A water level control (WLC) or pressure switch is usually mounted inside the control console directly behind the knob for setting it. It is a SPDT switch which is actuated by a diaphragm operated by air pressure via a small tube connected to the WLC's housing. The other end of this small tube attaches to an 'air dome' usually located somewhere on the outer wash tub.
There are usually 3 connecting terminals on a WLC; one for a common contact of the internal switch, one for a normally open switch contact and one normally closed switch contact terminal.
When at rest, there should be continuity between the common and normally closed contact terminal and no continuity between the common and normally open contact terminal. When activated by air pressure, the normally closed switch contact opens and the remaining normally open contact closes.
This arrangement is usually used to allow power to energize the water fill valve of a clothes washer until the internal diaphragm is activated. At that time power is discontected from the fill and switched to the timer to allow it to advance through the rest of the cycles (wash, spin, etc.).
The pressure switch's contacts may not be labeled C, NC, NO so you may have to rely on the machine's wiring diagram and wire color coding to determine which contact is which. The illustration to the right is what a WLC may appear as on a typical electrical diagram. The left most contact is the common and the lower contact the normally closed.
If the switch tests Ok but is still not being activated properly, there may be a problem in the air supply tube attached to it. These tubes can develop air leaks from rubbing against other internal washer components over years of use or the 'air dome' the other end attaches to can become clogged with lime, scale and soap residue hampering air flow to the pressure switch.