How Washers and Dishwashers Sense Water Level

Why this matters

"Won't fill", "won't stop filling", "fills and drains at the same time" and "water on the floor" are four of the most common washer and dishwasher complaints, and every one of them can be a level-sensing fault. They can also be an inlet valve, a drain pump or a siphon, which look identical from the front of the machine. A tech who knows which sensing method a unit uses, and what that method can and cannot get wrong, goes straight to the right test instead of swapping the inlet valve because it is the part that physically lets water in. This card explains the four methods appliance makers use, how each fails, and which procedure on this shelf tests it.

Two mechanisms on one page

WASHER (side section)
  [PRESSURE SWITCH or SENSOR] on console
              |
              | air hose: dry, runs uphill,
              | no dip where water can sit
 +------------|--------------+
 | OUTER TUB  |              |
 |~~~~~~~~~~~~|~~~~~~~~~~~~~~| water level
 |         +--+--+           |
 |         | air |  AIR DOME |
 |         |~~~~~|  trapped  |
 +---------+-----+-----------+
           water enters dome from below

DISHWASHER (side section at the sump)
      ~~~~~~ [ FLOAT ] ~~~~~~  water lifts float
                 |  stem
 ================|============ TUB FLOOR
                 |
           [MICROSWITCH]  lifted stem changes
                          state (per diagram)

The washer measures pressure, not water. Water rising in the tub rises into the open bottom of the air dome and traps the air above it. The deeper the water in the tub, the harder that air is squeezed, and the hose carries that pressure up to the switch or sensor. The pressure in the hose is roughly the height of tub water above the water surface inside the dome, expressed in inches of water column. Nothing electrical touches water; the whole system depends on a sealed column of air.

The dishwasher float measures water directly. The float sits in a cup or guard in the tub floor. Water lifts it, the stem rises through the floor, and the stem pushes the actuator of a microswitch mounted underneath. On most dishwashers this switch is an overfill guard, not the normal fill control: the normal fill is ended by time or by a flow meter, and the float only acts if water gets past that.

The four methods

1. Mechanical pressure switch (washers, mostly older and top-load). A diaphragm in the switch moves against a spring as hose pressure rises and snaps a set of contacts over at a fixed pressure. A single-level switch has one set of contacts; a multi-level or adjustable switch has several, with the knob or the board selecting which. The switch's contacts are drawn on the diagram at rest, meaning empty tub.

• How it fails: the hose, more often than the switch. A cracked or loose hose leaks the air, so the switch never sees full pressure and the washer overfills. A kinked hose can trap pressure, so the switch stays "full" and the washer refuses to fill or fills short. A slug of water sitting in a sag works the other way: it spends part of the pressure holding itself in place, the switch sees less than the tub level makes, and the washer overfills, as in 'The Washer That Kept Filling Past the Line'. Residue packed in the dome inlet blocks it. The switch itself fails with contacts that stick, or a diaphragm that leaks.
• Test: 'How to Test a Washer Pressure Switch and Air Hose'.

2. Electronic pressure sensor (most current washers). The same hose and dome feed a sensor that reports pressure as a frequency or a voltage to the board, and the board decides what "empty", "low" and "full" mean, usually against values it learns during a calibration routine. Many boards display the raw reading in service mode, which is the single most useful level reading on the machine.

• How it fails: every hose failure above, plus a sensor that drifts, a harness fault, or a board that has not been calibrated after a sensor or board change. The board usually codes it as a long fill, an overfill, or a sensor fault.
• Test: the hose half of the pressure switch procedure, then the sensor reading in service mode against the tech sheet's table, read empty and again as the tub fills.

3. Float switch (dishwashers, and the leak pan of many dishwashers). Described above. A second float in the base pan, where fitted, detects water under the tub from a leak and typically stops the fill and runs the drain pump.

