The pressure sensor tells the controller the wrong pressure. The controller never commands an unload because it thinks pressure is still low.

What you'll see

The controller display shows a pressure that doesn't match reality. It might show 5 bar when the actual sump pressure is 9 bar. The compressor keeps loading because the controller thinks it hasn't reached setpoint yet. Meanwhile, actual pressure climbs past the safety valve setting.
Before you assume this is the problem

Compare the controller reading with an independent pressure gauge on the separator vessel. If they match, the transducer is fine -- look at the inlet valve or separator instead.

See all causes of safety valve blows / too high pressure →

How to diagnose

  1. Compare controller reading with a gauge

    Install a test gauge on the separator vessel or check the existing mechanical gauge. Compare it with the controller display. A significant difference means the transducer is reading wrong.
    Result: Controller shows 5 bar, gauge shows 9 bar = faulty transducer.

  2. Check transducer location

    On some machines (like Atlas Copco GA55 FF with integrated dryer), the pressure sensor is AFTER the dryer. If the dryer freezes or blocks, the sensor reads low pressure while sump pressure climbs unchecked. Check if there's a restriction between the separator and the sensor.
    Result: Sensor after a blockage = sensor location problem.

How to fix it

  1. Replace the pressure transducer

    Install a new transducer with the correct range and output for your controller. Calibrate if required. These are not expensive parts.

  2. Fix upstream blockages

    If the sensor location is after a dryer or filter that can block, address the blockage. A frozen dryer evaporator is a common culprit on machines with integrated dryers.

Common mistakes

Don't ignore a mismatch between display and gauge. Even a small error can cause the compressor to overshoot its setpoint significantly.

Parts & tools

Replacement pressure transducer. Test pressure gauge. Multimeter for checking transducer output signal.

Review safety precautions before starting →

This issue can also cause

  • Compressor Won't Load Compressor runs but won't build pressure—the inlet valve isn't opening. Usually a control air or solenoid valv...