Washer not filling all the way

Have an approximately 10 year old Whirlpool top load washer. It's not filling as full as it used to on "Super size" setting. Where should I be looking to DIY repair this? Is the fill level based on a sensor or some kind of timer? Or....?
Thanks
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it has a pressure sensor that switches electricity off from the fill valve.Most failure, though, cause overfilling, rather than underfilling, as a hose leak does not build up pressure as the washer fills and so it takes a higher water level to build up the pressure to turn the fill off. Does it do this on all fills over all the various types of cycles, and on the rinse fill also???
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On Wed, 15 Jun 2011 17:11:10 -0700 (PDT), brassplyer

It's not a timer. It always gets to the same level even when it's only hot or cold water or both, even when the faucet is partly turned off, even when there are stones clogging the screens that protect the washer valves.

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There's usually a water level switch (aka pressure switch) that "senses" the water level in the wash tub. It's usually a plastic tube which runs from the switch in the control console down to the side of the outer tub, near the bottom. When the tub fills, water also enters the tube where it pushes on the air trapped inside and increases the air pressure. When the air pressure reaches a certain level, it triggers the switch. A defective pressure switch can result in both underfilling or overfilling of the tub.
Inspect the air tube for kinks, breaks or damage. The tube should also be free of water. The tubing should be securely connected to the bottom of the outer tub and to the bottom of the water level switch.
If you're comfortable using a multimeter, you can remove (after labeling!) the wires to the switch to test the switch for continuity test each of the possible pairs. You can test the switch by blowing air into the pressure switch to see if it causes the circuit to open and break the continuity of the appropriate switch pair. Or you could just take a leap of faith and replace it because that's an easy fix compared to getting to the sensor tube in the outer tub. That's what I would do since the cost of the switch is almost certain to be less than a service call.
-- Bobby G.
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