my swimming pool cover pump will work fine in sink,bathtub,wheelbarrow,but it will not turn on when put on top of cover with about 100 gal water on it

We understand your concern. The sensor on this pump requires there to be continuity in the water to turn the pump on and off. With "rain water" there is very little continuity in the water as it has not traveled through the rocks and soil to pick up the hardness. If you run a small amount of water on the pool cover it will allow the switch to engage. Strange process I know, but it does help. If you have any questions, please call our technical support line @ 1-800-742-5044. Thank you.