At present I can control a 12V DC submersible water pump using a transistor (TIP121) and PWM pin. Is it possible to control a 12V AC pump? I do not want to turn the pump simply on/off but control the flow of water from a value sent to PWM pin.
With the right circuitry in between the pump and arduino? Yes. What kind of circuitry? I don't know. That is beyond my electronic knowledge.
You may want to look around for pump controllers. Check there data sheets and see if you can find a way to interface the arduino with them. Chances are whatever means of input they have will run on either 5V or 12V.
Is it possible to control a 12V AC pump?
Well, it’s likely to be a synchronous motor, so speed control would be out. It could also be a DC motor with a rectifier, though, so a standard “light dimmer”-type triac circuit or solid-state relay might work.
Next step is to find out what sort of motor it is.
Thanks for the replies. I think I might just stick to using my 12v dc pump even though it is very loud! I think getting the ac pump to work is beyond my knowledge.