis. The red sealed brush retaining screw mentioned above on one side passed through the motor and chassis block to the other half in order to provide electrical connection. This is really difficult to insulate and if not done correctly, can cause shorts. Our experience with these models is that it is very easy to damage decoders unless the metal screw is replaced with a plastic equivalent. These models have virtually all been replaced by upgraded Bachmann examples and we are of the view that the really old Mainline locos with integral chassis motors are not worth the effort to convert and are better replaced with modern Bachmann versions. Fortunately, Mainline only produced a few locos of this construction so they are not very common.


Bachmann Split Chassis (only used on a few older models)

Bachmann chassis are slightly different in how the motor is connected to the chassis. With these locos, the can motor has two springs attached, one to each of the motor connections. These springs touch opposite chassis halves.
To isolate the motor from the chassis, the springs must be removed and insulation tape placed on the chassis block and/or motor terminals to prevent an electrical circuit being made between the block and motor terminals. This can be seen done here:


Once this modification has been made, wiring of the decoder is exactly the same as for the Mainline-type chassis as per these pictures:

And that's all there is to it!


Graham Plowman


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