Electrical Issue with Sprinkler System

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?

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