The compressor can't generate the internal control pressure needed to operate the inlet valve. Without it, nothing works -- even a perfect solenoid and inlet valve can't help.

What you'll see

The compressor runs, the controller gives the load command, the solenoid clicks -- but nothing happens. The sump pressure gauge shows very low pressure (close to zero or barely above atmospheric), instead of the normal 2.5-3.5 bar (35-50 psi) you'd expect during unload-running. The inlet valve has no pressure to work with.
Before you assume this is the problem

If the sump pressure IS at normal unload-running level (2.5-3.5 bar) but the compressor still won't load, the control air supply is fine -- look at the solenoid, wiring, or inlet valve instead. This cause page is specifically about situations where internal pressure is abnormally low or absent.

Could also be:

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How to diagnose

  1. Check the sump pressure gauge

    With the compressor running in unload, what does the sump pressure gauge show? Normal unload-running pressure is about 2.5 to 3.5 bar (35-50 psi). If it's near zero or much lower than normal, the compressor can't build up internal pressure. That's your control air source -- without it, nothing downstream works.
    Result: Sump pressure near zero = no internal pressure buildup. Normal sump pressure = control air supply is fine, look elsewhere.

  2. Check the bypass hole in the inlet valve

    Every screw compressor has a small bypass hole in the inlet valve. Even when the valve is fully closed, this hole lets a tiny amount of air get sucked in and compressed. That's how the machine builds initial pressure from a dead start. If this hole is plugged with dirt, no pressure builds at all. Find it, inspect it, clean it.
    Result: Plugged bypass hole = no initial pressure. Clean hole = look at blow-off and MPV next.

  3. Check the blow-off valve / orifice

    There's a small blow-off hole (or valve) usually just above the inlet valve, between the valve and the air filter. During unload, the balance between air sucked in via the bypass and air blown off through this hole determines the unload-running pressure. If the blow-off is too large (worn, damaged, or tampered with), pressure never builds up high enough.
    Result: Excessive blow-off = pressure stays too low. Normal blow-off = check MPV.

  4. Check the minimum pressure valve (MPV)

    The MPV sits after the oil separator, before the compressed air outlet. It only opens once pressure reaches a certain level (usually 3.5-4.5 bar / 50-65 psi). This prevents all internal air from escaping into an empty downstream system. If the MPV is leaking, the air you're building up internally escapes into the piping -- and you never reach the pressure needed to operate the control system. Note: this mainly happens when the downstream system is depressurized (after a long shutdown, or on a new installation).
    Result: MPV leaking = air escapes to piping. Sump pressure stays low. Close the outlet valve as a test -- if pressure now builds up, the MPV is your problem.

  5. Check the control air source line

    The control air line typically taps off the top of the separator vessel. Disconnect it there and check if you have pressure. If the sump shows good pressure but nothing comes from the tap point, that specific fitting or port may be blocked.
    Result: No pressure at tap point despite good sump pressure = blocked port.

How to fix it

  1. Clean the bypass hole

    Use a small wire or needle to clean the bypass hole in the inlet valve. It's a tiny hole -- sometimes just some oil sludge or a piece of debris is enough to block it completely. After cleaning, the compressor should be able to build initial pressure again from a dead start.

  2. Fix or replace the blow-off

    If the blow-off orifice is worn or damaged, replace it. If someone has tampered with it (drilled it larger, for example), restore it to the correct size. The blow-off size is calibrated to maintain the right unload-running pressure.

  3. Re-kit the minimum pressure valve

    If the MPV is leaking, strip it down and replace all O-rings and seals. An MPV overhaul kit from the manufacturer is usually inexpensive and straightforward. The MPV is located on top of the separator vessel on about 99% of all screw compressors -- you can't miss it.

Common mistakes

The most common mistake is not understanding where control air comes from. Many technicians trace the solenoid and inlet valve and forget that the whole system depends on the compressor building up internal pressure first. If there's no internal pressure, nothing downstream can work -- not the solenoid, not the inlet valve, nothing. Always check sump pressure first when nothing else makes sense.

Parts & tools

Small wire or needle for cleaning bypass hole. MPV overhaul kit (O-rings and seals). Sump pressure gauge should already be on the machine -- if it's broken, replace it. You're flying blind without it.

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