The pneumatic control system has no air to operate the inlet valve and engine speed actuator. Without control air, the inlet valve defaults to closed and the engine stays at idle or an incorrect speed. The compressor appears dead even though the engine is running.

What you'll see

Engine starts and runs but the compressor produces no pressure. The engine may stay at idle speed instead of ramping up. On most portable diesel compressors, both the inlet valve position AND the engine speed are controlled by the same control air pressure. If there is no control air at all, neither system works -- the inlet valve stays closed and the engine does not speed up.
Before you assume this is the problem

If the engine does ramp up to full speed but no pressure builds, the control air may be working for the engine speed but not the inlet valve -- check the inlet valve actuator specifically. See: Intake Valve Not Opening.

See all causes of not building pressure / won't load →

How to diagnose

  1. Understand the control air source

    Control air is taken from the compressor output (separator vessel) via the regulating valve. When the compressor first starts, there is no control air because there is no output pressure yet. The system is designed to start unloaded and gradually build control air as pressure increases. On many machines, the load button provides a bypass that keeps control air pressure low during warm-up.
    Result: Understanding where control air comes from and how it is distributed.

  2. Check for control air at the regulating valve

    The regulating valve is the central component. It receives compressor output pressure and modulates it to create control air pressure. Check if output pressure (from the separator) is reaching the regulating valve input. If the compressor has never built any pressure, there is no source for control air -- you may need to manually load the machine (load button) or check if the minimum pressure valve is stuck closed.
    Result: Pressure at regulating valve input = control air should be available. No pressure = MPV stuck or other issue.

  3. Trace control air lines to actuators

    Follow the control air tubing from the regulating valve to the inlet valve actuator and the engine speed actuator. Look for disconnected tubes, cracked or kinked lines, or tubes that have vibrated loose from their fittings. A single disconnected tube can disable the entire control system.
    Result: Disconnected or damaged tube found = reconnect or replace.

How to fix it

  1. Reconnect or replace damaged control air lines

    Reattach any disconnected tubing. Replace cracked or hardened tubes. Control air lines are typically small-diameter nylon or polyurethane tubing with push-in fittings. Carry spare tubing on site -- it is cheap and this is a common failure point.

  2. Check the regulating valve

    If the lines are intact but no control air is being produced, the regulating valve may be stuck or maladjusted. Try adjusting the regulating valve (turn the adjustment screw). If no change, the internal diaphragm or valve mechanism may be damaged. Overhaul or replace the regulating valve.

Common mistakes

Do not disconnect control air lines to 'test' them while the compressor is running at full pressure. Loss of control air causes the inlet valve to open fully and the engine to go to maximum speed -- which can over-pressurize the system if the safety valve is also faulty. Always test with the compressor stopped or at low pressure. Also: when replacing tubing, use the correct diameter. The wrong size changes the control response time.

Parts & tools

Replacement control air tubing (correct diameter). Push-in fittings. Pressure gauge for measuring control air. Screwdriver for regulating valve adjustment.

Review safety precautions before starting →

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