• How it fails: a float stuck up on debris or a grease build-up holds the switch in its "full" state, so the machine will not fill. A float stuck down, or a switch that no longer changes state, removes the overfill guard, which nobody notices until something else fails and the floor gets wet. A base pan float that trips means water is already in the pan; finding the leak is the job, not resetting the float.
• Test: power off and verified dead, switch disconnected, continuity across the switch with the float down and then lifted by hand. Near zero ohms in one position and OL in the other, in the sense the diagram shows. Then free the float and confirm it drops under its own weight.

4. Flow meter (dishwashers, and some washers alongside a pressure sensor). A small turbine or vane in the inlet path sends pulses to the board as water passes, and the board closes the inlet valve after a set count, so the machine fills by volume regardless of water pressure.

• How it fails: a fouled or seized meter sends no pulses, so the board sees no water arriving. Depending on the board, that produces a no-fill code after a timeout, or a fill that continues until the float or the pressure sensor ends it. A meter that miscounts gives a consistently low or high fill.
• Test: the tech sheet's flow meter check, usually a pulse or voltage reading at its connector during fill, plus a measured fill volume compared with the sheet's figure.

Matching symptom to method

The first check on any of these is whether the inlet valve is getting voltage when it should and is shutting off when the voltage goes away. A valve that leaks through with no voltage overfills every machine regardless of what the sensors say, and no sensor test finds it. The inlet valve test is in 'Replace a Washing Machine Inlet Valve'.

Worked example: a dishwasher that will not fill

A built-in dishwasher. The customer reports a hum, no water, and the cycle times out with a fill error after a few minutes. House water is fine at the sink.

Step 1, which method. The tech sheet shows a flow meter on the inlet and a float switch in series with the inlet valve, normally closed with the float down, so the float opening the circuit removes power from the valve.

Step 2, valve voltage. Toe kick off and the floor under it checked dry first, since this is a live line-voltage reading inches from a water connection; if there is standing water, the reading waits until the breaker is off and the water is cleared. The tech starts a cycle and reads at the inlet valve terminals with the meter on AC volts, shrouded probes on the terminals, hands clear of the water line: 0 V during the fill period. So the valve is not being told to open, and the valve itself is not the first suspect.

Step 3, float switch. Power off at the breaker, locked, and absence of voltage verified at the valve terminals with a meter proven on a known live source before and after. Float switch disconnected and read across its two terminals: OL with the float as found. The diagram says normally closed with the float down, so it should read near zero. Inside the tub, the float is sitting high in its guard with a wad of label paper and grease under it. Cleared, it drops freely, and the switch now reads near zero with the float down and OL with the float lifted by hand.

Step 4, the sense check. OL as found meant the stuck float was holding the switch in its "full" state. That broke the circuit to the valve, which matches the 0 V reading at the valve. The switch itself works in both positions once the float moves.

Step 5, finish test. Switch reconnected, toe kick refitted, breaker on. The tech starts a cycle: the valve gets line voltage, water enters, the fill ends on its own with the water level just below the float's lift point, and the wash motor starts. He then runs the sheet's fill check and confirms the flow meter is counting, since a flow meter fault could have produced the same timeout and he cleared only one of the two candidates by observation.

The fork. Had the float switch read near zero with the float down, the next check was the board's valve output and the flow meter, in that order, and the valve itself only after both.

What changes the answer

• Front-load washers often combine an electronic pressure sensor with a flow meter and an overfill level on the board, so a single failed component may only show up as a code.
• High-efficiency top-loads fill to a sensed load level rather than a fixed level, so "it barely fills" is often correct behavior. Read the expected level from the tech sheet before treating it as a fault.
• A drain hose with no high loop or air gap can siphon water out during fill, so the machine fills continuously while the sensors are working perfectly. Check the drain before the sensors when water level never rises; see 'How to Install a Dishwasher Drain with a High Loop or Air Gap'.

References

• Manufacturer tech sheet for the specific model: sensing method, switch contact states, sensor table and fill volume
• See related: How to Test a Washer Pressure Switch and Air Hose
• See related: The Washer That Kept Filling Past the Line
• See related: How to Install a Dishwasher Drain with a High Loop or Air Gap
• See related: Appliance Schematic Symbols and Wire Color Abbreviations