I have used two of these fully-auto pumps
for over 10 years ago, and they both work well, but I have experienced a quirk now and then. I have one for the main cabin bilge, and one for the lazarette, with two different wiring setups. The reason I have the "fully-auto" pumps
is so that if I accidentally leave the "auto" wire energized and go home, I don't have a dead battery and/or burned-out pump
next weekend, since it only comes on for a second or two every couple of minutes. These pumps
are also good if you have a relatively flat, shallow bilge, where a float switch won't trigger until you have a ton of water in the bilge, or sloshing might turn the pump on and off frequently.
I don't have anything connected to the "always-on" wire of the lazarette pump; just a toggle for the "auto" wire. That's because water in our lazarette is rare -- we only get it when surfing hard, but since the outboard is mounted in there, I want to be able to keep the motor above water in the event we go out and play hard, scooping water into the lazarette at the transom. We store so much junk in the lazarette that fouling a float switch would be a real problem, so the fully-auto pump works out very well.
The cabin-bilge unit is wired with a more complex setup. the "auto" wire is energized via a toggle in the panel, so it operates just like the lazarette pump. The panel is de-energized when the master switch is off (assuming I remember to turn it off when leaving the boat), so I wanted a "fail-safe" float arrangement per ABYC standards as well. So I have a separately-fused, full-time-hot, wire to a float switch that activates the "always-on" wire of the pump. I also went one step further, and mounted the float a little high in the bilge, and added a "high-water" alarm to it. So in the event of unusually high water in the bilge, the pump will run whether the master switch is on or not, and it will trigger the high water alarm if it does. The boat is pretty dry, so the only time the high water circuit has triggered has been when hosing down the bilge to clean it out.
As for the quirks I mentioned... First, if you get any debris in the impeller, it will add load to the pump, and the load-sensing circuit will think that it's still pumping water, so the pump will run continuously. Also, last year, I had one of the pumps run continuously, as in your case. I didn't find anything fouled at the impeller, but after cleaning everything, it's working fine again this year.
Since I have been selling these in our store, I have only had one return from a customer (who had the same issue as you do), and the warranty was honored, no questions asked. I would verify that your wiring is indeed correct, and if the pump is definitely running all the time, have it replaced under warranty.