Air Compressor Governor: What It Controls and How It Works

Ask a CDL student what the air compressor governor controls and the answer is simple: it controls when the compressor pumps air into the storage tanks, keeping the pressure inside a safe range. That one sentence covers the exam question, but it hides some useful detail about how the governor actually does its job, and why the same idea shows up on a shop air compressor under a different name. This guide explains both, so whether you are studying for an air brake endorsement or maintaining a workshop compressor, you know exactly what is happening.

The short answer

The air compressor governor controls the cut-in and cut-out pressure of the system. It watches the pressure in the tank and switches the compressor between two states:

• Loading (cut-in): when tank pressure drops to the low limit, the governor tells the compressor to build air again.
• Unloading (cut-out): when tank pressure reaches the high limit, the governor tells the compressor to stop pumping useful air.

In a truck air brake system the compressor is gear-driven off the engine and never actually stops spinning, so “cut-out” does not mean it switches off. Instead the governor sends a signal to an unloader that lifts the inlet valves, letting the compressor pump air back and forth across its own head without adding pressure to the tank.

Cut-in and cut-out pressure on a truck

On a typical truck air brake system the numbers look like this:

• Cut-out (unload): usually around 120 to 130 psi.
• Cut-in (load): usually around 100 to 110 psi, and always at least 20 psi below the cut-out point.

As you use air for braking, the pressure falls. When it hits the cut-in level, the governor loads the compressor and it rebuilds pressure. When it reaches cut-out, the governor unloads it. This cycle repeats constantly and keeps the tanks in a safe working band. A common truck part for this is the Bendix D-2 governor, which uses a spring-loaded piston to open and close the unloader signal.

Checking those settings is a standard part of the pre-trip air brake test: you build pressure, watch for the governor to cut out at the top, then fan the brakes down and confirm it cuts back in around the low limit. The California DMV air brakes handbook walks through that check.

The same job on a shop compressor

A workshop or garage air compressor does the identical job with different hardware. Here the governor’s role is filled by the pressure switch, and on many units an unloader valve as well.

• The pressure switch senses tank pressure and physically cuts power to the motor at the cut-out setting, then restarts it at the cut-in setting. On a shop compressor, cut-out really does stop the motor.
• The unloader valve vents the pressure trapped over the piston when the motor stops, so it can restart against no load.

So on a truck the governor unloads a spinning compressor, while on a shop machine the pressure switch switches the motor off entirely. The principle, holding tank pressure between a high and a low limit, is exactly the same. If you run a shop compressor, our air compressor pressure switch guide and the walkthrough on how to adjust a pressure switch cover the practical side.

Why the governor matters for safety

The governor is a safety device as much as a convenience:

• If it fails to cut out, the compressor keeps building pressure. On a truck the safety relief valve eventually blows to protect the system; on a shop compressor an overpressured tank is dangerous. Either way, something is wrong.
• If it fails to cut in, pressure keeps dropping. On a truck that means the low-pressure warning, then a loss of brake air, a critical failure. On a shop compressor your tools simply run out of air.

A governor that cycles at the wrong pressures, cuts in and out too close together, or lets the safety valve pop is telling you it needs adjustment or replacement. Do not ignore it.

Adjusting a governor

Truck air governors like the Bendix D-2 can be adjusted, but the cut-out and cut-in settings are safety-critical and should be set to the manufacturer’s specification, usually by a qualified technician. On a shop compressor, the pressure switch has adjustable cut-in and cut-out screws, but changing the cut-out above the tank’s rated pressure is never safe. When in doubt, leave the top limit where the manufacturer set it.

In one line

The air compressor governor controls when the compressor loads and unloads, keeping tank pressure between a cut-in and a cut-out point. On a truck it signals an unloader; on a shop compressor a pressure switch does the same job by cutting the motor. Understanding it means you can spot a fault before it leaves you without air.

Frequently asked questions

What does the air compressor governor control? The air compressor governor controls when the compressor pumps air into the storage tanks. It monitors tank pressure and loads the compressor when pressure drops to the cut-in point, then unloads it when pressure reaches the cut-out point, keeping the system within a safe pressure range.

What are typical cut-in and cut-out pressures? On a truck air brake system, cut-out is usually around 120 to 130 psi and cut-in around 100 to 110 psi, with the cut-in always at least 20 psi below the cut-out. Shop compressors vary by model, but the same two-limit principle applies, set by the pressure switch.

Does the governor stop the compressor? Not on a truck. The compressor is gear-driven off the engine and keeps spinning, so at cut-out the governor signals an unloader to lift the inlet valves, and the compressor pumps air across its own head without building tank pressure. On a shop compressor, the pressure switch does stop the electric motor.

What is the difference between a governor and a pressure switch? They do the same job in different machines. A truck uses a governor to load and unload a continuously spinning, engine-driven compressor. A shop compressor uses a pressure switch to turn the electric motor on and off. Both hold tank pressure between a cut-in and cut-out setting.

How do I test the air compressor governor on a truck? Build pressure and watch for the governor to cut out at the top of its range, then fan the brakes to lower pressure and confirm it cuts back in near the low limit. This governor check is a standard part of the pre-trip air brake inspection.

What happens if the governor fails? If it fails to cut out, pressure keeps rising until the safety relief valve blows or, on a shop tank, becomes dangerous. If it fails to cut in, pressure keeps falling until you lose air, which on a truck means the low-pressure warning and eventual brake failure. A faulty governor needs adjustment or replacement promptly.