Pressure Switch Faults: Not Switching Off, Chattering, Not Starting the Pump

Is the pressure switch broken, or just mis-set?

Short answer. In most of the enquiries we get, a "dead" pressure switch has not failed at all. It is one of three things: a set point put outside what the datasheet allows for that particular range, a switching differential set below the physical floor for that sensing element, or a contactor coil hung straight on the contact, which burns it under the inductive category instead of the resistive one. Below is symptom, datasheet cause, and the check you can run on site. Every number comes from the datasheets of the switches we actually sell: Wika PSM-520 (PV 35.01), Wika PSM02 (PV 34.82) and Dwyer Series DX (bulletin IN-DX). Page references for every claim are in the sources paragraph at the end.

If the switch is not faulty but simply unadjusted, you want a different article — how to adjust a water pressure switch. If the question is supply and contactor, that is pressure switch wiring. Here we assume the switch is already installed, already wired, and behaving wrongly.

Eight symptoms and what sits behind each one

The pump never switches off and the pressure keeps climbing

The first thing to check is not the contact, it is the range on the label. The PSM-520 datasheet prints two different numbers, not one: the setting range, and the permissible switch point on rising pressure. For the 6…15 bar version that switch point sits between 7.5 and 15 bar. Which means something very simple: a pump that makes 6 bar will never switch that unit off, and turning the screw will not help. For the -0.4…+7 bar version the same column gives 0.2…7 bar, and for 6…30 bar it gives 9…30 bar.

The second cause is a blocked pressure port or deposits on the sensing element. In the PSM-520 the pressure is taken by a copper-alloy CuSn6 bellows; in the PSM02 it is a diaphragm (up to 16 bar) or a piston (from 10 bar). If the pressure does not reach the element, the switch sees zero and keeps the pump running.

Only as a third hypothesis do we assume a welded contact. That shows up on a continuity check of COM–NC and COM–NO with the power off. PSM-520 terminals are marked 1 (NC), 2 (COM), 3 (NO).

The pump will not start even though there is no pressure

This one is the reset point. The PSM-520 datasheet carries a footnote almost nobody reads: both the switch point and the reset point have to be within the setting range. The rule is universal: switch point minus differential must stay inside the range. On the 6…15 bar version the lowest permissible switch point of 7.5 bar together with the widest differential of 5 bar gives a reset of 2.5 bar, below 6. On the -0.4…+7 bar version it is even easier to violate: a 0.2 bar switch point with a 6 bar differential gives -5.8 bar. It is safe only where the difference stays inside the nameplate range: a 7 bar switch point with a 6 bar differential gives a 1 bar reset.

The other variant is NC and NO swapped during a replacement. The DX bulletin states the logic in a table: above the set point NO is closed and NC is open, below it the other way round. A pump switch needs the contact that is closed at low pressure.

The switch chatters and the pump short-cycles

Short cycling is nearly always too small a differential. But the differential has a floor, and it is not estimated by eye. PV 34.82 gives the formula for the PSM02: diaphragm on gauge pressure is 0.1 bar + 5…20 % of the switch point, and the datasheet prints its own worked example — at a 3 bar switch point the hysteresis comes out at 0.25…0.7 bar. For the piston version the formula is different and much coarser: 5 bar + 5…15 %, so at a 100 bar switch point the hysteresis is 10…20 bar. A narrow window on a high-pressure piston switch cannot be set at all.

On the Dwyer DX the deadband is fixed. For the 2.5…10 psid version it is 1.5 psi (0.10 bar) at the low set point and 2.5 psi (0.17 bar) at the high one; for 10…25 psid it is 2.5 psi (0.17 bar) and 3.5 psi (0.24 bar). There is nothing to turn: if the window is too narrow for your process, you need a different switch or different logic, not an adjustment.

And a word on frequency. The PSM02 is rated for a maximum of 1 Hz, and its service life is printed as more than 2 × 10⁶ load cycles per ISO 10771-1 — a lower bound, not an exact figure. Work from that bound: two million cycles at a continuous 1 Hz is roughly 23 days. So a system that holds the switch at its rated frequency limit uses up the guaranteed datasheet minimum in about three weeks. Two caveats at once: the real service life is often considerably longer, and ISO 10771-1 itself is a pressure-pulsation fatigue test, so it speaks about the body and the sensing element rather than contact wear. The conclusion does not change: if the switch clicks once a second, the switch is not the cause.

