I'm new to the forum and have a mystifying problem with (I assume) the electrical of my well pump.

What I have checked/changed: The breaker on the main panel that goes to the subpanel at the well house has been changed, as well as the entire subpanel (with all new breakers) at the well house. The wiring between the main panel and the well house and between the subpanel and the pump controller (square-D pressure switch) is both continuous for both legs and not shorted between the legs.

Symptom: With the breaker at the main panel on and the pump breaker off, the busbar at the subpanel in the well house reads 240V; the resistance between the legs of the well pump (I'm not sure of the well depth or the horsepower of the well pump) reads 4 ohms, which seems to be approximately right for a 240V 1/2-HP pump. When I switch on the pump breaker, the busbar voltage at the subpanel goes to zero (about 100 mV AC), and the individual legs at the well house *both* power lights, etc. at 120 V, but of course will not power the pump. What could possibly cause such a thing ?!? The main panel busbar (at the house) still reads 240V, i.e., no change at all when the well pump breaker is turned on.

I am well versed in electricity, but I have little experience with this equipment, and would be forever indebted to whomever can shine a light on this mystery.