Installing a DCC decoder in a Heljan Class 23 Diesel Loco
Click on each image for a more detailed view
Last updated 5 April, 2020
Class 23 loco and TCS DP2X-UK decoder


This has to be the most infuriating Heljan loco to install a decoder in. Ignore the instructions which say that the chassis drops away from the body when you have released the lugs - it doesn't. The body fits really tightly. I have now done a couple of these locos and it doesn't get any better with experience - each loco seems to be slightly different in how securely the body is fixed to chassis.


Class 23 loco - trying to release bodyshell


Class 23 loco - trying to release bodyshell

Ignore the instructions supplied by Heljan about the MU cable as it's already detached.

You will need 4 VERY stout pieces of card or plastic to keep the moulded lugs on the bodyshell away from the chassis whilst you try to get some movement between the two parts.

You also need to make sure the buffers are pulled forward otherwise they lock the body to the chassis - not mentioned in the Heljan instructions!!! Don't pull too much on the buffers, though, or you will pull them out completely - with springs! If you do manage to pull the buffers out be careful you don't lose the spring attached to it - if they come off they are extremely difficult to find again and could have travelled some distance!!!!

Also be aware that there are some very fine detailing parts on the underside of the chassis and it is EXTREMELY easy to break them. However, some parts just fall off without too much effort, so beware!!!

As stated above the tightness of fit of each body seems to vary slightly. This particular loco shown in the photos was especially difficult. Once I had achieved a little movement it was possible to coax the body end upwards by gently levering with a small screwdriver using the two holes that the back ends of the buffers reside in. Note that it had already been necessary to remove the buffers.


Class 23 loco - blanking plug Class 23 loco - DCC socket



Once the body has been removed the DCC socket is easily visible. Gently lever or pull out the blanking plug.

As seems to be normal on recent Heljan locos the location of pin 1 is not marked as such, though the letters NMRA do appear next to it.




Class 23 loco - resistor installation problem



Current draw is nearly 0.5 amp when at full speed so you need a decoder that has a continuous output of 0.8 amp or greater.

In some models it is possible to go for the easy option and install a plug and play decoder like the TCS DP2X-UK. However, as can be seen left, on this particular loco installation of the resistors on the main board was so poor that they stuck up too far to allow the installation of this decoder as they would have come into contact with components on the decoder and caused a short circuit. A TCS DP2X would be a better option as the decoder is orientated across the board rather than longitudinally and would avoid the potential shorting problem, but unfortunately none of these were to hand when this installation was undertaken.



Class 23 loco - resistor installation problem

As a result a wired decoder was installed, in this case one of the new range of M2SAX decoders from DCC Concepts.

Once installed, test the loco on DCC before replacing the body to ensure everything works as it should, then program the loco with its running number and replace the bodyshell, snapping it into place.

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