I have a five zone sprinkler system that includes a controller
(Richdel model 446PR) and a pump that pulls water from our backyard
pond.

The controller can turn the pump on and off by toggling a relay switch
connected to the pump. The relay switch has not been working. When I
disconnect the wires from the relay and test them with a voltage
meter, it registers 28.5vac. Since it's a 24vac relay, things should
be fine. But when I connect the wires to the relay the voltage drops
to 8.5vac and cannot flip the switch. Does anyone know why the
voltage drops like that?