HOW-TO GUIDE
How to Test a Float Switch With a Multimeter
By the OhmSavvy Research Team ·
Water and mains don't mix: isolate before you touch anything
- Most sump and condensate pumps run on mains: 230 V in the UK and Europe, 120 V in North America. Unplug the pump, or switch off and lock off its fused spur or breaker, before putting hands near the water or the switch. Prove it dead with a voltage tester if it is wired in.
- Keep the meter and your hands dry, and never stand in water while probing a live connection.
- Sump and condensate water can be dirty or contaminated. Wear gloves and wash up afterwards.
Mechanical vs reed float switches
Float switches all do the same job, opening or closing a contact as liquid rises, but the inside differs, and that affects how you test them.
Tethered (tilt)
A float on a cable that tips as water rises; a ball or weight inside flips a mechanical switch. Common on sump pumps.
Vertical mechanical
A float slides up a rod or arm and presses a microswitch. Often fitted to sump and condensate pumps.
Reed float
A magnet in the float passes a sealed reed switch in the stem. Typical for aquariums, tanks and low-current controls.
Electronic
Pressure, capacitive or optical sensors with no contacts. These need power and cannot be checked with continuity.
A reed float is really a reed switch in a waterproof housing, so the checks in how to test a reed switch apply. Reed contacts are usually rated for small signal or relay currents only, so they rarely switch a pump motor directly.
Normally open vs normally closed float switches
"Normal" refers to the float resting at the bottom. A normally open (NO) switch is open when the float is down and closes as it rises: the action used to start a pump that empties a sump. A normally closed (NC) switch is closed when down and opens as it rises, which suits filling a tank or a top-off reservoir. Some tethered switches have three wires (COM, NO and NC) so they can do either.
1Disconnect the float switch from power
How you isolate the switch depends on how it is wired.
- Plug-in piggyback type (common on North American sump pumps): the pump plugs into the back of the float switch's plug. Unplug both from the wall and separate them. Then test from one pin of the switch's wall plug to the matching socket slot on its back; the other pin passes straight through.
- Wired-in type (more usual in UK installations): isolate at the spur or breaker, prove dead, then disconnect the float leads at the pump's terminal block or control box. Testing with the pump still connected adds the motor winding in parallel.
- Low-voltage controls (aquarium top-off units, condensate alarm contacts): unplug the controller and pull the float's connector.
2Lift and lower the float while reading continuity
Set the meter to continuity and clip or hold the probes on the two switch leads. Start with the float hanging at its lowest point, then raise it slowly by hand to the top of its travel, and lower it again. Watch where the reading changes. Repeat three or four times: the trip point should be the same each time. Then switch to the ohms range and note the closed reading.
| Switch action | Float down | Float up |
|---|---|---|
| Normally open (emptying, pump-up) | OL | Beep, near 0 Ω |
| Normally closed (filling, top-off) | Beep, near 0 Ω | OL |
| Three-wire, COM to NO | OL | Beep |
| Three-wire, COM to NC | Beep | OL |
On a tethered switch, the float has to tip past horizontal before it flips, and it switches back at a lower angle, so there is a gap between the on and off levels. That gap is by design: it stops the pump from short-cycling. OL is just the meter showing an open contact; see what OL means if the display is unfamiliar.
01
Isolate
Unplug or lock off
02
Disconnect
Free the switch leads
03
Float down
Note beep or OL
04
Float up
State should swap
05
Repeat
Same trip point
Sump pump, condensate pump and aquarium notes
- Sump pumps: a tethered float can catch on the pit wall or the discharge pipe and fail to rise even though it tests fine. Check the tether length and clearance as well as the switch.
- Condensate pumps: many have two switches, one that runs the pump and a high-level safety switch, often with NC contacts that open to stop the boiler or air conditioner if the reservoir overfills. Test both.
- Aquariums and tanks: top-off floats are usually reed types on a low-voltage controller. Salt creep and algae can stop the float sliding, so test it out of the water after cleaning.
If a plug-in pump has stopped completely, check its plug fuse before condemning the switch; our guide to testing a 13 amp plug fuse covers UK plugs.
Reading the float switch results
| What you find | Likely cause |
|---|---|
| Clean swap at the same point, under an ohm closed | Switch is good; look at the pump, wiring or controller |
| Never changes from OL | Open contact, broken reed, or a cable broken inside its sheath |
| Closed at both ends of travel | Welded contacts or a reed stuck closed |
| Trip point changes each time | Float rubbing, debris on the stem, or a worn mechanism |
| Reading flickers when the cable is flexed | Damaged lead, often where it enters the float |
| Float sits low and feels heavy | Waterlogged float; replace it |
Cleaning a float switch before you replace it
Many float switch failures are mechanical, not electrical. Silt, grease, limescale, biofilm and algae stop a float sliding or tipping freely. With power isolated, wash the float and stem in warm soapy water with a soft brush, and use a mild descaler for limescale if the maker allows it. Avoid harsh solvents that can attack the plastic or the cable seal, and never open a sealed float.
Retest after cleaning
Meters for pump and float switch jobs
The float test itself is a continuity check, but pump circuits run on mains, so the same meter often has to prove the outlet or the pump cord is dead before you touch it. That makes a CAT III rated meter with a voltage range you trust the better choice for sump and condensate pumps; a budget meter is fine for low-voltage aquarium and tank-level switches. Basement pump outlets are often GFCI protected; if yours keeps cutting the pump off, see why a GFCI outlet keeps tripping.
Meters for float switch and pump checks
Hands-on tested. CAT III 600V with built-in NCV to confirm a pump circuit is dead before you start.
CAT IV 600V rating, bar graph and Min/Max recording to watch a switch change state as the float rises.
CAT II 600V budget meter with NCV, suited to aquarium and low-voltage tank switches rather than panels.
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Frequently asked questions
How do I know if my sump pump float switch is bad?
Signs include a pump that never starts as water rises, one that runs continuously, or one that cycles erratically. Unplug the pump, isolate the float switch and check continuity while raising and lowering the float. If the reading never changes, changes at random points, or the float is stuck or waterlogged, replace the switch.
Can I test a float switch without removing it from the tank?
Often you can. With power isolated, disconnect its leads at the plug, terminal block or controller, then lift the float by hand or with a tool while someone watches the meter. For a submerged reed type on a fixed stem, raising the float by hand is the test. Remove it only if you need to clean it or it cannot move freely.
Why does my float switch read closed in both positions?
Either its contacts have welded, often after switching a motor's start current for years, or you are still measuring through the pump or other wiring. Disconnect the switch fully and recheck. A reed float switch can also stick closed after a current surge. If it still reads closed at both ends of travel once isolated, replace it.
Can I turn a normally open float switch into a normally closed one?
Many vertical reed float switches can be converted by removing the retaining clip, turning the float upside down on its stem and refitting it, which moves the magnet relative to the reed. The instructions usually say whether this is supported. Tethered and mechanical types generally cannot be reversed, so buy the correct action for filling or emptying.
What resistance should a float switch have when closed?
Close to zero, typically well under an ohm for a reed or mechanical switch measured at its leads, plus the resistance of a long cable. A closed reading of several ohms or more, or one that drifts as you move the float, suggests worn or contaminated contacts, or a damaged cable where it enters the float.
