I had a similar problem with my tabs. Each tab on my set up goes direct to a battery and is fused. Although I had voltage, it couldn't handle any current. It turns out that inside the fuse holder was corrosion. When I tried to activate the tab, nothing happened sometimes just a click. I thought the pump motor was bad. Glad I didn't jump the gun and buy a new motor. I just replaced the fuse holders. This happened on both tabs 2 weeks apart from each other. Each tab went to a separate battery with a separate fuse.
May not be the same, but it's worth a look. And cheap to fix.