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QUOTE (Geoff Booth @ 20 Jul 2008, 20:52) <{POST_SNAPBACK}>Hi folks,

One of my locos has developed a strange problem that has left me puzzled. The loco (Arnold SBB RE4/4ii) was running OK but the lights at one end weren't working so checked all the settings and everything fine so decided to check the bulbs. Having removed the body once I put it back on the track it immediately started lurching about an inch along the track at about half second intervals - a bit like some locos do on a programming track as it goes through programming. The speed was set at zero and it did this irrespective of any loco selected on the hand controller.

I have checked it out on the programming track and everything appears fine there - address etc. Tried a full reset which it took and it did the same on address 3 and then reprogrammed the address (to prove the decoder was OK) but it still keeps on lurching away whenever I put it on the main track! It has a TCS M1 fitted but I suspect that isn't the problem. All the wiring looks fine so I'm stumped. Any clues as to the problem and how to fix it?

Hi Geoff.
I don't know this loco but it sounds like perhaps you upset something on the chassis. Did you do anything to the quartering of the wheels or upset the valve gear in any way.Sometimes sticking valve gear will give this type of problem. Carefully examine the loco to make sure that nothing has got bent out of shape in any way.Its very easy to do. I know because I have done it myself.first off try the loco on the track without the body. Putting the body back may have put pressure on something.

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QUOTE (Richard Johnson @ 21 Jul 2008, 08:35) <{POST_SNAPBACK}>Hi Geoff


If its only 1 loco then I really don't think is a command station issue.

OK... I'm presuming being N scale the bulbs are using white/yellow but no blue wire/grounding thru the chassis. I have no easy answer so lets eliminate what we can....

As an experiment, take out both bulbs.... does this fix it running-wise?

If it does then it may be a bulb with a semi faulty filament creating an occasional intermittent resistance from rail voltage to fuction wire - that'll be a "back door" into the decoder as the Fn wire is after all rectification - so if its broken away and contacting occasionally it will be creating all sort of interesting interference as its impedance rapidly changes as it heats/cools and lets go with vibration/movement.

Either way, bulbs can bea nuisance later on so I'd replace both with LEDs using both blue and function wire if U have wiring room.... if not, use white LEDs via function wire with the + leads to chassis and 500ohm to 1k resistors.

There's not much else it can be if the wiring is right....

** recheck wiring just in case for the tiniest strand of copper can create havoc.
** motor - but thats very very unlikely as the symptoms aren't right
**mechanism - ditto - doesn't match symptoms


If its not fixed by removing the bulbs it may well be a decoder fault.... try another perhaps

fingers crossed


Hi Richard.
Well done. Would the bulbs be causing too much of a drain on the decoder ? or is it simply a bulb fault as Geoff mentioned that with only one bulb working at the start it ran ok. It was when he connected up the second bulb that the problem appeared.
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