Burnt or welded contacts

Utilization category decides this, and almost nobody looks at it. The PSM-520 datasheet gives two different currents per DIN EN 60947-1: resistive load AC-1 is 10 A at AC 230 V, inductive load AC-15 is only 6 A. A contactor coil is AC-15, not AC-1. The category designations themselves come from the IEC/DIN EN 60947 series rather than from the switch datasheet: AC-15 is the control of electromagnetic loads above 72 VA.

The PSM02 numbers are smaller still: AC-12/DC-12 is 4 A, AC-14/DC-14 is 2 A. There is no AC-15 row in the PSM02 table at all: if the coil is heavier than those same 72 VA, it goes on an interposing relay rather than on the pressure switch. Then the part hidden in the columns: AC 250 V applies only to the angular connector, on the M12 × 1 connector and the cable outlet the AC ceiling is AC 48 V, and DC 24 V is the ceiling for all connection variants, not just for M12. A PSM02 with an M12 connector carrying 230 V is outside its datasheet, and it will not last.

The opposite case exists too. The PSM02 minimum switching power is 25 mW on its solid silver contacts. An ordinary PLC digital input clears that comfortably: 24 V at a few milliamps is already over a hundred milliwatts. A high-impedance input drawing tens of microamps does not reach 25 mW, and the contact's self-cleaning action does not work there. If the switch feeds a PLC digital input directly, see how to wire sensors to a PLC — the same article covers where the supply comes from (ours are 24 V power supplies).

The set point drifts: 4 bar yesterday, 4.6 bar today

Part of that scatter is allowed. PSM-520: non-repeatability of the switch point ≤ 2 % of span. For the -0.4…+7 bar version the span is 7.4 bar, so a calculated ±0.15 bar is normal, not a defect. The PSM02 prints the scatter row by row: ±0.04 bar for 0.2…2 bar, ±0.16 for 0.5…8 bar, ±0.32 for 1…16 bar — but the footnote under that table limits those figures to versions with an adjustable set point; a fixed factory setting is specified differently, at ≤ 4 % of the set switch point.

The second cause is a set point in the wrong part of the range. PV 34.82 states it plainly: the switch point should lie within 30…70 % of the setting range end value. For a 1…16 bar unit that calculates to 4.8…11.2 bar. A 1.2 bar set point on a 16 bar switch is formally possible, but repeatability there is gone.

The third is temperature. The PSM-520 is rated for a medium at -20…+100 °C and an ambient of -40…+70 °C; the PSM02 depends on its seal (NBR -20…+80 °C, FKM 0…+100 °C, EPDM -40…+100 °C). The Dwyer DX only lives between -1 and +60 °C, so an unprotected outdoor install in winter is already outside the datasheet.

Water inside the housing, rust on the terminals, oxidation

The usual cause is an ingress protection rating that was never there. The PSM-520 is IP30, and the datasheet adds a condition: even that IP30 only applies if all the mounting holes on the rear are covered, or the instrument is panel-mounted on a flat surface. The cable enters through a rubber grommet for Ø 6…14 mm cable. Outdoors or in a washdown area, that instrument belongs in an enclosure.

The PSM02 is better, but also conditional: IP65 with the angular DIN 175301-803 A connector, IP67 with M12 × 1 and with the cable outlet, and only when the mating connector carries the same rating. The Dwyer DX has a weatherproof UL type 4X (IP65) housing, and its bulletin separately requires the conduit connection to be arranged so that condensation cannot enter the housing, with a breather drain in a separate conduit body if needed.

The medium has attacked the sensing element

This is the case where there is nothing to repair. The datasheets name their media very narrowly. PSM-520: non-corrosive media — oil, water and air; wetted parts are the CuSn6 bellows and a tin-plated free-cutting steel process connection. PSM02: compressed air, neutral and self-lubricating fluids, neutral gases. Dwyer DX: compatible gases and liquids, wetted parts brass and fluoroelastomer, plus its own CAUTION — make sure the process fluid is compatible with the wetted materials.

