Shorts

You state that the "insulated wheels are correct" on the lead and trailing trucks. I assume you took the wheel sets out of the lead and trailing trucks before you painted the trucks and then put them back in. From experience I know you can make a mistake putting the insulated wheels on the wrong side. I have done that before. If you are in the cab looking forward, the insulated wheels, on most brass I have dealt with, are on the left side or fireman's side of the locomotive, including the trucks. Take a careful look that the insulated wheels are on the left side of the truck as the trucks are installed on the locomotive.

I am not familiar with the Sunset O-8 Mikado and how different the lead and trailing trucks look but also make sure you have not installed the front on the rear and vice versa. I know that sounds silly but I did that on a PFM Long Bell loco. I had put the wheels on correctly but then reversed the trucks. Duh!! Too much beer that night.

John

Agreed

Dear Sancho,

Firstly, good-on-you for insisting "it must run-right on analog DC before going DCC"!!!
(This minor but significant procedural step may well have saved the life of untold decoders!)

Second, point of clarification, when you can "constant shorts", do you mean:
- literally a "Dead Short, continuous in nature",
as if the entire loco is acting like a piece of wire soldered accross the rails
(a continuity beeper meter would sound like "Beeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeep")

OR

- a series of rapid, momentary shorts which seeem to continuously happen anytime the loco attempts to run down the track
(a continuity beeper meter would sound like "Bee be bee bee be be bee bee bee bee be be be be beep",
like some demented Morse-Code key)

???
(this is actually important and key to determining what kind of short can cause either condition)
 

Given the diagnostic info, I tend to agree with John above. Either:
(In no particular order of preference or likelyhood)

- The leading/trailing truck wheelsets were removed from said assemblies, and were re-installed backwards.
(I know you say this didn't happen, and I believe that you believe this to be the case,
but the diagnostic evidence is clearly indicatig something is amiss,
and Wiilly Occum says this is the most-likely/most-plausible cause...)

- The assemblies themselves have been re-installed on the locomotive flipped end-for-end
IE Leading truck is installed in the trailing position, and vice versa.
(unlikely, but some loco models do have visually-similar leading and trailing unit designs...
...or it could have just been a particularly late-night at the workbench...   )

- The insulation in the insulated wheel
(often a ring of paper slipped between the wheel and the tyre/tread/flange component)

has been bridged/shorted/broken-down-to-the-point-of-no-longer-insulating
(This is a common issue on older brass, and can be extremely hard to spot by-eye.
Strategic probing of LH Tread <> centre-of-axle
AND SEPERATELY centre-of-axle <> RH Tread
with a digital multimeter set to "continuity" or "Ohms" will reveal the truth.).

It is arguably possible that some paints can conduct accross the (literally paper-thin) insulating ring,
causing a insulated-wheelset to become a "dead short" wheelset,
but that's a how (did the fault-condition happen) question, not a why (is the fault symtom occuring) question.

- The wheelset has been "bumped" out of gauge or alignment, such that there is a glancing-short between the wheel-treads, via the truck framing assembly (would expect to see a matching issue with mechanical binding, unlikely to be experienced by both Leading and Trailng trucks time-co-incident)

- The leading/trailing truck mounting had an insulating piece (sleeve, bushing, etc) which has been lost/omittted in the dis/re-assembly, thus leading to a "sneak path" where there previously wasn't one.
(again, a stretch, but included for completeness).

Rest assured, you can diagnose and resolve this,
just gotta follow the diagnostic evidence...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

I now feel foolish...

Thanks for the replies, guys. My issue was that I reversed the drivers on assembly and hadn't even thought about them as the issue. It's up and running, although I have some inconsistent pickup from the tender. Work for another day.

I _knew_ you could find it...

Dear Sancho,

File this one under "Trust, but Confirm"...

Good to hear you got it sorted...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

@Sean.........

........that is probably the most common mistake made by anyone who is brave enough to take apart, paint and re-assemble a brass steam engine!  It’s a pretty big club. LOL

If you can’t see which wheel is insulated, use a meter to check the continuity.  I stick a small bit of tape on the insulated wheel to remind me.  The second most common mistake is getting the wheel-sets reversed in the tender.

Peter