ZW Problem

I have a 275 watt ZW transformer.  When I check the voltages on all the outputs on the voltmeter, it shows that all are working properly measuring 20 volts.  As  soon as I hook it to a track and place an engine on the track, it goes into a dead short.  The red light comes on.  I’ve tried multiple engines that I know work perfectly on the layout, but as soon as I put it on that zw, that’s what happens.  Any ideas?


Original Post

I would try using a couple test leads to hook the ZW to an engine that is laying on its side off of any track by touching the roller with on test lead and the wheels with the other lead to see if a short occurs. Would definitely let you know if the ZW has the problem internally.

Is there anything else possibly powering the track? Accessories you ground via the outside rails, etc. If so, it may be that your ZW is out of phase with the other source.

So based on the additional information you supplied for my clarity, I would have to agree with @cjack that is sounds like a faulty circuit breaker in the ZW. If you look at the ZW Schematic below, within the red box, you will see the circuit breaker in parallel with the short circuit lamp/resistor. The circuit breaker and lamp/resistor are connected from the U terminals to the ground of the winding, thus all the current  out of the A-B-C-D terminals must flow back into the U through the circuit breaker. When the circuit breaker opens due to a short, the current must now flow through the lamp/resistor, which severely limits the current, but enough to light the lamp. When the circuit breaker cools, it closes again, bypassing the lamp/resistor, and is ready to protect again.

The circuit breaker is supposed to handle around 10 amps, but your ZW's circuit breaker seems to be opening a lot less current passing through it. 

Try an illuminated car (or two) of some type, and see if the ZW can handle that. Just trying to get a better idea that it is a faulty breaker and not something else.

ZW Schematic



Photos (1)

I can't comprehend a stock ZW breaker tripping as quickly as described.  Have you tried another transformer on your test track?  Or try an ohmmeter with nothing connected.  You didn't say what kind of track it is.  If it's the ancient Lionel 3-rail track, the insulation between center rail and tie could have failed; I've had it happen.

Interesting analysis Rob.  When he said he had 20 volts at terminals, I assumed that the breaker opened as soon as power is applied.  But using a voltmeter and there being no other load, 20 volts would be coming through the red bulb.  If he were to remove the bulb, absence of any voltage would indicate that is the issue.

 If you think it is a defective circuit breaker, jumper the circuit breaker terminals so the transformer senses that the circuit breaker is closed. Then slowly bring the voltage up to the test track and see if the locomotive acts normally. If you do this fairly fast, the ZW can handle any short circuit that may be created by another problem in the transformer or the track.


ZW Breakers are pretty simple DIY Repair job if you can use a screw driver and a soldering iron. There are three styles Early PW, Late PW and Modern. You can adopt any of the three in to any ZW, but staying with alike types is convienient.

Z22: Image result for lionel Z 22

ZW-232:Image result for lionel zw-232

Modern Automotive Bussmann 19115 assembly: 

RJR posted:

Personally, I'd be more inclined to find a modern self-resetting breaker than use an OEM style breaker.  And I'd still use external breakers also, which would protect against shorts among ABCorD

@RJR, just to be clear, the Bussmann 19115 assembly referenced above  is a modern self-resetting breaker that most parts providers use as a modern alternative to the OEM style.

I fully support external breakers on each circuit/bus as well as TVS diodes, but that brings on scope creep to the initial diagnosis of a potentially faulty internal breaker and the presumed corrective measure of replacing it.

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