I got this used 1996 Caldera Spa that needed a cir pump. The pump was tested and works fine but when plugged into the circuit board the GFCI breaker trips instantly. Without the cir. pump connected the tub works fine. The original owner said the tub works fine other than a burnt cir. pump. Any thoughts on what may be the problem.

I'm not familiar with Calderas older than 2000 but I'll throw out a guess.

You may have a problem with the heater.

With the circ pump disconnected, you may have a flow switch or pressure switch that will not allow the heater to come on. Without voltage going to the heater, it will not trip the system.

Try this:

Kill power to the tub.
Disconnect the heater.
Connect the circ pump back up.
Turn power back on and see if it runs

Thanks for your help...but still NO GOOD. I ran the circulator pump for a couple of minutes by plugging it in seperate from the tub and I seemed to get warm water so I assume the heater is good. Another question...the cir. pump ran when plugged into a regular outlet so I guess it's OK (it should be, it's brand new) but should there be resistance between the black and white wires? I get like 35 Ohms when I put a tester on it (the new pump that is)? It seems there would be un equal current on the hot and nutral then.

Oh...I should also mention that I tested the cir pump with ALL other items disconnected from the panel and it still trips the breaker.