You need for the transformers to share a common ground. If you get a spark when you try to connect the U terminals then the transformers polarity are reversed. I think the VW is an older transform (and so might be the ZW) and does not have a polarity selecting plug (one blade wider than the other) nor a 3-prong plug. If you get a spark just flip the VW plug. Then you can connect the TMCC base to the ground.
Jan