Model Railway Forum

I started building my DCC layout when I moved house and like 'Topsy' it has grown to the limit of space available over the past 9 years. I discovered the hard way that the track and wheels need to be gleaming to ensure good operation of the locos on DCC, but then started to get locos hesitant to start or stopping at odd locations. I have also experienced some locos suddenly moving without instruction, and on occasion locos suddenly stopping with a short circuit notification (usually when crossing points or slips ) but after switching the supply off and clearing the 'problem' then switching on again a completely different loco code appears in the display.
This odd situation has deepened when examination of the locos involved has revealed that they are all fitted with the same brand of decoder only sold by one well-known supplier to the hobby. Before I change the decoders en-masse it has occurred to me to enquire if any others have encountered the same problems.

The usual cause of runaways is interference resulting from locos running on dirty track. Don’t forget the wheels need to be just as spotless as the rails.
It can be mitigated by turning off DC running in CV29.
Shorts through points will almost certainly be an issue with the back to back distance allowing the wheels to touch the rail and the blade.
Maybe the brand of decoder you are having trouble with has DC on as a default while others have it turned off as the default?

this will help you select the setting for CV29
http://www.2mm.org.uk/articles/cv29%20calculator.htm
leave bit 2 unchecked and select what you need, usually bit 1 and bit 5

Thank you for the responses, I will investigate further with the information you have provided.
As you say binary maths are essential and the instructions assume that you are familiar with it, sadly I am a grumpy old git and am finding it difficult to understand the processes. It's a long time since I was at school or further education.

I have seen a problem recently where a loco goes unresponsive as it stops in a station, with all functions shutting down.
Recovery is by way of simply lifting a bogie off track and replacing it whereupon the decoder reboots, the functions re-enable automatically and all is well until the next time, so it is not losing its memory, just losing power.

Hi
Firstly, welcome to the forum.

Secondly, Binary Maths are not necessary! The link provided, while to N gauge, applies to all scales and gauges and offers both Binary and conventional values. So do not be put off by Binary.

As has been said...
Uncontrolled DCC runaways are caused by the decoder "Seeing" the full DCC rail volts as being full DC power and the loco takes off at full speed. This can be stopped by turning Off DC operation in the decoder, this done by reducing the current value set in CV29 by 4. i.e. typically reduce CV29 value to 2 or if long address used reduce it to 34. But ideally read the current value set in CV29 then reduce whatever is read by 4.

Shorting on points and crossing is frequently caused by out of gauge wheel sets, called the Back to Back distance (B2B). It not uncommon for brand new items to have incorrectly set B2B. In OO this is 14.45mm +/-0.5mm measured between the inside faces of the wheels on an axle. A special B2B wheel gauge is available to set the wheels correctly apart on their axle. Sold by good model shops or available from such suppliers as The Double O Gauge Association. Their Intermediate standard gauge is needed for UK OO https://doubleogauge.com/doga-shop/

Another thing that can be done is to fit Insulated Rail Joiners to problem points fitting them on the ends of points Vee rails, as this stops any back feeding occurring and having opposing polarity rails being shorted by metal wheel rims at the frog, where the two metal rails are at their closest. See ,,, https://www.brian-lambert.co.uk/DCC-Pag ... l#problems