Will not drain.

Elimination Of Problems From Most To Least Likely:

I know you said the drain's not blocked, but i feel obligated to mention it anyways ;)

1. Connection from discharge hose to drain pipe is blocked/corroded.
a) Loosen the gear clamp (5/16" Nut Driver/Phillips/Standard) holding the discharge hose to the barbed drain fitting.
b) Check that the barbed fitting is not blocked by poking at the opening wth a screwdriver.
c) If the barbed connection is Galvanzied and it has gotten heavily corroded, replacement of that fitting is recommended.

2. Discharge hose is blocked/kinked from dishwasher to drain connection.
a) After 1b), place discharge end of hose into a bucket and run the dishwasher through a cycle.
b) If flow is poor, examine hose for kinking or parts (such as couplings) that may have corroded shut. Removing front panel and using a flashlight will be required for this (1/4" nut driver/phillips) If such corrosion found, 1c).
c) If hose and fittings are ok, there may be an inline filter whose screen is clogged or a filter screen where the dishwasher ties into it's discharge line.
3. Pump has failed internally . Usually held in place by large squeeze clamps, screws, gear clamps. Will most likely removal of dishwasher from alcove to repair. Any reputable dealer should stock parts. i.e. Kenmore => Sears
N.B. May be under warranty - check. May be cheaper just to get another dishwasher at this point, especially if the dishwasher is more than 10 years old.

GE dishwasher, will not drain completely. Leaves about 1/2 inch above bottom screen. What controls when water pump shuts off? Is there some type of float?