The control air line to the engine speed actuator is disconnected, cracked, or blocked. The actuator receives no pneumatic signal and cannot increase engine speed. The engine stays at idle regardless of compressor demand.

What you'll see

Very similar to speed actuator failure -- the engine does not rev up under load. But the root cause is the control air supply, not the actuator itself. This is often the result of a tube coming loose from its push-in fitting during transport or from vibration. The inlet valve may or may not open (depends on whether its control air line is also affected -- they often share the same source but split to separate actuators).
Before you assume this is the problem

If both the inlet valve and engine speed are affected, the problem is likely at the regulating valve or a common control air line upstream of the split. See: No Control Air to Valve Actuator, Regulating Valve Failure.

See all causes of engine won't rev up under load →

How to diagnose

  1. Trace the control air line to the speed actuator

    Follow the small-diameter tube from the pneumatic junction (where control air splits between inlet valve and speed actuator) to the speed actuator. Look for disconnected fittings, cracked tubes, or kinks. The tubes are typically nylon or polyurethane and push into compression fittings. Vibration can work them loose over time.
    Result: Disconnected or damaged tube found = reconnect or replace.

  2. Check for air at the actuator

    Disconnect the tube at the actuator end. With the compressor loaded, does air come out of the tube? If yes, the supply is fine and the actuator is faulty. If no, the blockage is in the line.
    Result: Air present = actuator issue. No air = line blockage or disconnection upstream.

How to fix it

  1. Reconnect or replace the control air line

    Push the tube firmly back into the fitting until it clicks. If the tube end is damaged or enlarged (preventing a good seal), cut off 10mm and re-insert the fresh end. If the tube is cracked or kinked, replace the entire length. Secure with cable ties to prevent vibration damage.

Common mistakes

Do not use a tube diameter different from the original. The control system response depends on the correct air volume. Also: push-in fittings require the tube to be cut square and clean. A ragged cut will not seal properly.

Parts & tools

Replacement nylon or polyurethane tubing (match diameter). Push-in fittings if needed. Cable ties. Tube cutter.

Review safety precautions before starting →

This issue can also cause