Coolant pump on storage tank operation

I am hoping someone can help me with this issue. I replaced the coolant pump that is attached to the storage tank (front left) and I know that pump works as I bench tested it. For whatever reason, when I park the car after use that pump is not coming on and moving coolant into the tank. I am hoping someone knows what triggers that pump to turn on. I know the ECU turns it on but there must be a sensor etc. that tells the ECU to send power to that pump. I just want to know the sequence of events that turns that pump on. Thanks to all!