Help with Athearn Blue Box problem

I'm gotten somewhat comfortable with decoder installations, but I've got one loco that's giving me fits. It's an Athearn Blue Box GP38-2 that a while back installed a Digitrax DH163D in. I isolated the motor and soldered all the connections, and the loco ran fine. Its sat for about a year, and when I tried to run it I got nothing. Attempts to read any CVs resulted in a no data message on my Zephyr.

I took it apart, verified that the motor was still isolated, then popped in another DH163D. Nothing.

I hooked up test leads from my Zephyr, and when I make contact with the red and black wires the loco runs. If I put it on the track it will pick up on the "red" side, but not through the frame. In other words I have to run a jumper to the black wire, which is soldered to the metal tab that was originally used for the headlight.

If I remove one truck and touch the test lead to the frame bolster...nothing.

It's not shorting out, it's just not conducting through the frame.

I hope I've explained that clearly enough, I may shoot a quick video to demonstrate the problem.

Any thoughts?
Is there rust or oil on the contact strip between the truck and the frame? The power is not getting through the truck, right?
That's how it seems, however the frame bolster is nice and clean, and I even polished it a bit. But yet when I touch a lead to the bolster I get nothing. I'm going to try soldering a wire from the plate on the trucks directly to the decoder, bypassing the frame. Not sure what else to try at this point, I have an article that I found on the net that talks about soldering a wire to each bearing block, although I'm hoping not to have to go to that extreme!

Thanks for the reply!
Is the frame clean as well where the truck would contact it? Another spot to look is the rivet that holds the inside and outside metal pieces on each half of the truck. I have had that come undone one more than one loco. I used an automatic punch, the spring loaded one, to smash it back together. It definately sounds like the contact trouble is in the truck so it is a matter of looking at the few places things can go awry.
Paul Mack
Hillsboro, OR

Thanks for the reply. What's weird is I replaced the trucks with another set from a brand new BB Geep, same thing. It's really perplexing, it makes you want to think the frame is bad! This weekend I'm going to try what I mentioned above and eliminate the frame from the equation...

Will keep you all posted, thanks for the input.