Inhibited glycol, ammonia, condensate with chemistry in it, aggressive cleaning solutions: that is a different class of instrument. Steam belongs in the same group — a copper bellows and brass do not survive it. We cover that separately in our article on industrial pressure sensors. For steam and chemistry specifically, see pressure sensors for steam and aggressive media: syphons to DIN 16282, cooling elements and the temperature limit at the process connection.

The differential switch does not see a clogged filter

Two classic mistakes, both from the bulletin. First, swapped ports: on the DX the high-pressure port is marked "+" and sits on the left with the label side facing you, the low-pressure port is "−" on the right. Reversed, the switch sees the differential with the opposite sign and never trips.

Second, continuous single-side pressure. There are two different numbers in the bulletin here and they get mixed up. The static limit of the instrument itself is 200 psig (13.8 bar), printed in the bulletin as "Pressure Limits". Separately, continuous single-side pressure is capped at 1.25 × the full differential range. For the 10…25 psid version that works out to 31.25 psi if "full range" is read as the upper range value of 25 psi — the most likely reading, but also the most liberal one. The bulletin never defines the term, and if it is read as the span (25 − 10 = 15 psid) the limit comes out lower, at 18.75 psi. That is the figure to use when you want margin. If full line pressure sits upstream of the filter while downstream is vented, that limit is easy to exceed and the mechanism suffers. One more: DX mounting orientation is strict — ports on the horizontal plane, ±10°. For the task class itself, see our article on the differential pressure sensor.

The check: a gauge, a multimeter, twenty minutes

Safety first, and this is not boilerplate: the DX bulletin requires you to make sure the system is not pressurised before installing or removing the device, and that power is off while wiring connections are made. The same applies to any switch with a coil on its contact.

Then, in order. 1) Isolate the supply and vent the pressure. 2) Check continuity COM–NC and COM–NO, and confirm the contact changes over at all. 3) Apply pressure from a separate source with a gauge and watch the switch point — that is exactly the procedure the DX bulletin describes: a pressure source plus a gauge, while monitoring the contact output. 4) Compare both the switch point and the reset point with the datasheet table for your range. 5) Measure the actual load current and compare it against the inductive category row, not against the biggest number on the label. 6) Look at the cable entry and the housing: water, condensation, corroded terminals.

One detail about the adjustment screw. The DX bulletin warns outright that the screw can be turned far enough to fall out, and forbids turning it to the point where no female threads are visible in the body. On a switch with two screws — the PSM02 has one for the set point and one for the hysteresis — set the switch point first and the hysteresis after.

Three datasheet limits that actually break switches

When the switch is not the culprit

Some symptoms are only displayed by the switch. Short cycling on a booster set often comes from the accumulator rather than the differential — we covered air-cushion pressure in the article on adjusting a water pressure switch, and without your pump's own datasheet we will not quote numbers here. Pressure dips on starting are a question for the drive, not for the switch: we discuss variable frequency drives and soft starters for pumps separately, along with choosing a pressure sensor for a booster set.

And separately, what a switch cannot do by definition. The DX bulletin carries a requirement worth quoting in full: design a fail-safe system that takes into consideration the possibility of switch failure, power failure and operator error. In other words, if your process cannot tolerate a stuck contact, the answer is not a better switch but a second, independent layer of protection. One pressure switch is not simultaneously dry-run protection, protection against its own contact failing, and protection against loss of supply.

What we carry in pressure switches

Industrial mechanical switches from Wika, in two families. PSM-520, with the set point adjustable under a protection cap: the -0.4…+7 bar version and the 6…15 bar version. PSM02, compact, with a separate hysteresis screw: 0.5…8 bar. Dwyer Series DX differential pressure switches for filters and ventilation: 2.5…10 psid and 10…25 psid. Those three Wika items — PSM-520 at -0.4…+7 and at 6…15 bar, and the PSM02 at 0.5…8 bar — are the ones whose product pages carry a price, so look there. Everything we bring in under that brand is collected on the Wika page. If none of the ranges covers your set point, send us the switch point and the hysteresis you need: both families have more versions than we list, and we will source the right one, or exchange it if the one already bought does not fit.

