Installing a backup light in a sound equiped 2-8-0

[prr steam]

I attempted to install a back up light soldering the backup light leads to where the yellow and blue wires are soldered to the decoder. Now no lights work but the engine runs and sounds fine. Any suggestions?

[Jim Banner]

Many possible reasons.  Here are a few.
(1) did you test the bulb you used for the tender light?  What current did it draw?  Is this current compatible with the decoder?  Did you include any resistor to protect the decoder?
(2) did you instead use an LED for the tender light?  Did you use the proper value of resistor? 
(3) do either of the lights work when you disconnect them from the decoder and test them with a 9 volt battery?  Note that if you used LEDs, you will have to use the correct polarity and the ballast resistors to get the lights to work.
(4) if the lights work with the battery but neither works connected to the decoder, I would suspect a blown positive supply (blue wire.)  This is normally the same supply as for the rest of the decoder in a silent decoder, but it is possible your sound + motor decoder has a separate supply for lighting.  If that is the case, do the lights work when you return them to one rail instead of to the blue decoder wire?  This would confirm a blown positive lighting supply and suggest a solution.

[richG]

I attempted to install a back up light soldering the backup light leads to where the yellow and blue wires are soldered to the decoder. Now no lights work but the engine runs and sounds fine. Any suggestions?

It may be a defective solder joint for the blue lead. The blue lead is common for the front light, white, and rear light, yellow.
Look for an inexpensive volt meter. Here is what I use for a volt meter and resistance checks.
http://cgi.ebay.com/Digital-Volt-Meter-DC-Current-Multi-Tester-No-Reserve_W0QQitemZ380010042854QQihZ025QQcategoryZ42291QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Search ebay for digital volt meter. There are a lot available for reasonable price.
You might even find one at Home Depot or Lowes, plus Radio Shack,

Rich