The Aqualink rs uses a regular relay and a two speed relay. The regular relay connects one leg of 240 to the pump and connects the other leg to the input of the two speed relay.
The two speed relay switches power to the high speed terminal at the pump or the low speed terminal.
I suspect that the relay is stuck and not able to switch from low to high. The circuit board sends 24 volts dc to switch from high to low. High speed is the default so it should happen with no power. If it wouldn't switch from high to low, it might be that the relay coil wasn't being energized with 24 vdc.
With no power, the relay should default to high speed. Maybe 24 volts is constantly being sent and not allowing the relay to switch.
Check for 24 VDC at the two speed relay (small control wires at the top). There should be no power when high speed is selected and 24 VDC when low speed is selected.
Then, turn off power at the breakers and connect power directly to high speed by bypassing the relay. Then turn on power and see if the high speed works.
The schematic shows the filter pump relay. It does not show the two speed relay. The two speed relay should be connected to AUX2.
The filter pump relay has two low voltage wires at the top that get 24 VDC, which power an electromagnet that pulls the contacts closed. The bottom 4 connections are high voltage.
Two wires from the breaker go to contacts 1 and 3. When the switch closes, 1 connects to 2 and 3 connects to 4. Terminal 2 should go to the pump “Common” terminal.
Terminal 4 of the filter pump relay should go to the “Common” terminal of the two speed relay. The two speed relay has 3 total terminals labeled “Common”, “Normally Closed”, and “Normally Open”. The “Normally Closed” terminal should go to the pump High speed terminal. The “Normally Open” terminal should go to the pump Low speed terminal.
When the two speed relay is not receiving 24 VDC, the relay should be connecting the “Common” and “Normally Closed” terminals, which sends power to high speed. When the two speed relay is receiving 24 VDC, the relay should be connecting the “Common” and the “Normally Open” terminals, which sends power to low speed.
If you post pictures with the control panel access cover off, I can give more specific instructions.