Rotary Screw Compressor - Control Air Supply Chain Problems

You're here because you checked for control air at the inlet valve and no air was coming out (or very little air).

But first - is there actually control air available?

Before diagnosing the supply chain, you need to confirm there IS control air at the source (top of the separator vessel). If there's no control air supply at all, the problem is different.

Quick check: Carefully disconnect the control air line at the separator vessel (or at the solenoid inlet). With compressor running in unload mode, is air blowing out (2.5-3.5 bar / 35-50 PSI)?

If NO - no control air at the source:
→ Go to /troubleshooting/screw-compressors/no-control-air-supply - you have an MPV leaking or bypass hole plugged problem

If YES - control air IS available at the source:
→ Stay on this page. The problem is somewhere in the supply chain between the separator and the inlet valve.

This page covers problems in the control air supply chain:

Separator vessel (control air source) → Solenoid valve → Lines → Inlet valve

Let's diagnose this systematically, starting with the most common cause: the solenoid valve.

Step 1: Check the Solenoid Valve

The solenoid valve is the #1 most common cause of "won't load" problems in the control air chain.

###What the Solenoid Does

The solenoid is an electrically controlled air valve:

When controller signals "LOAD" → solenoid opens → control air flows to inlet valve
When controller signals "UNLOAD" → solenoid closes/vents → inlet valve closes

Common problems:

Burned electrical coil - No longer energizes
Stuck valve internally - Dirt, oil sludge, corrosion
No electrical power - Wiring, controller, safety interlock

Test 1A: Does the Solenoid Click?

Listen carefully when you manually load/unload the compressor (via controller):

You should hear a distinct "click" sound from the solenoid when it activates
Click when loading, click when unloading

If you hear the click:

Solenoid coil is working (electrically OK)
But valve may still be stuck mechanically → Go to Test 1B

If you DON'T hear a click:

Either no electrical power OR burned coil → Go to Test 1C

Test 1B: Does Air Pass Through the Solenoid?

If the solenoid clicks but compressor still won't load, the valve may be stuck internally.

How to test:

Locate the solenoid valve
- Usually mounted near the separator vessel or on the compressor frame
- Has electrical connection (cable/wires) and two or three air ports
Identify the ports:
- Inlet - Connected to separator vessel (control air source)
- Outlet - Connected to inlet valve actuator
- Vent (if 3-way valve) - Vents to atmosphere when unloading
Disconnect the line at the solenoid OUTLET (line going to inlet valve)
Manually load the compressor via controller
Does control air blow out of the solenoid outlet?
- YES: Solenoid is working! Problem is downstream (plugged line or inlet valve) → Go back to main troubleshooting tree
- NO: Solenoid is stuck internally → Replace solenoid

Common solenoid problems:

Dirt or oil sludge inside valve preventing operation
Corrosion from moisture
Worn valve seat or piston

Solution: Replace solenoid valve

Cost: $80-$300 depending on model
Make sure voltage matches (24VDC, 110VAC, or 230VAC)

Test 1C: Does the Solenoid Have Electrical Power?

If solenoid doesn't click, either the coil is burned out OR there's no power getting to it.

How to test:

Use a multimeter (set to AC voltage or DC voltage depending on your compressor)
Measure voltage at solenoid terminals while compressor should be loading
- Typical voltages: 24VDC, 110VAC, or 230VAC
- Check your compressor manual or solenoid nameplate
Is voltage present?

If YES - Voltage is present:

Solenoid coil is burned out
Test coil resistance: Disconnect solenoid electrically, measure resistance across coil
- Should be 50-500 ohms typically (check solenoid specs)
- If infinite resistance (open circuit) → coil is burned out
Solution: Replace solenoid

If NO - No voltage:

Problem is electrical → Go to Step 2

Step 2: Check Electrical Power to Solenoid

If there's no voltage at the solenoid, the problem is upstream in the electrical system.

Possible Causes:

A. Controller Not Sending Signal

Controller may have fault or safety interlock active
Check for alarm codes or warning lights on display
Check if controller is actually commanding "LOAD" mode

B. Safety Interlock Active

High temperature shutdown
Low oil pressure
Emergency stop not reset
Door/panel switches
Check: Reset all safety switches, E-stop, check for alarm indicators

C. Wiring Problem

Loose connection
Broken wire
Bad connector
Check: Trace wiring from controller to solenoid, look for loose/damaged connections

D. Blown Fuse or Bad Relay

Some compressors have fuses or relays in solenoid circuit
Check: Control panel for blown fuses
Check: Relay operation (listen for click, measure voltage before/after relay)

Solution: Fix the electrical issue

Reset safety interlocks
Repair wiring
Replace fuses
If controller fault → may need technician

Step 3: Check Control Air Source (Is There Control Air Available?)

