My Whirlpool AWM328 washes OK but stops at wash/rinse?

Machine completes wash part of cycle but stops when it gets to the rinse/spin point. Have checked all obvious problems. Inlet valves seem OK, motor is fine (replaced brushes about 6 months ago). heater is fine, etc. Slightly confused by level sense switch - this has 4 contacts wired back to the controller, but only 3 of them seem to do anything - when you blow into the tube, you get a click and the 3 connections work as a changeover switch (ie. single pole, double throw). The 4th contact seems to be permanently open-circuit. Could be faulty, but don't understand how this could cause problem - wouldn't this also affect the main wash part of the cycle?

Help! Any advice much appreciated.