Fixing Whirlpool part 3378207 to make dishwasher start washing reliably

A dishwasher doesn't simply recirculate water while washing. It tries to separate solids coming off the dishes from the water, and recirculate only the water. Some dishwashers have filters. This dishwasher has a soil separator. First, a chopper breaks up any large particles. Then part of the water being pumped by the wash impeller goes through the plastic doughnut surrounding it, where baffles try to trap particles. Later, when the dishwasher pumps out water, it pumps from the soil separator, sending the particles down the drain.

The outlet of the soil separator connects to the drain pump via the drain pump cover. Also, the outlet of the soil separator has a simple valve operated by water pressure. Under that little cover to the right is a diaphragm which is pushed down by a spring to open the passage. When pressure builds up, it acts against the spring, pushing the diaphragm upwards, and pulling a rubber cone which blocks the passage.

But, how does that pressure build up in the first place, while the passage is open? I'm assuming it builds up due to the drain impeller spinning the wrong way. Both the wash and drain impellers are on the same shaft, and the current function depends on the rotation direction, determined via the motor start winding.

However, if water leaks out elsewhere, then pressure may not build up enough to push up the diaphragm and seal off the passage. That Whirlpool 3378207 drain pump cover has a rubber gasket on top. It has a thin plastic ridge surrounding that gasket, to keep the gasket in place. That ridge breaks off, and then pressure stretches the gasket, allowing water to leak out. The result was that sometimes the dishwasher would fill with water normally and not start washing, even though the pump was running. It's surprising that enough water can escape that way to cause this, but apparently it can.

If I stopped and restarted it while full of water, it always started properly. I guess the big pulse happening when the pump starts provided enough pressure to move the diaphragm, but the slow pressure increase happening as it was filling didn't.

I cut two thin slices from a copper water line, and soldered them into the appropriate shape for holding the gasket in place. The gasket had been stretched a lot, but I managed to squeeze it into place.