R8249 Loco Decoder Problems?

[Oldhst]

Recently fitted a R8249 decoder to a non DCC ready, China made Fowler 2-6-4 tank. At same time fitted a new motor. Pick up contacts are OK and all wiring insulated at soldered joints. Tested for short circuits with meter - no problems.

On occasions, loco "shoots off" when even a small amount of power is applied. I do not get this with other locos fitted with this Hornby chip.

Any ideas of cause / solution.

[walkingthedog]

Is it a new loco, if not are the wheels clean?

[Howbi]

Could be what is known as DC runaway........you need to change CV29 to a value of 2 which will disable DC running..........HB

[Fishmanoz]

DC Runaway is caused by poor contact somewhere down the chain distorting the DCC signal such that the decoder thinks it is DC not DCC and takes off as full DCC power is going to the track all the time.  Some individual decoders are more prone to this hence some locos taking off and others not.  Decoders often become more sensitive if they have been subjected to shorts on the track, even for a brief time.

[Oldhst]

 Thanks to Walkinthedog, Howbiman and Fishmanoz for your comments.

The loco is not new but the wheels are clean.

The comments re DC runaway are very intresting as I have 2 locos fitted with elderly (15 yre old) Lenz decoders which normally run well but are prone to setting off at speed causing a mega disaster following eg a small derailment of another loco. I will certainly be looking at CV29 for those two and making them DCC only.