How to Install a Pressure Switch (2026 Guide) | Boss
Installing a pressure switch on an onboard air system takes under an hour with basic hand tools, and it's the single component that decides when your compressor turns on and off. Get the plumbing and wiring wrong and you'll either overwork the compressor or never build enough pressure to run your gear.
- A pressure switch mounted on the air tank controls compressor cut-in and cut-out automatically.
- Boss pressure switches come in 110-135psi and 150-180psi ranges for different onboard air setups.
- Thread sealant, a tank port, and a relay-protected 12V feed are the only real requirements.
- Skipping the relay is the most common mistake - it burns out switch contacts within months.
- Buy the switch rated to match your compressor's working pressure, not your tank's maximum rating.
Why this matters
A compressor running without a pressure switch either runs constantly, which shortens its life, or needs manual shut-off, which nobody remembers to do on a trip. The switch senses tank pressure and breaks the circuit once the set point is reached, then reconnects it once pressure drops.
For 4WD, ute, van and caravan owners running onboard air for tyre inflation, air lockers or airbag suspension, this is the part that turns a compressor into an actual system. Get the pressure range wrong and you'll either starve an air locker of pressure or overload airbags designed for lower working pressure.
Boss Air Suspension stocks pressure switches in two ranges for this reason - 110-135psi and 150-180psi - because different onboard air setups need different cut-out points. Matching the switch to the job is more important than any other part of the install.
What you'll need
- A pressure switch rated for your compressor and tank (Boss stocks 110-135psi and 150-180psi models)
- An open port on your aluminium air tank, or a T-fitting if all ports are used
- PTFE thread tape or liquid thread sealant
- An automotive relay rated at 30A minimum
- Wire (minimum 4mm² for the compressor feed), crimp terminals and heat shrink
- A multimeter
- An adjustable spanner or socket set for the tank fitting
- 30 to 60 minutes, working alone
The steps
1. Choose the correct pressure switch range
Check your compressor's maximum output and your tank's rated working pressure before buying anything. A 150-180psi switch on a compressor that tops out at 140psi will never cut out, and the compressor runs until it stalls or trips its own thermal cutoff.
Most 12V onboard air setups running tyre inflation or general air tools sit comfortably on a 110-135psi switch. Setups feeding air lockers or higher-pressure accessories often need the 150-180psi range. If you're unsure, the compressor's data plate or manual will list a maximum duty pressure - match the switch below that figure, not above it.
Common mistake: buying based on tank pressure rating rather than compressor output. The tank might handle 200psi; the compressor decides what's achievable.
2. Select a tank port and prepare the fitting
Aluminium air tanks typically ship with 3 or 5 ports. Pick a port that's easy to reach with the tank mounted - you'll want to check and adjust this switch later without pulling the tank out.
Wrap the switch's threaded fitting with 3-4 turns of PTFE tape, working in the direction of the thread so it doesn't unwind on installation. Liquid sealant works as an alternative on NPT fittings but takes longer to cure.
Common mistake: over-tightening the switch into an aluminium port. Aluminium threads strip far more easily than steel - hand-tight plus a quarter turn with a spanner is usually enough.
3. Mount the switch on the tank
Thread the pressure switch into the chosen port and tighten until snug, checking the switch body sits at an angle that keeps its terminals clear of moving parts, brackets or airflow.
If you're running a system built around a Boss Auto Pressure Controller with Fittings and Water Trap, the switch typically threads into a spare port on that manifold rather than directly into the tank - check your kit's supplied diagram before committing to a port.
4. Wire the switch through a relay
Never run the compressor's full current straight through the pressure switch contacts. Pressure switches are rated for a set current draw, and most 12V compressors pull more than that under load - running the compressor directly through the switch burns out the contacts within months.
Wire the pressure switch to trigger a relay instead: switched 12V feed through the pressure switch to the relay coil, and the relay's heavy-gauge output feeding the compressor. This is the same wiring principle covered in the guide on wiring a 12V air compressor for onboard air.
Common mistake: running the compressor directly off the switch to save a step. It works for a few weeks, then the switch contacts pit and fail.
5. Ground the circuit and check polarity
Run a dedicated earth wire from the relay back to a clean chassis ground point, not through an existing accessory earth. Onboard air setups draw enough current that a shared earth causes voltage drop and erratic switch behaviour.
Use the multimeter to confirm 12V is present at the switch terminals before connecting the relay, and check continuity on the earth run. A reading above 0.5 ohms on the earth path usually means a poor connection somewhere along the run.
