Dcc loco issues

[Michaelaface]

I have two locos that short circuit my dcc system as soon as their pickup wheels touch the track, i doubt its a decoder issue as I’ve tried several different ones to no avail and I’ve completely dismantled one loco to find the issue and found nothing

Can anyone explain this? As it makes no sense to me

[Bigmet]

What are these specific locos (maker and type) that give this trouble? (Some model constructions are apt to be troublesome. Aspects like half insulated wheelsets as an example, where it only takes one wrong way around to make the loco a dead short.)

Did these locos run on DC before any decoder was installed?

[Michaelaface]

Bachmann 4mt 4-6-0 factory DCC fitted, worked fine for 7 years, until one day this issue occurred and then my hornby select blew its decoder, just replaced it with a different decoder which works and it just instantly shorts on DCC, dismantled the whole thing to try to work out if there was a short anywhere, could find nothing, disconnected the motor and put it through DC power and it worked fine, unless at high speeds where it’d start to smoke

The other is a bachmann G2A which runs perfectly fine on DC but as soon as the decoder is added I get the exact same scenario as above

[Bigmet]

The smoke you mention isn't good. Was it from the motor itself or from soldered on capacitors?

However, the smoke problem isn't going to cause a dead short on DCC, and you have only seen this on the 4-6-0 anyway so it's nothing to do with the G2A's problem. Immediately shorting out a DCC system says that the two rails are being connected before the decoder.

I would take a look at the pick ups and wiring up to and including the decoder socket, ideally using an ohmmeter to check the integrity of every connection, and no connection where there shouldn't be one. Quickest way to blow a decoder is for the power from one rail to contact one of the two motor feeds. This fault may not affect DC operation at all. Where these wires run together and are clipped onto the loco to tender link on the G2A (is the 4MT the same?) is a location I would look at very closely.

Had you recently done any servicing, alteration or adjustments to these two locos before the trouble was evident? Used any conductive lubricants or switch cleaners?

[Michaelaface]

Was from the motor itself, literally dismantled the whole thing looking for a short and that was the only issue that arose, going to try a new motor and completely rewire the 4mt

Have looked through the G2A and can’t see any shorts, and it it was shorting before the decoder, why would it still work on DC?

Both are in out of the box condition

[Chops]

A useful thread, but sorry for your troubles. Most frustrating. I've been leery about getting back into DCC as I like it simple and reliable. DCC is great when it works, but when it doesn't...

[Dad-1]

This is not a silly question.

Did you try running before you put the body back on ?
I have a Bachmann 57XX that ran without the body, initially O.K, but then constantly shorted with it back on.
It turned out to be the motor rocking and touching the metal ballast weight all around the inside. Covered
in masking tape and all now O.K.

A hornby M7 gave a similar problem where with a slight rocking of the motor it allowed a motor terminal to
touch the driver flange, result constant shorting.

Off out will continue to 'think'

Geoff T.

[Michaelaface]

Yep, tried that still shorts, really frustrating

[boxbrownie]

If the smoke came from the motor itself and it now shorts without apparent cause it could be that the windings onnthe motor armature have melted the insulation and are causing a huge load to the controller/chip.

You could try a resistance readout on each motor winding and see if one (or more) give a very high reading.

[Mike Parkes]

I had a short on Standard 4 Mogul - traced it to a contact strip on the baseplate having too big a soldered joint to a wire and it was breaching the gap to the adjacent contact strip. Remove the decoder and trace the circuitry with a multimeter on resistance.

[Mountain]

Oil on the motor commutator plates can cause these symptoms.

[Bigmet]

... I've been leery about getting back into DCC as I like it simple and reliable. DCC is great when it works, but when it doesn't...

I don't think this problem is anything to do with DCC as such. DCC does not make motors smoke...

...Have looked through the G2A and can’t see any shorts, and if it was shorting before the decoder, why would it still work on DC?...

DCC has full power on the track at all times, and overload protection is built in to the system of necessity. If you have a short with some resistance in it - for example conduction through damaged insulation or carbon track conduction from burnt wire insulation - It may still allow the motor to run on DC for a while. DCC will typically quickly shut down.

I had such a 'mystery' a couple of years ago, this was a friend's loco on which a 'mate' had applied a wonder electrical cleaner some time previous. The wire insulation from loco to tender (Bachmann 4F, very similar construction to the G2A) had gone conductive, and was like chewing gum.

I think your plan to fully rewire may be the winner, whatever the root cause of the trouble. And as above, it's not a DCC fault, it's DCC uncovering some other problem. The smoking motor is nothing to do with DCC. Not being able to see and test the trouble means one has to guess and can only suggest lines of enquiry.

[Chops]

Interesting observations, clarification please:

The "full power" is the twelve DC volts transmitted to the rails, yes?

The DCC current is a low voltage, 1.5 volts (?) sent in simultaneously which activates the decoder, yes?

The decoder is then a resistor, or throttle, if you will, allowing more or less DC current to turn the motor?

If those two things are true, then does it stand that the AC current, being quite low, be sensitive to any light oxidization or grime that would not usually cause a DC application to falter?

[Roger (RJ)]

Interesting observations, clarification please:

The "full power" is the twelve DC volts transmitted to the rails, yes?

The DCC current is a low voltage, 1.5 volts (?) sent in simultaneously which activates the decoder, yes?

The decoder is then a resistor, or throttle, if you will, allowing more or less DC current to turn the motor?

If those two things are true, then does it stand that the AC current, being quite low, be sensitive to any light oxidization or grime that would not usually cause a DC application to falter?

Sorry, wrong on virtually all counts.

Full power is often around 14 or 15 volts AC* (see below) but can be a bit higher or lower and it actually is the DCC signal boosted up to the higher voltage and current that the locos run on.

The decoder rectifies the DCC power to dc and processes it then sends a pulse width modulated (PWM) DC voltage to the motor at full DCC voltage, minus a bit of voltage that is lost in the decoder when being rectified.

The NMRA symbol

dccs-1.gif (3.11 KiB) Viewed 2872 times

shows a loco over a representation of a DCC waveform.

*It's not true AC like household power. It's a modified square wave with unequal mark space ratios that change between positive and negative.

[Michaelaface]

second question, does anyone know of anywhere where I could purchase a replacement motor for my 4MT, accidentally broken mine looking for the cause of the short and the smoke (one of the brushes came out and a tiny spring flew off)

I've given bachmann an email asking them, but any other options would be helpful!