We do not carry domestic booster-set switches of the Italtecnica type and we do not select them. But if a booster set is exactly what brought you here, we have a better answer than a third new switch: replace the on-off logic with a 4…20 mA pressure sensor plus a variable frequency drive with built-in PID. The pump then holds the set point on speed, starting becomes gentle, and the switch — with its hysteresis and its burnt contacts — leaves the circuit together with the problem. For picking the sensor for that, see choosing a pressure sensor for a booster set. If you need a 4…20 mA signal for a controller or a drive rather than a contact, that is a pressure transmitter — those live in the pressure sensors section; by brand, Wika and Dwyer Instruments, and the full catalogue is all industrial sensors. The current status of a specific part number is checked by a manager on request.

Frequently asked questions

Can I use a 6…15 bar switch if the pump makes 6 bar?

No. In PV 35.01 the permissible switch point on rising pressure for that version starts at 7.5 bar, so it will simply never switch off. You need a different version, and the two table rows should not be mixed up: on the 0…7 bar version the switch point starts at 0.6 bar, while 0.2 bar applies only to the -0.4…+7 bar version, the one with the vacuum part of the range.

I reduced the differential to its minimum and it still chatters. What is wrong?

The differential has a construction floor: 0.1 bar plus 5…20 % of the set point for a diaphragm PSM02, 5 bar plus 5…15 % for the piston version. If the window is already at minimum, the cause is outside the switch: either the damping volume is too small, or the switching frequency has gone past the rated 1 Hz.

Will a pressure switch carry a 230 V contactor coil?

Read the inductive category row, not the biggest number. PSM-520: AC-15 is 6 A at AC 230 V. PSM02: AC-14 is 2 A, and AC 250 V is only declared for the angular connector; on M12 and the cable outlet the AC ceiling is AC 48 V, while DC 24 V is the ceiling for every connection variant. There is no AC-15 row for the PSM02 at all: a coil above 72 VA goes through an interposing relay. Contactor circuits are covered in pressure switch wiring.

The switch is mounted outdoors. What happens to it?

The PSM-520 is IP30 with a rubber cable grommet, so outdoors it belongs in an enclosure. The Dwyer DX has a UL type 4X (IP65) housing, but its bulletin requires the entry to be arranged so condensation cannot get in; check the temperature separately, since the DX range is -1…+60 °C.

The differential switch will not trip although the filter is clearly clogged

Check the ports. On the DX, "+" is the left port with the label side facing you and "−" is the right one; swapped tubing gives the differential the opposite sign. Then check orientation: the ports must be on the horizontal plane within ±10°.

What to do next

Send us five things and we will tell you what is wrong with the switch: the part number from the label, the symptom in words ("will not switch off", "clicks every five seconds"), the set points as they are now, exactly what is connected to the contact (voltage, current, load type), and the medium with its temperature. If it turns out the switch is healthy and the problem is the range or the load category, we will say so rather than sell you a new one. Write to us via the contacts page.

Related reading: adjusting a water pressure switch, pressure switch wiring, differential pressure sensor and gauge, industrial pressure sensors, pressure sensor for a booster set, pressure sensors for steam and aggressive media.

Sources. WIKA data sheet PV 35.01 ∙ 03/2026, "Pressure switch, model PSM-520" — p. 1 (media, description), p. 2 (setting ranges, permissible switch points, differentials, non-repeatability, AC-1/AC-15 categories, temperatures, IP30, cable entry, terminal assignment), p. 3 (wetted materials, approvals). WIKA data sheet PV 34.82 ∙ 02/2026, "Compact pressure switch, model PSM02" — p. 1 (media, two adjustment screws), p. 2 (setting ranges and per-row non-repeatability, the 30…70 % rule, 1 Hz switching frequency, service life > 2 × 10⁶ cycles per ISO 10771-1, solid silver contacts), p. 3 (AC-12/AC-14 categories, voltage ceilings per connector, 25 mW minimum switching power, hysteresis formulas with the datasheet's own examples, temperatures per sealing material, vibration and shock), p. 4 (wetted materials per thread, IP65/IP67). Dwyer Instruments, Bulletin IN-DX, FR# 443822-00 Rev.1 (3/17), "Series DX Differential Pressure Switch" — p. 1 (specifications, fixed deadbands, pressure and temperature limits, the 1.25 × rule, vibration), p. 2 (installation, "+"/"−" ports, switch action table, set point adjustment, cable entry and condensation, the fail-safe requirement).