Installing a DCC decoder in a Hornby Class 31 Diesel

Click on each image for a more detailed view.

For additional notes about adding a sound decoder click here.

Class 31 and decoder


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. You will need a decoder with a sheath to protect the decoder from short circuits. If the decoder of choice does not have a protective sheath Hornby have thoughtfully provided a sheath for you to slip over the decoder before installation. A TCS M1-UK decoder was used in the installation detailed here, which already has a protective covering to ensure against short circuits.

This installation is quite straightforward - getting the body back on can be the most difficult part of the operation.

You will need two small screwdrivers for this installation, one for flat head screws and one for cross head screws.

Releasing body from Class 31
There are four screws securing the body to the chassis. You will need the small flat headed screwdriver for this part of the operation. Once the screws have been undone (and they sit behind the bogies, so may not drop out easily) the body can be gently lifted away from the chassis.
Class 31 dummy plug


If the decoder you are using is fairly large you will need to remove the PCB in order to locate the position under the board which has been left specifically for storing a decoder. There are two cross-head screws holding the PCB in position. Undo these screws and lift the PCB clear of its mounts, making sure you don't lose the very small washers that sit between the PCB and the securing mounts.

Installing the decoder itself is really very straightforward. Gently remove the PCB in the NMRA socket by levering it out from each end with a flat screwdriver.


Class 31 with decoder installed

Class 31 - securing the lights

Once the existing PCB has been removed it is obvious which way round the new NMRA board should plug into the socket as pin 1 is labelled clearly on the underlying PCB. Push the NMRA board firmly into the socket, being careful not to bend the pins. The M1-UK decoder installed fitted neatly under the PCB, with the wires carefully positioned to avoid being trapped when the body was replaced.

As with all of the latest Hornby diesel locos, the board has 12 pins available, but only the 8 pin socket is required for the decoder to plug in to.

Before replacing the body, program the CV settings as required on the programming track and test the loco on the main line or a rolling road. You CAN test the lights before replacing the body by removing the lighting units from within the body shell, but experience shows that this is not really recommended unless absolutely necessary as the body is extremely difficult to replace if the lighting units are not firmly fixed within the body shell - a case of if they haven't been moved and don't need to be, leave them alone!

Replacing the body and ensuring the lights work can take some time with certain versions of the Class 31, especially if the light assemblies come away with the body. Getting them correctly seated can be a nightmare! You will need some time (and patience!) to get it back together and in working order. YOU HAVE BEEN WARNED!!

One solution is to use some double sided tape with a strong adhesive - a small piece of carpet tape has been used in the photograph opposite. This holds the lighting unit firmly in place and you can test the lights without having to replace the body. It needs to be placed in the centre as the moulding of the lighting unit underneath has a number of irregularities and the front of the unit is the only flat surface. If the lights still do not work, try prising the ends of the copper contact strips away from the plastic by a small amount.

One other solution to help relocating the lighting units is to insert a small screwdriver through the cab doors and press them carefully into place after the body has been fitted.

Hornby need to work on this to ensure that removing the loco body on future models does not create such a problem when it comes to fitting the body back on to the chassis.


Adding a sound decoder  
Class 31 with small speaker

It was decided to replace the TCS-M1UK decoder with a Loksound decoder from South West Digital, pre-programmed with sounds for the Class 31.

The decoder is plug-and-play with a 8-pin NMRA plug ready to plug into the loco and with a speaker attached. Unfortunately all Loksound decoders come with a standard size speaker. This is too big to fit into the Class 31 in the available space, so this was replaced with a LS50441 ESU miniature round speaker 20mm 100 ohms with sound chamber. This fits perfectly into the space between the fan and the cab end.

The decoder itself had to be located under the large circuit board and above the motor. Undo the two screws holding the board in place and lift slightly. The decoder will lodge itself between the board and the motor, there being two lugs (one each side) jutting inwards in this void which if the decoder is positioned carefully, prevent it dropping on to the motor.



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This page was last updated 29 March, 2020
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