The pump looks like a miniature of a submerged style basement sump pump. At the bottom is the pump housing, and impeller. It's all glued together, but by carefully prying with a screwdriver I managed to open the lower part containing the impeller. I experimented a bit with it open this way, and found that I could get the pump to start and run fine if I pushed a bit on the motor axle. When put back toghether again the pump would run a few more times, but then die. At this point I suspected either bad contact in the commutator, or that one or more, but not all, windings were bad.
To get any further I needed to get access to the electrical motor in the upper, sealed part of the pump. This proved harder to do than opening the lower pump housing. Prying with screwdrivers and knives just broke off pieces of plastic, but wouldn't move the top cap off. In the end I ended up sawing carefully around the flange of the top cap, and managed to saw away the glued part without wrecking the flange itself. Once I got the flange off to my surpise and horror black, rusty water pured out. It's a wonder the pump had at all turned this way. I couldn't get the dc motor out of the plastic housing. Maybe it's integrated into the housing somehow. Maybe it's just rusted in there.
The good news is that I got the motor working again by flushing out the water and rust with "liquid wrench". I also took out the water seal on the shaft between the upper motor part and the lower impeller housing. There is just one, somewhat flimsy looking rubber seal. I'm no longer surprised my motor was full of water. Infact I wonder how that seal could ever had kept the motor dry through natural pressure variations caused by different tank water levels, and varying outside temperatures.
For a while I though that since the motor appeared to be able to run even when full of water, and it would be hard to keep the water out of there if the housing was filled with air (since air is easily compressed, allowing water to enter), maybe it would be a good idea to fill the housing with a noncorrosive fluid instead. I contempleted filling it with liquid wrench for a while. Then I realized that it would be worse to have the liquid wrench leak into the watertank, than having water in the pump.
Before reassemblying I stuffed the water seal with grease, and lubricated the motor bearing with a little light oil. The housing was glued toghether with epoxy. The pump has worked fine since this "overhaul". If I were to do it again I would use a simpler procedure. Since the motor wont come out anyway it would be enough to drill an access hole in the top of the motor housing. There is a 20mm emty space between the upper lid and the motor. The motor can be drained and lubricated through this hole. To get to the seal, the lower part has to come off though.
/Martin