6. Test cut-in and cut-out pressure
Run the compressor and watch the tank gauge. Note the pressure at which the switch cuts the compressor out, then bleed air down through a tyre valve or accessory until the switch cuts back in.
Compare both figures against the switch's rated range. A 110-135psi switch should cut out somewhere near 135psi and cut back in around 110psi - if it's cutting out well outside that band, the switch may be faulty or wired to the wrong relay terminal.
Common mistake: testing once and walking away. Run this cycle twice to confirm it's repeatable before calling the job done.
7. Secure and weatherproof all connections
Heat shrink every crimp joint and route wiring away from exhaust heat, moving suspension components and sharp chassis edges. Cable tie the loom at 150-200mm intervals so nothing sags onto hot or moving parts.
A switch that's wired correctly but left with exposed crimps will corrode within a season of Australian conditions, especially near the coast.
Troubleshooting
- Compressor won't cut out at all - check the switch is wired to the relay coil, not directly in the compressor's main feed. A switch wired backwards through the relay stays permanently closed.
- Compressor cuts out too early - you may have the wrong pressure range fitted. A 110-135psi switch installed where a 150-180psi was needed will cut out well below your target pressure.
- Switch clicks but nothing happens - test the relay separately with the multimeter. A failed relay is a more common cause than a failed switch.
- Pressure reading is unstable on the gauge - check for a loose fitting or unsealed thread at the switch port. A slow leak shows up as pressure that never settles.
- Switch overheats or discolours - this points to the compressor being wired directly through the switch contacts rather than through a relay. Rewire immediately; the switch is likely damaged and needs replacing.
- No 12V reaching the switch - trace the feed back to the ignition or accessory circuit and confirm the fuse hasn't blown under load.
Tools and resources
- Boss Auto Pressure Controller with Fittings and Water Trap for a complete in-cab control setup
- An aluminium air tank with a spare port if your current tank is fully used
- A guide to choosing the best air compressor pressure switch for onboard air setups if you haven't picked a switch yet
- A multimeter, crimp tool and heat shrink kit from any auto electrical supplier
- Full Boss Air Suspension product range for onboard air and load assist components
What to do next
Once the pressure switch is installed and tested, the next logical step is confirming your whole onboard air system runs at the right working pressure for its job - whether that's tyre inflation, an air locker or load assist airbags. The guide on testing and adjusting airbag suspension pressure covers how to set and check pressure once the compressor side is sorted.
FAQ
How do I install a pressure switch on an onboard air system?
Thread the switch into a spare tank port with PTFE tape, wire it through a relay to the compressor, then test cut-in and cut-out pressure against the switch's rated range. The whole job takes under an hour with basic tools in 2026.
What pressure switch do I need for onboard air?
Match the switch to your compressor's maximum output, not your tank's rating. Boss stocks 110-135psi switches for standard setups and 150-180psi for higher-pressure accessories like air lockers.
Can I wire a pressure switch directly to the compressor without a relay?
No. Pressure switch contacts aren't rated for a compressor's full current draw and will burn out within months. Always run the compressor through a relay triggered by the switch.
Why does my compressor never cut out?
This usually means the switch is wired incorrectly through the relay, or the switch's cut-out pressure is set higher than the compressor can actually produce. Check wiring first, then confirm the switch range matches the compressor.
How much does a pressure switch for onboard air cost in 2026?
Boss pressure switches list at $30.45 in 2026, covering both the 110-135psi and 150-180psi ranges. Fittings, relay and wiring add a modest amount on top depending on what's already on hand.
Is a pressure switch the same as a pressure controller?
No. A pressure switch is a simple on/off component that triggers the compressor at set pressures, while a controller such as the Boss Auto Pressure Controller manages airbag suspension pressure adjustment from inside the cab.
Where should the pressure switch be mounted?
Mount it on a spare port on the air tank or manifold, in a position that stays accessible for future checks and away from heat sources or moving suspension parts.
What happens if the pressure switch range is wrong for my compressor?
A switch rated above the compressor's maximum output never cuts out, running the compressor until it stalls or overheats. A switch rated well below the compressor's capability cuts out too early and limits usable pressure.
One last thing
The relay is the part people skip to save fifteen minutes, and it's the part that decides whether the switch lasts one season or five. A $30.45 pressure switch wired through a relay in 2026 will outlast one wired straight to the compressor by years, not months.
