Diagnosing a short in a BLI Reading T1 4-8-4 (N scale)

[peteski]

Interesting.  I would have expected a decoder causing a breaker of a DCC system to trip to give lower resistance readings.

Did you trace the wires of the decoder's rear plug?  Are any wires connected to the tender's wheel/track pickups? Earlier you mentioned that the rear plug is connected to decoder- speaker and rear headlight.  Is that all (only 4 wires)?

[thomasjmdavis]

Interesting.  I would have expected a decoder causing a breaker of a DCC system to trip to give lower resistance readings.

Did you trace the wires of the decoder's rear plug?  Are any wires connected to the tender's wheel/track pickups? Earlier you mentioned that the rear plug is connected to decoder- speaker and rear headlight.  Is that all (only 4 wires)?

It appears to me that the rear truck is also wired to the rear plug (there are a total of 6 wires- 2 speaker, 2 rear light, 2 that go under the chassis and appear to be connected to the rear truck). I am hoping not to tear this thing down any farther than I have,  The wiring is clearly very delicate. 

EDIT: I was wrong about where those wires were going.  Detached to decoder again to get a better look.  All 6 wires from the rear plug go to the circuit board below the decoder.  From there, 2 go to the speaker, 2 to the rearheadlight, and two appear to go to the rear truck.  The BLI diagrams are no help here, as they do not show the wires.
http://www.broadway-limited2.com/support/N%20Rdg%20T1%20Expl%20View%20-%20Tender%20(12.16.2021).pdf

[thomasjmdavis]

I also tested a few other DCC equipped locos- I got 400 ohms on a Digitrax equipped FT, and 40k on a Bachmann decoder equipped Also S4.  Note the edit of the comment above about the wiring, if you read that one earlier.

[peteski]

That makes more sense as to why you need to unplug both decoder connectors to get rid of the short. 
If  both the front and rear connectors have wires running to track pickups then the decoder must have both the front and rear electric pickup connector pins connected internally.  That's why both connectors have to be unplugged to get rid of the short.  That to me pretty much confirms that the problem is in the decoder (even if the meter does not indicate a "real" short).

I still think t hat if you do contact BLI, you could inquire if they would sell/give you a stealth PC board instead of replacing the faulty decoder. That  way you could install a decoder of your choice. This would be even more beneficial if BLI will not replace their faulty decoder under warranty and will charge you for new decoder.

[thomasjmdavis]

Thanks Peteski.  That confirms my suspicions.  And I was thinking the same thing (replacing with the 'stealth' board.  I don't think the warranty would apply in any case- since this is second hand, and the previous owner removed the smoke unit (opening a loco usually voids a warranty, especially if you remove parts).

And.... with 2 more BLI locos on order (E8s), I think I may invest in a meter with smaller probes and designed more for electronics that house wiring, to get into all the tiny plugs and wires, so I have a better idea of what is going on inside.
'

[peteski]

You don't need an expensive digital multimeter, and even the high-end ones will likely not have probes fine enough for what we do in N scale DCC.  I own several multimeters and none had probes fine enough. I ended making my own super-fine probes from items I had lying around my home/workshop.







Without these I wouldn't be able to do all those ESU decoder write-ups I posted in the DCC section, but I also use them for many other  tasks (like probing miniature connectors).

My super-fine probes are made from sewing needles, some styrene tubing, decoder wire (purchased in long lengths) and alligator clips I had hanging around.

Using cutoff wheel in a Dremel tool I cut the needles to length and ground a flat area on the dull end. I did that because it is impossible to solder to the chromed surface of the needles. I then soldered about a foot length of wire to  each needle. I then glued them into  the sytyrene tubing.  I thing I used 5-minute epoxy.  I soldered the other ends of the wire to alligator clips.  I also used some flexible nylon tubing (extracted from empty aerosol cans) to make protecting sleeves for the needles  so I don't stab myself when they are in storage.  With the alligator clips I can clip my test leads onto any meter's test leads to be able to problem in very small or tight areas.

I'm not sure of your meter's measuring capabilities, but you shoudl be able to find an inexpensive (probably under $20) meter for your workbench which would be more than adequate for most of model railroader's or DCC installer's needs.

Here is one for $20: https://www.harborfreight.com/9-function-digital-multimeter-with-audible-continuity-59410.html
And another: https://www.harborfreight.com/dm300-pocket-sized-digital-multimeter-64018.html
And one for $8: https://www.harborfreight.com/7-function-digital-multimeter-59434.html[url=http://]]https://www.harborfreight.com/7-function-digital-multimeter-59434.html[url]]]]https://www.harborfreight.com/7-function-digital-multimeter-59434.html[url][/url]

Either one of these has more than enough for modeler's needs. If the meter you own has similar  capabilities then I don't think you need to buy another meter.