Installing a DCC decoder in a Heljan Class 17 Clayton Diesel Loco

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Class 17

Before installing chip into any loco it is advisable to check the current being drawn by the loco. You should test the current drawn when the loco is stalled to find the peak current and select a decoder that will cope with that maximum current requirement.

This has to be one of the most difficult and frustrating DCC ready locos to install a decoder into, so be warned before you start!! You will need patience, a keen eye, care with VERY small parts and some time - it isn't a five minute job!

There is little space inside the loco and it was soon discovered that a variety of decoders will not fit, including:

TCS DP2X, TCS DP2X-UK, TCS T1, TCS MC2, Digitrax 123P, the latter 3 all having JST type connectors on the decoder, are all too big to fit into any of the available spaces.

This installation used a TCS M1-UK decoder, which does fit, but isn't located where all good DCC guide books would advise putting a decoder, as you will see below!

Removing buffers from Class 17

The loco comes with a leaflet which describes, with pictures, how to remove the body and install a decoder. Simple.

In theory yes, but in practice more of a challenge.

First remove the cab. This can be done by gently inserting a very small flat screwdriver under one corner and levering until the cab releases from the chassis. Repeat on the other side.

Then it is necessary to remove the buffers at one end of the loco. BEWARE, each buffer has a tiny spring inside and a very small plastic terminator on the end underneath the loco. Both of these small parts are so small that they can be easily lost - HEED THE VOICE OF EXPERIENCE!

Once the buffers have been removed the body can be VERY gently levered off by running a fingernail between the edge of the body and the chassis on the underside. Raise the body at an angle towards the other end and push gently to release the body. It is a tight fit and does come into contact with the circuit board so proceed slowly.

Remove dummy plug Pin 1

Location of decoder

Once inside with the body separate from the chassis the body remains attached to the circuit boards by the lighting wires. It is advisable NOT to remove the lighting modules from the body (unless you want another challenge in getting them back into exactly the right position!).

Remove the dummy plug from the decoder socket. You will note that the socket is naked and that pin 1 is not marked. If you insert the decoder plug the wrong way round it will do no harm but you will find that the lights don't work. It is therefore advisable to place the loco on your DCC track with the controller set to loco 3. Try switching the lights on and off to see if they work. If not remove the loco from the track, remove the decoder plug and insert it the other way round. Repeat the exercise with the loco on the track to check that the lights do work.

Finding a location for the decoder is not easy. Ideally it should be located on top of the circuit board, provided the decoder is insulated (all TCS wired decoders come ready insulated). However, with the foam inside the body shell there isn't enough space between the circuit board and the inside of the body shell to enable this to happen, even allowing for compression of the foam. In the end the decoder was located alongside the motor, as shown in the Heljan notes, but this isn't really a desirable location and it wasn't possible to use any sticky fixers to fix the decoder in position as these, too, would have been too thick.

However, you can't fix the decoder into position until the body shell has been replaced as the decoder needs to fit in between the lower part of the body shell and the motor bracket.

Program the decoder as required BEFORE replacing the cab and check that everything works as expected. On this example the motor was extremely noisy when running at full speed.


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