Why Your Compressor Keeps Tripping The Oil Differential Pressure Switch (And What Most Techs Miss)

If you’re reading this, you’ve probably been there: a critical compressor on a refrigeration rack trips on oil differential pressure. You swap out the Danfoss pressure switch—maybe an MP 55 or a KP 35—and reset the system. Three weeks later, it trips again.

The immediate reaction is to blame the switch itself. But in my four years reviewing pressure switch specs and handling field rejections for a major industrial controls supplier, I’ve found the switch is rarely the primary problem. It’s the messenger telling you something else is broken.

Here’s what most technicians miss when they focus on the switch rather than the system behind it.

The Surface Problem: “The Switch Keeps Failing”

When I receive a returned Danfoss pressure switch from a frustrated customer, the complaint is usually identical: “This switch is defective. It tripped prematurely.”

We bench-test every single returned unit. In our Q1 2024 quality audit, we tested 187 returned oil differential pressure switches. Seventy-two percent of them functioned perfectly within their rated specifications. They did exactly what they were designed to do: they protected the compressor.

So why did they trip in the field? Not because the switch was bad, but because the conditions it was sensing were bad.

The Deeper Issue: What That Trip Actually Means

An oil differential pressure switch (like a Danfoss MP 55 or a KP 15 configured for oil) measures the difference between oil pump pressure and crankcase pressure. It's designed to protect the compressor from running without adequate lubrication.

When it trips, it's saying: “The pressure difference is too low. I am shutting down the compressor to prevent catastrophic failure.”

The switch is the safety guard. But most people look at the guard and say, “This guard is broken.” They should be asking, “Why was the guard triggered?”

1. Oil Pump Wear (The Obvious One, But Often Ignored)

This is the classic cause. A worn oil pump can't maintain adequate pressure at the differential port. But here’s the problem: a pump can lose 20% of its efficiency and still “look fine” during a visual inspection. You don’t know it's worn until you put a gauge on it and see the numbers under load. I’ve rejected dozens of field returns where the technician swore the pump was fine—until we tested it and found pressure drops of 15-25 PSI below spec under normal operating conditions.

That pressure drop is exactly what a Danfoss RT 260 or similar differential switch will catch every time.

2. Incorrect Switch Settings (You’d Be Surprised)

I have a mixed feeling about this one. On one hand, Danfoss documentation (which is honestly some of the best in the industry) clearly explains how to set differential and cut-in pressures. On the other hand, I’ve walked into facilities where the switch was set based on the previous technician’s “gut feel” rather than the compressor manufacturer’s spec.

I remember one case: a customer replaced a KP 35 three times in six months. I asked them to read me the differential setting. It was 45 PSI. The compressor spec demanded 60 PSI minimum differential. The switch was doing exactly what it was told—it just wasn’t told the right numbers. A simple adjustment (and a five-minute read of the wiring diagram) solved the problem permanently.

Here’s something vendors won’t tell you: a pressure switch isn't a “set it and forget it” component. Every time a compressor undergoes major service—especially a rebuild or replacement—the oil differential settings should be verified against the new compressor’s specifications. Don’t just copy the old switch’s settings.

3. Crankcase Pressure Fluctuations (The Hidden Culprit)

This is the one most technicians don’t think about. The oil differential pressure switch measures difference. That means a problem on the crankcase side of the equation can trigger a trip just as easily as a problem on the oil pump side.

What can cause crankcase pressure to spike or fluctuate?

• Flooded starts (refrigerant migrating to the crankcase overnight)
• Worn rings (blow-by pressurizing the crankcase)
• A clogged or malfunctioning crankcase breather
• An undersized or blocked oil return line

I had a situation where a system kept tripping the differential switch on startup every morning. The switch was replaced twice. It turned out the compressor was sitting in a cold warehouse with a refrigerant migration issue. The crankcase heater was undersized for the ambient temperature. The fix wasn’t a new switch—it was a larger heater element and a time delay relay to ensure the compressor wasn’t starting with liquid refrigerant in the oil.

The Real Cost of Ignoring the Root Cause

Let’s talk about what happens when you keep swapping switches without finding the root cause.

The obvious cost is the switch itself. A standard Danfoss KP series switch might be $80-150. An RT series or MBC 5100 might be $200-400. Over multiple trips, that adds up. But that’s the small cost.

The bigger cost is the compressor damage. Every time the differential switch trips, it means the compressor ran with inadequate lubrication for at least a moment. Even if the shutdown happened “in time,” the bearing surfaces have taken wear. Do that three, four, five times, and you’re significantly shortening the compressor’s lifespan.

I’ve seen a $22,000 compressor lost because nobody stopped to ask why the oil differential switch kept tripping. The switch was protecting the compressor, but the diagnostic approach never went deeper than “replace the switch.” The compressor eventually seized. The client paid for the replacement and the lost production time.

The Fix (It’s Simple, But Harder to Sell)

The solution isn’t a better pressure switch. Danfoss switches (which, again, I review hundreds of annually) are reliable when applied correctly. The solution is a proper diagnostic procedure.

1. Don’t touch the switch first. Put manifold gauges on the oil pump port and the crankcase port. Record the differential pressure at idle, at full load, and at startup.
2. Verify the switch settings against the compressor manufacturer’s spec. Don’t guess. Look it up.
3. Check the oil condition and level. Foamy oil or oil contaminated with refrigerant won’t generate proper pressure.
4. Inspect the crankcase heater and plumbing. Is it working? Is the line clear?
5. Only then, if everything checks out, consider whether the switch itself has failed. (And at that point, it might be a relief to find it is the switch—because the real problems are usually more expensive.)

An informed customer (or technician) asks better questions and makes faster decisions. If you take one thing from this, let it be this: the pressure switch is doing its job. Treat it as a source of diagnostic information, not as a consumable part to be replaced reflexively. Your compressor—and your service budget—will thank you.