I helped someone troubleshoot a Lionel Trainmaster ZW transformer this week, because my Lionel ZW transformer had exactly the same problem. It seems like it’s pretty common, so maybe others have the problem too.
The Lionel ZW, as you may know, has four pairs of posts on the back for power. You can use it to run four trains, but what many people do is use the inner posts to power accessories and fine-tune the voltage output. The problem with my ZW was that one of the pairs of posts didn’t work. Sometimes two of them might not work.
Here’s a slick trick to try, and if it works, the fix is super cheap and easy and doesn’t require you to open it up.
A much more professional approach is to open up the ZW and replace the posts. But this trick works in a pinch, especially if you’re just trying to get your old train running for the holidays.
The ZW has four posts labeled A-D, and four posts labeled U. The trick is that all of the U posts are interchangeable. So if post A or D doesn’t seem to work, try hooking one of the wires up to a different U post. If it starts working, the problem is that one or more of the U posts has separated from its bus.
I fixed mine with a nice thick piece of bare, solid copper wire. I think it was 12 gauge. At any rate, I loosened up all of the nuts on the U posts, looped the piece of wire tightly around the post underneath the A post, ran it along the tops of the U posts underneath the B and C posts, then looped the other end tightly around the U post underneath the D post. Then I just tightened the nuts back on. In a pinch, you don’t necessarily even have to use copper, but it’s better. As tempting as it may be to use a paper clip, steel gives a lot more resistance, and it’s not thick enough.