How to check your outside AC low voltage lighting transformer 

Nice explanation!!

Thank you! Very helpful and I'm off to buy a new transformer...and off to see how to test a gfci first, as I don't know how to test if it is bad or going bad.

There's a good chance there is a fuse inside which is blown when something outside shorted out (happens often enough to mention). Higher-end units use GFCI circuit-breakers so you don't have to replace fuses. Actual transformers inside the box don't go bad that often, but if they do, they likely heated up enough to melt the coil windings together, again because something they are connected to shorted out, drew too much current and the heat melted the insulation of the winding wires. The winding type transformers are heavy so your control box will be more than a couple pounds. These are becoming less common as copper is more expensive. Electronic transformers are a different animal...if your lighting control box is not very heavy, you likely have an electronic transformer (less than a pound or so on residential units). Usually, testing the output of these requires a load ( a light bulb, etc) attached to have an output which in a way, is a catch 22. If something else is wrong and you have 120V+/- verified supply voltage, the load may not work for that reason, not a bad e-transformer. I do carry around a small, known good controller to troubleshoot with...easier than component level circuitry troubleshooting and allows me to test one lighting run at a time if multiples are present.

Thanks

12 volts AC or DC???? Please always state ac or dc volts