What has gone wrong with my locomotive?

[AyJay]

Can anyone please suggest what might be going on with one of my locomotives?

I have a moderate sized OO gauge layout.  DCC courtesy of NCE Powercab.  Feed from heavy gauge bus to droppers for each length of rail.

The errant locomotive in question is a Hornby Black 5 with contacts 4-wire connection to the tender. The decoder chip is probably a Zemo, in the tender.

 

The locomotive recently started juddering, stop - go - stop - go - stop - go.  at two specific places on the layout; and nowhere else; on a gentle radius left curve on the innermost (anti-clockwise) mainline.

It passes this place without problem if going in reverse, or forwards in the wrong direction.  It only happens when running the correct way, slowly, not when moving fast.

It only happens to this locomotive, all the others behave perfectly.

This locomotive also behaves perfectly when running the clockwise line.

 

I can see no faults with the track.  This is not a short-circuit according to the controller. Neither is the current reading on the controller showing a current surge.

The locomotive wheels are clean. They do not appear to be jamming mechanically.

I put the locomotive on my programming track and reprogrammed the address.  No change!

 

Now as an added bonus this evening,  when I was investigating what was happening, the locomotive just came to a sudden stop when crossing a live-frog point (Peco) and refused to move any more.

The wheels are not jammed, I power-cycled the controller, there was no short.

This locomotive has been used a lot less than others and has not been mis-treated.  Can anyone suggest what may be behind this crazy behaviour?

Thank you,   Alan 

[Nigelcliffe]

As its in one direction around the curve, and the loco is fine going forwards "the wrong way", plus the stalling at the turnout, I'd start by looking at the pickups and electrical connectivity.

 

Pickup issues can be quite subtle: as the loco goes around a curve, the wheels slide left-right on their axles.  This can lead to momentary loss of pickup as the wheels move, then they move back again and re-make contact.  The way the gears and rods work in a loco can contribute to this problem (in some designs there is a small pressure moving the axle up/down depending on direction of travel).  

 

Check that the plug between loco and tender is conducting on all pins - some movement in the plug might mean it fails on some curves, but is fine on others.    

 

If it is pickup adjustments, go very carefully as its easy to make it much worse.   

 

 

[ISW]

13 hours ago, Nigelcliffe said:

Pickup issues can be quite subtle: as the loco goes around a curve, the wheels slide left-right on their axles.  This can lead to momentary loss of pickup as the wheels move, then they move back again and re-make contact.   

 

If it is pickup adjustments, go very carefully as its easy to make it much worse.   

Turn the model over, and 'wiggle' each axle with a finger, watching the wiper pickup contacts to see if any lose contact with the back / tread of the wheel. If any gaps appear, that's where you are losing power pickup.

 

I find that the only reliable way to adjust pickups is with the wheel/axle removed. It 'can' be done with the wheel/axle in place, but is very easy to make it worse (as previously noted in an earlier posting).

 

Ian

[SamThomas]

Follow Nigel's good advice & maybe look at any chemical blackening on the wheel treads &/or pickups (as you have said that this locomotive has not had much use).

 

Check for pickup continuity with a test meter or test lamp.

 

A tiny amount of Track Magic can help with pickups.

[BoD]

An off the wall suggestion - check the screw holding the tender drawbar in place underneath the loco.  Too tight or working it’s way loose can result in similar behaviour.

[tarifa]

One more check would be the 4 wire plug into the tender socket.  Check the wires are not touching on the plug, creating an internal short.

 

Mike

[Michael Hodgson]

Long shot if all else fails, but flexible leads on connections between vehicles can be a problem if the wire is broken internally, and this isn't apparent on inspection - at certain angles the wire can be becomes discontinuous but it still looks OK.  I have known the strain on bending very thin wire in one particular direction to cause this.  It's not a very common fault but like most intermittent problems, it's really difficult to find when it does.  

[AyJay]

Thank you all.  Sometimes it takes someone else to suggest the things that should be obvious.

The one idea that comes to my mind is if I can find an 8-pin blanking plug, I think I'll try disconnecting the decoder and see what happens when run on analogue.  Is there any chance the decoder itself could have become 'corrupted'?

[Nigelcliffe]

34 minutes ago, AyJay said:

Thank you all.  Sometimes it takes someone else to suggest the things that should be obvious.

The one idea that comes to my mind is if I can find an 8-pin blanking plug, I think I'll try disconnecting the decoder and see what happens when run on analogue.  Is there any chance the decoder itself could have become 'corrupted'?

 

Seems very unlikely to be a decoder issue from the reported symptoms.   How would the decoder know it was going clockwise round a curve  and thus go faulty, yet work normally on other pieces of track  ?   

 

Testing on analogue is often easier than digital, but you've got to look very carefully to diagnose intermittent contact issues.   

[AyJay]

That fixed it!

I lifted the plate under the chassis between the wheels to get better access to the contacts.

I was able to re-tension a few of them. Also, with the side of a scalpel, wipe the gunge that had accumulated on the wiping contacts.

Lastly, picked up a lot of muck from the inner surfaces of the driving wheels, with IPA-soaked cotton buds.

Connecting wires and plug got a good eyeballing and seen to be OK.

75005 now runs perfectly.

Thank you all for your suggestions.