DCC erratic running

[john_ramsbottom]

I'm just coming back to this after a 10 year lull caused by lack of space to construct a layout. Some of the kit I am using is this age and older, and I'm not sure exactly of the provenance. Latest step has been to install a decoder by hard-wiring in a Hornby 9F Evening Star loco which has a Ringfield motor in the tender. The decoder is I think a Bachmann 8 pin 4 function - it had a separate harness with 8 pin plug which I have cut off.  I think I have the wiring correct - red wire to track via the coupling to the loco, black to track via the body of the tender. Purple and pink to motor brushes. It will run, but very jerky. Some of this may be due to dirty track, but a new Hornby class 40 diesel loco runs fine on same track, so it cannot be all bad.  The strange thing is that if you lift one side of the tender off the track (the insulated tyred wheels) but keep the other side in contact, the motor seems to run fine. I am completely flumuxed by this - does not seem to make sense, so wondered if anyone had any ideas where I may be missing a trick?

[RDS]

Hello John

I cannot comment on the colours of the Bachmann Decoder for the Motor connections (but they are Orange and Grey on the Hornby Decoder).  However, a clean track is essential.  IPA on a cotton bud is excellent for cleaning track and wheels.  Also ensure your Loco is lubricated using only Model Railway lubricant.

[96RAF]

Take a look on this site in the dcc section and you will see the standard 8 pin decoder colours.

 

purple is normally a fly lead not connected to the plug and  used for switching function 2 output so if you have it connected to the motor you are probably trying to run the motor from a limited current source causing it to cut out and in.

 

If your decoder has non standard colours then look on the manufacturers web site for details of what is what.

[john_ramsbottom]

Although I cannot locate any post on this site which gives the standard 8 pin decoder colours, I think I have it correct. My earlier description of "purple" is misleading, as there is an un-connected wire of deeper purple and what I described is probably meant to be grey. (Bit colour blind). So orange and grey to motor, red and black to track. Evidently not the issue of incorrect wiring.

I still cannot understand though why the motor runs OK when you hold it against the track, but stutters when you let go, especially when a different loco seems to run fine on the same track...

Thanks for the suggestions however.....

[john_ramsbottom]

Isn't IPA a sort of beer???

[Chrissaf]

I still cannot understand though why the motor runs OK when you hold it against the track, but stutters when you let go.

.

Probably poorly aligned pickups on the wheel backs.

.

Standard Decoder colours are here (scroll further down the page that opens):

.

http://www.brian-lambert.co.uk/DCC.html#Basic

[Fishmanoz]

John, if you look even further down Chris's referenced Brian Lambert DCC pages, you'll find conversion methods for the 3 different Ringfield motors.  First determine what type you have, then follow the conversion method.

 

I believe it is important you do that his due to the fact tha lifting the insulated tender wheels makes a difference.  It should of course work this way as you have track power via the loco on one side and the still in contact tender wheels on the other.  The fact it doesn't then work well with both sides of the tender in contact means something is amiss on the insulated side of the tender.  The most significant problem that can arise here is when the left hand motor brush is connected to chassis leading to a short between decoder track input and motor output, a sure fire way to decoder destruction.  So you should read up on Brian's conversion methods before you go any further.

[RDS]

... My earlier description of "purple" is misleading ...

 

and Pink?

[96RAF]

I have some decoders with pink wire, it is faded orange, dodgy chinese eletlic stling.

[john_ramsbottom]

I have done some more investigating with this. I am now pretty certain that the decoder is wired correctly, despite earlier confusion over the wire colours.  If I run the tender off the track using a wire feed from the controller, it is as smooth as anything, so it must be to do with connectivity with the track power.  This loco does not have pickups against the wheels but seems to rely on metal to metal through the loco body. I am wondering about the link coupling between loco and tender which transmits one side of the power. Should there be some sort of spring here to ensure that a swivel link also connects electrically?

[Wobblinwheel]

Could either the track pickups in the loco or tender be reversed? I had this issue once where two of the wires from the loco to the the tender were backwards. Motor would run fine as long as loco and tender were not on the track at the same time.

[john_ramsbottom]

My understanding of DCC is that the polarity from the track does not matter. If you lift the loco off the track and place it on the other way, it still runs forwards. So having the internal connections reversed would not make any difference.

However I have seen somewhere a comment on this 9F loco, referring to the pickup from the tender. Effectively only 2 wheels collect the power and when running over a diamond crossing, there is a time when both these wheels are on an insulated section.  This user overcame this by a modification to the loco section. The 5 "driving" wheels on one side pick up power. The wheels on the other side are insulated from the axles. But there is a continuity via the valve gear and connecting rods, so this person connected a wire to the crosshead and ran it through the loco body and across to the tender to double up with the connection from the tender wheels.  I might give this a try to see if it helps with my problem....

[96RAF]

Whilst DCC does not have a distinct positive and negative like analogue, it does have a polarity (A and B on your controller) which must be matched rail to rail, loop to loop around the layout and wheel to wheel along a loco else a short will occur.

 

e.g. if your loco wheels pickup is left and right orientated (A-B) and your tender is right and left (B-A) orientated when you join them together you will see a short.

 

The DCC signal is like an AC mains sinewave but square in waveform and in that there is a potential both positive and negative of around 15-20 volts about a zero volts baseline.

[Fishmanoz]

While adding extra pickups as described is a good idea, let me repeat what I suggested in my previous post - if the loco is running fine with just the non-insulated tender wheels in contact and becomes jerky when the insulated wheels are in contact too, you have a problem with the tender that needs investigation first.  Either, there is an intermiitant short in the tender (probably unlikely, would cause a complete cutout), or the wheels aren't making good contact with the track, or the pickups aren't tension properly against the wheels (only make good contact when the tender is tipped and wheels pushed towards the pickups).

 

I would be fixing the first problem before adding anything else.

[2e0dtoeric]

If the tender has been dismantled, one or more of the sets of wheels could have been re-fitted the wrong way round, so the insulated wheel is on the wrong side.

[Fishmanoz]

Certainly worth checking 2e0, but I suspect that is likely to be a complete cutout scenario, not a rough running one unless it is combined with poor contact on the reversed wheel set.