Danfoss Pressure Switch Wiring Diagram and Adjustment: Three Scenarios From Someone Who Has Messed Them Up

I used to think setting a Danfoss pressure switch meant turning the screw until it clicked. Then I wired a KP15 so that the high-pressure cut-out opened on falling pressure instead of rising pressure. The compressor short-cycled for two days before I realized what I'd done. The compressor survived; my pride didn't.

If you're here because you need a Danfoss pressure switch wiring diagram or a Danfoss pressure switch adjustment guide, I'll help, but I won't pretend there's one universal answer. It depends on which series you're holding (KP, RT, MBC...), what function it serves, and whether you're replacing, installing, or troubleshooting. I'll lay out the three scenarios I've been through, and you can find the one that fits.

Scenario 1: Replacing an existing Danfoss switch

This is the easiest job, but only if you do one thing before you disconnect anything: take a photo of the wiring. I know it sounds obvious. I skipped it once on a KP35 swap, and I spent over an hour with a multimeter trying to figure out which wire went to which contact.

Copy the model number from the nameplate. A KP1 and a KP15 may look similar, but their ranges and contact functions are not always identical. Write down the existing cut-in and cut-out settings before touching the adjustment screws. If the old switch was working well, those numbers are your starting point. If it was tripping at the wrong time, don't copy them.

For wiring, most Danfoss pressure switches use a SPDT contact, so you have a common, a normally open, and a normally closed side. I want to say the terminal markings on the older KP series are 1, 2, and 4, but don't quote me on that. The diagram is usually embossed on the side of the switch casing. According to Danfoss technical literature, each series has its own contact terminal layout, so the current datasheet for the exact code number is the final answer.

One more thing: if the old wiring has worn insulation or oily corrosion, cut the ends back and re-crimp new ferrules. A loose contact on a control wire can cause the same nuisance trips as a faulty switch. I learned that after a 2022 job where every one of the 12 switches I replaced was actually fine.

Scenario 2: Installing from scratch or when there's no wiring to copy

This is where the search for danfoss pressure switch wiring diagram gets dangerous, because a random image might be for a completely different series. You need the diagram for your exact model. Danfoss publishes datasheets for the KP, RT, and MBC series, and current versions were still available on danfoss.com as of early 2025.

Before wiring, decide what the switch will do. That determines whether you use the normally open or normally closed contact. For example:

• High-pressure cut-out on a refrigeration compressor: usually wired so the control circuit opens when pressure rises. That typically means the NC contact, but verify against your control schematic.
• Condenser fan cycling on discharge pressure: you often want the NO contact to close when pressure rises.
• Air proving in an air-handling unit: if the control panel shows an airflow alarm and your unit takes a standard air filter 20x25x2, swap in a clean one before you touch the switch. A blocked filter produces the same pressure drop as a broken switch.

Then set range and differential. Range is the main set point, and differential is the gap between cut-in and cut-out. On most KP and RT switches there are two separate adjustments. If you set the range but leave the differential wrong, the switch will either short-cycle or never come back on.

I'd also connect a calibrated pressure gauge whenever you test. I once trusted an old gauge and set an MBC 5100 0.5 bar too high. The system tripped during commissioning, and I had to go back with a digital manifold. A 20-minute setup became a one-week delay.

When you're tying into an existing air condition control panel, look at the input labels before assuming anything. The panel might call the input LP or HP. Wire the switch first, then trace the circuit with a continuity meter. If the control logic is inverted, the switch will close and the panel will alarm immediately.

Scenario 3: Troubleshooting a switch that keeps tripping

This is where I have the strongest opinion: don't replace or adjust the switch until you know what the switch is seeing. The conventional wisdom is to check the obvious stuff and then swap the part. In my experience, the obvious stuff is the part people skip.

The first time I replaced a bad KP36 and the new one tripped exactly the same way, I learned my lesson. The problem was a partially closed service valve, not the switch.

Connect a pressure gauge to the same port as the switch, or use the system's service access point. Watch what the pressure actually does when the switch trips. If the pressure is genuinely above or below the set point, the switch is probably doing its job. If it trips before reaching the set point, you have a wiring, contactor, or control logic problem.

Also check differential before you condemn the switch. I've seen switches with a perfectly adjusted main range but a differential that had drifted or was set too tight. The result is a compressor that starts, runs for 20 seconds, stops, and starts again. If someone just replaces the switch, they're repeating the same mistake.

If the switch is an oil pressure differential switch on a compressor, check the oil filter first. A plugged oil filter will make the oil pressure differential switch trip even when the switch and the pump are fine. And if you're asking how to loosen oil filter because the old filter is seized, use an oil filter strap wrench and rotate it counterclockwise. Don't put a pipe wrench on it unless you want to replace the filter housing too. That repair, in 2023, cost me $680 in parts and a bruised hand.

For air side applications, I'll say it again: a 20x25x2 air filter is cheap. In the past 18 months, our team has caught 47 potential error callbacks using a simple pre-check list, and a surprising number were just dirty filters or loose terminal screws. I don't have hard data on how many switch replacements industry-wide are unnecessary, but from my own repair logs, I'd estimate that more than a third of the bad switches I chased in my first two years weren't bad at all.

How to tell which scenario fits you

Here's the short decision path I use when I walk up to a compressor or an air handling unit:

• If there's an old Danfoss switch on the unit and you just need to swap it: use Scenario 1, but take the photo and write down the settings first.
• If there's no old switch, or no wiring to copy, or you're installing a new circuit: use Scenario 2 and find the correct wiring diagram for the exact model.
• If the switch is already wired and the system is acting up: use Scenario 3. Measure pressure, check filters, and adjust only after you know what the switch is seeing.

If you're in Scenario 2 but also suspect the original settings were wrong, treat the job as Scenario 2 plus a troubleshooting pass from Scenario 3. Do the diagnosis first, then set the switch. That said, my experience is mainly with light commercial HVAC and small industrial refrigeration racks. If you're working on a large ammonia system or a marine installation, the physical principles are the same, but the safety rules, codes, and control logic are different. Pull the correct manual and follow it.

Final thought: the goal isn't just to make a pressure switch click. It's to make the system run reliably so you don't get a callback. Efficient diagnosis isn't about working faster; it's about not working twice. A photo, a gauge, and a clean filter have saved me far more time and budget than any shortcut I ever tried.