If the solenoid is working (electrically and mechanically), but there's still no air at the inlet valve, you need to check if there's actually control air AVAILABLE at the solenoid inlet.

How to test:

Locate the control air supply line - This connects from the top of the separator vessel to the solenoid inlet
Carefully disconnect this line at the solenoid inlet (or at the separator vessel if easier)
- ⚠️ Be careful - if control air is present, it will blow out
With compressor running in UNLOAD mode, is air blowing out?
- Should be 2.5-3.5 bar (35-50 PSI) compressed air

Result A: YES - Control Air Is Present

Diagnosis: Control air source is OK. Problem is the solenoid (even though you thought it was working).

Double-check:

Solenoid inlet port - is it plugged?
Solenoid internal passages - stuck or blocked?
Replace solenoid

Result B: NO - No Control Air

Diagnosis: There's no control air supply available. The problem is either:

MPV (Minimum Pressure Valve) leaking - Air escapes, can't build unload pressure
Bypass hole plugged - No air enters compressor, can't build unload pressure

Next step: Go to → No Control Air Supply Problems

This gets into the harder diagnostics (MPV and bypass hole).

Step 4: Check Control Air Lines for Blockage

If control air is available at the solenoid but not reaching the inlet valve, the line between solenoid and inlet valve may be plugged.

How to check:

Disconnect line at solenoid outlet (already did this in Test 1B)
- Air should blow out when solenoid is energized
If air blows out of solenoid BUT you checked earlier and no air at inlet valve:
- The line between solenoid and inlet valve is plugged

Solution:

Disconnect line at both ends
Blow compressed air through it (reverse direction)
Check for kinked tubing
Replace line if old/deteriorated
Check for water/ice in line (drain, improve drainage)

Real-World Case: Atlas Copco GA200 Not Loading

Symptoms:

Solenoid valve was replaced (new part)
But still won't load
Air flows to solenoid inlet but NOT to solenoid outlet

Diagnosis: Even though solenoid was "new," it was defective OR wrong voltage

Root cause in this case:

Turned out to be no control air supply (plugged air intake filter → bypass hole couldn't build unload pressure)
But the lesson: Don't assume new parts work - test them!

What we learned:

Always verify: Does solenoid pass air when energized?
New doesn't mean working (wrong voltage, defective, wrong type)
Check control air supply availability

Common Causes in Control Air Chain (Ranked)

Based on hundreds of cases:

1. Solenoid Valve Failed - 60% of control air chain problems

Stuck mechanically (dirt, sludge)
Burned coil
Wrong voltage / defective new part

2. No Electrical Power to Solenoid - 20%

Safety interlock active
Controller fault
Wiring issue
Blown fuse

3. Control Air Line Plugged - 10%

Dirt, oil sludge in line
Moisture/ice
Kinked tubing

4. No Control Air Supply Available - 10%

→ This leads to the No Control Air Supply page

Summary Checklist

Work through these in order:

Does solenoid click when loading/unloading?
- NO → Check electrical power, check for burned coil
- YES → Go to next check
Does air blow out of solenoid outlet when energized?
- NO → Solenoid stuck internally, replace it
- YES → Go to next check
Is control air available at solenoid inlet?
- NO → Go to No Control Air Supply
- YES → Check line between solenoid and inlet valve for blockage
Is line from solenoid to inlet valve clear?
- NO → Blow out line, replace if needed
- YES → Re-check solenoid, or go back to main troubleshooting (inlet valve problem)

When to Call for Help

Most solenoid and electrical problems can be diagnosed with basic tools and a multimeter.

Call a technician if:

Complex electrical diagnostics (controller programming, PLCs)
Safety interlocks you're not familiar with
You're not comfortable working with electrical systems

Typical service call: $200-$600

Recommended Resources

In-Depth Training:

Industrial Compressed Air Systems Course - comprehensive training covering control systems, solenoid valves, electrical troubleshooting, and pneumatic control circuits

Related Troubleshooting:

Main "Won't Load" Troubleshooting - start here if you haven't already
No Control Air Supply Problems - if control air isn't available (MPV, bypass hole)
Inlet Valve Problems - if control air IS reaching the valve

Bottom Line

Control air supply chain problems are usually the solenoid valve (60% of cases).

Diagnostic flow:

Does it click? → NO = electrical problem
Does it pass air? → NO = stuck internally, replace it
Is control air available at solenoid inlet? → NO = go to "No Control Air Supply" page
Is line clear from solenoid to inlet valve? → NO = blow it out

Most of the time, you'll find it's a stuck or burned-out solenoid. Replace it and you're back in business.

Good luck! Let me know in the Q&A forum if you found the problem.