How to Test a Washer Pressure Switch and Air Hose
Why this matters
A washer that overfills, underfills, keeps filling, or will not move past fill gets a pressure switch ordered more often than it needs one. The switch is a diaphragm and a set of contacts that only reports the air pressure that reaches it. If the hose between the tub and the switch is split, kinked, packed with sludge, or holding a slug of water, a perfect switch reports the wrong level. Tracing the hose and testing the switch on its own takes fifteen minutes and separates the two. How the different sensing systems work belongs to 'How Washers and Dishwashers Sense Water Level'; this card is the hands-on test of the hose and the switch, on mechanical switches and, briefly, on electronic sensors read through the board.
Water first, then power
A washer that will not stop filling. Close both supply valves behind the washer before anything else, then unplug it. Water at the floor and line voltage at the valve and switch terminals is the combination to separate before you reach in. If water has reached the floor or the outlet, kill the circuit at the breaker instead of pulling the plug from a wet receptacle.
Isolate before every reading. With the washer unplugged, prove the switch terminals dead with a meter checked on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition; check the current edition for its number). Every reading on a mechanical switch in this card is taken dead. The electronic-sensor check runs through the board's diagnostic mode with the cabinet closed.
Dirty water. The air hose and the dome can hold gray water and soap sludge. Never suck on a hose or a switch port to test it, and use a short length of clean tubing of your own for blowing, not the machine's hose. Gloves on when you pull a hose that has been sitting in the tub's water.
The path the air takes
side view, top-load or front-load tub
PRESSURE SWITCH (high, near the console
or at the top of the cabinet)
^
| air hose: rises the whole way,
| no sag, no kink, clamped or
| pushed on at both ends
|
+-----------+-----------+
| TUB | |
| water rises -> |
| +------+------+ |
| | AIR DOME | | air trapped in the
| | (or trap at | | dome is compressed as
| | tub bottom)| | the water level rises
+----+-------------+----+
As water rises in the tub, it rises into the dome and compresses the air trapped above it. That pressure travels up the hose and pushes the diaphragm in the switch, and at a set pressure the contacts change over. The switch never touches water. It reads air, so anything that lets air leak out or stops air reaching it changes what it reports.
What each hose fault does. A split or a loose end bleeds the pressure away, so the switch sees less than the real level and the washer fills past where it should. A pinched or kinked hose, or a dome packed with sludge, can trap a little pressure and delay or block what reaches the switch. Water lying in a sagging section of hose adds or subtracts its own column depending on where it sits, which makes readings erratic, so the hose must run upward the whole way with no low point for water to collect. A hose pulled off entirely gives no pressure at all, and the fill only stops when the board's time limit or an overflow safeguard steps in, if the model has one.
Tracing the hose
- Get to it. On most top-loaders the switch is behind the console and the hose runs down the back or side of the cabinet to the tub; on front-loaders, the top panel lifts off and the switch sits near the top with the hose dropping to the tub's bottom. The tech sheet shows which.
- Follow it end to end by hand, feeling for splits and soft spots, especially at the dome end where it bends and at any clip that holds it to the cabinet.
- Look for low points. Any loop or dip where the hose runs downhill and back up again can hold water. Reroute it so it climbs continuously, using the original clips.
- Pull it off at the switch end and look in. Water coming out of it, or sludge in the end, means water has been getting into the hose. On many machines that is a dome clogged with suds and lint, or a hose sagging into a trap.
- Check the dome. Pull the hose off the dome and look into the dome nipple. Flush a sludge-packed dome with warm water through the nipple, or remove the dome where the design allows and clean it. A partly blocked dome nipple is a common cause of a slow, wrong reading that comes and goes.
- Blow through the hose with your clean tubing on one end to confirm it is clear end to end. Air should pass freely.
A hose with any split, a hardened section that will not seal on its nipple, or a kink that has set, is replaced, and new clamps go with it where it was clamped.
Testing a mechanical switch
- Identify the terminals. The switch label or the tech sheet names them. A single-level switch on a front-loader usually has a common, an empty contact, and a full contact. An older multi-level top-load switch can have several sets for several levels, and the tech sheet tells you which pair belongs to which level.
