Network Rail Class 31 Mid-life make-over:

Paul Chetter has now created a Class 31 Protodrive sound project for Zimo decoders. An MX645 with Paul's new software will be fitted.

The speaker will be inverted, in an attempt to reduce the high frequency output.

An attempt will be made to "weather" the previously pristine loco, to closer resemble the last units used by Network Rail.



With thanks to Kevin Bates for his photo above, showing the condition of the real Class 31 locomotives.


Updated Circuit Diagram to accommodate the Zimo decoder with minimal change


The only changes are 3 new series diodes to protect the end 2 running lights from reverse breakdown when disabled via FO4.


The change-over from LokSound V4 to Zimo went without any dramas, so the next task is to define the new CV listing.

Zimo MX645 wired in


Function Map: (After minor tweaks)

F Key



Directional running lights


Engine start


Brake key


Hi Horn (playable)


Lo Horn (playable)


Light Engine Mode 


Coast On/Off


Ramp up to full power


Buffering (on) / coupling (off)   


Flange Squeal


Brake Release


Detonator LED / Brake Squeal


Spirax Valves 


Brief 2 Tone Horn




Wagons clanking




Guard’s whistle


Door slam


Cab light


Disable End 2 running lights


Loco IR Floodlights (Infra Red illuminators)


Reserved for train lighting (DBSO I-R Floods)


Reserved for train lighting (Internal train lighting)


Reserved for train lighting (Measurement car lights)


Reserved for train lighting (DBSO cab light)


Parking lights (rears at each end of train)


Volume down


Volume up



First the lighting control via Zimo's Swiss Mapping:

Description CV Value
Group 1  (running lights)    F-Key (0) 430 29
M-Key 431 0
1st fwd FO 432 14
2nd fwd FO 433 0
1st Rev FO 434 15
2nd Rev FO 435 0
Group 2  (detonator flash LED) F-Key 436 11
M-Key 437 0
1st fwd FO 438 3
2nd fwd FO 439 0
1st Rev FO 440 0
2nd Rev FO 441 0
Group 3  (cab light) F-Key 442 19
M-Key 443 0
1st fwd FO 444 2
2nd fwd FO 445 0
1st Rev FO 446 2
2nd Rev FO 447 0
Group 4  (disable end 2 lights) F-Key 448 20
M-Key 449 0
1st fwd FO 450 4
2nd fwd FO 451 0
1st Rev FO 452 4
2nd Rev FO 453 0
Group 5  (IR Flood Lights)    F-Key 454 21
M-Key 455 0
1st fwd FO 456 1
2nd fwd FO 457 0
1st Rev FO 458 1
2nd Rev FO 459 0
Group 6  (parking rears end 1) F-Key 460 26
M-Key 461 0
1st fwd FO 462 15
2nd fwd FO 463 4
1st Rev FO 464 15
2nd Rev FO 465 4

Next a few essential housekeeping issues:

Description CV Value
Remove conventional mapping entries 33,34 0
BEMF sampling 9 87
BEMF set-up 56 55
Set motor control freq to 40kHz 112 32
Remove fade-out from F19 313 0
Set max volume to 65 395 65
Set Light engine momentum 390 128
Cab light auto-off when moving (F2) 128 60


Good! Motor control is fine and all the lighting works as planned.


Inverting the speaker:

The original speaker, an ESU 20x40mm drive unit, sealed in its standard enclosure, was glued and sealed to the roof underside, venting upwards via the circular fan grill aperture in the roof.

However, the audio properties were not great!  There seemed to be an excess of treble sounds when compared to a fuel tank mounted downward facing speaker in e.g. a class 57.

The Hornby Class 31 fuel tank is full of metal and even if this was milled away, the enclosure would be too small for a 20x40mm unit. So as a compromise I've kept the speaker in the same location, but inverted it, so that the sound emerges downwards, escaping through the side grills and the air gaps surrounding the bogies.

Showing the new downward facing speaker glued to the roof unerside.


Plan B:

Although the inversion perhaps made a slight improvement, the sound tonal quality was still disappointing. (Worse than I normally hear from an ESU 20x40).

So..... I removed the original speaker completely and substituted a downward facing 57 x 22 x 9mm bass reflex unit, fitted to the chassis assembly:

Showing the downward facing bass reflex speaker glued via plasticard supports to the chassis block.

(The markings on the speaker case are glue and outer wiring insulation left over from its last application.)


This sounds much better, and is the solution I will run with! 



Adding a bit of weathering:

The Roof has been treated with a mix of dilute matt black paint and rubbed-in black pastel. Majoring on the exhaust ports and immediate surrounding area, but addressing most of the roof surface. A few very slight smudges of pastel have also been applied to the body sides and cab fronts, while the side grills have all received diluted matt black paint.  Its not a great example of weathering technique, but hopefully it gives at least a suggestion of a working locomotive.


 Finally the infra red flood light LEDs were recoated with deep crimson water colour paint, to simulate the visible light spectrum produced by these illuminators.

Forward running lights, cab light and infra red illuminators all on.



Running on the loft layout:

The good news is that Paul's new sounds and Zimo's unique driving capabilities mean that the makeover has been very successful and the locomotive provides a rewarding driving experience.

A curious glitch occurred when I attempted to read the address CV prior to re-programming it for loft layout operation..... The Lenz controller froze and after a re-boot still failed to read the Class 31 CVs, although it was able to change the address without reading the CV first. (It behaved quite normally when checked other locos.)  However, whatever the problem was, it's now cleared as the 31 is now behaving normally again with the controller.

Laptop now up and running again, so a video sequence is getting closer!


To watch a video of the 31 negotiating the loft layout,

 at the head of the Network Rail measurement train:

Click here