- Read it empty. Washer unplugged, connector off or wires labeled and removed, no pressure on the port. Common to empty: near zero ohms. Common to full: OL.
- Apply pressure gently. Push a short length of clean tubing onto the switch port and blow slowly, or squeeze a small bulb. You will hear and feel a soft click as the diaphragm snaps over. Do not blow hard; a diaphragm takes very little pressure to trip and a hard blast can damage it.
- Read it full. Hold the pressure. Common to full: near zero ohms. Common to empty: OL. Release the pressure and the click comes back, with the readings returning to empty.
- Judge it. A switch that does not click, clicks but does not change contacts, reads anything other than near zero ohms or OL across a closed or open pair, or sticks in the full position after you release the pressure, is replaced. A switch that passes all of this is good, and the fault is the hose, the dome, the inlet valve or the board.
What this bench test cannot tell you is the exact pressure at which the switch trips, because breath is not a calibrated source. If a switch passes and the washer still fills to the wrong height with a sound hose and dome, the switch can still be the cause; the finish test below is what settles it.
Electronic level sensors
Many current washers use an electronic sensor instead of contacts: a pressure transducer on the same air hose, sending a frequency or a varying voltage to the board. There is nothing to read with an ohmmeter that tells you the level. Trace and clear the hose exactly as above, because the sensor reads the same air. Then put the board into its diagnostic mode, as the tech sheet describes, and read the water level the board reports: empty should show the empty value, and the reading should climb smoothly as the tub fills. The tech sheet also names the sensor's supply voltage at its connector, which you can back-probe with insulated probes, placed with the unit unplugged and hands clear during the run, if the board shows no reading at all.
A worked case
A top-load washer that sometimes fills nearly to the top of the tub. The customer turned it off twice before it spilled over.
Water off at both valves, unplugged, proved dead at the switch terminals. The hose runs from the bottom of the tub up the back of the cabinet to the switch under the console.
Tracing it by hand, the hose is fine along its run, but at the dome end, where it pushes onto the nipple, the last half inch is split along its length. With the washer agitating, the hose flexes and the split opens and closes, which explains why the overfill is intermittent rather than every load. Pressure bleeds out of the split, so the switch sees less pressure than the real level and keeps the valve open.
The switch is tested anyway, because finding one fault does not prove there is not a second: empty, common to empty near zero ohms and common to full OL; blown gently, a click, common to full near zero ohms and common to empty OL; released, back to empty. The switch is good. The dome nipple is clear when looked into.
A new hose is cut to the original length, pushed onto both nipples with its clamps, and routed up the same clips with no dip.
What would have changed the call. If the hose had been sound and the switch had stuck at full after release, the switch is the part. If both had passed and the washer still overfilled, the next suspect is the inlet valve not closing when commanded, which shows as water still flowing with the unit unplugged, and 'The Washer That Kept Filling Past the Line' works an overfill case end to end. If the washer had underfilled instead, a dome partly packed with sludge or water lying in the hose is the more likely cause than a split.
The finish test
- Restore water, check every connection you touched, restore power.
- Run a fill on one setting and watch it stop at the level the maker specifies for that setting; on many top-loaders the tech sheet or the tub itself shows the line, and on front-loaders the tech sheet gives where the water should sit relative to the door glass.
- Run a second fill on a different setting or load size, and confirm it stops at its own correct level. Two different fills that both stop correctly prove the switch is tripping at the right pressure; one good fill can be luck.
- Let it drain fully and confirm the board moves on, which proves the switch returns to empty.
Write on the ticket: hose condition and any reroute, dome condition, switch readings empty and full, the two fill heights observed, and any part replaced.
References
- See related: How Washers and Dishwashers Sense Water Level
- See related: The Washer That Kept Filling Past the Line; How to Find and Read the Tech Sheet Before the First Reading
- NFPA 70E, Standard for Electrical Safety in the Workplace, Article 120, current edition (verifying absence of voltage)
- Manufacturer tech sheet for the model: switch terminal identification, fill levels, and diagnostic mode