Applies to: STS Series (5–100HP) · STSH Series · STSD · STSP · STSP2
This guide covers fault diagnosis and corrective actions for South-Tek rotary screw air compressors (STS, STSH, STSD, STSP, and STSP2).
⚠ Warning — Before Any Troubleshooting Mechanical and electrical troubleshooting must only be performed by qualified, trained personnel. Before inspecting:
1. Press STOP and wait for the compressor to fully stop. Close the air outlet valve.
2. Open manual discharge valves in the compressor system.
3. Press Emergency Stop. Disconnect power at the mains panel.
4. Loosen the separator tank oil filler plug 1–2 turns to vent remaining pressure.
5. Confirm the compressor system is fully depressurized before opening any connection.
1. Press STOP and wait for the compressor to fully stop. Close the air outlet valve.
2. Open manual discharge valves in the compressor system.
3. Press Emergency Stop. Disconnect power at the mains panel.
4. Loosen the separator tank oil filler plug 1–2 turns to vent remaining pressure.
5. Confirm the compressor system is fully depressurized before opening any connection.
Fault Reference Table
| Symptom | Possible Cause | Corrective Action |
|---|---|---|
| Compressor does not start | Main disconnect switch off or faulty | Activate or replace |
| Line fuses blown or faulty | Activate or replace | |
| Supply or control connection fault | Inspect and correct wiring | |
| Thermal protection relay tripped or faulty | Reset or replace relay | |
| Motor contactor fault | Inspect and replace if faulty | |
| Low or unbalanced line voltage / single-phasing | Check all three phases with a voltmeter; correct supply | |
| Controller shows running, but compressor does not operate | Electrical cabinet heavily contaminated with dust | Inspect and clean the electrical cabinet interior |
| Compressor runs continuously at load and cannot reach setpoint pressure | Air demand exceeds compressor capacity | Calculate total system demand. Add capacity if needed |
| Air leaks in the compressed air installation | Locate and repair all leaks | |
| Cabin suction filter dirty, reducing airflow | Inspect and replace if contaminated | |
| Suction control valve system fault | Inspect valve and control circuit; replace faulty parts | |
| Load/unload pressure settings distorted | Verify and readjust pressure setpoints on controller | |
| Excess oil in outlet air line | Oil level in separator tank too high | Drain oil to the correct level |
| Scavenge line or check valve clogged | Inspect; clear blockage and root cause | |
| Separator element clogged | Inspect and replace if contaminated | |
| Oil foaming due to incorrect oil type | Drain system completely and refill with the correct approved oil (S-Tek 46 or S-Tek 46 FG) | |
| Safety valve opens on load | Upper pressure setpoint too high or distorted | Check and correct compressor operation settings |
| Safety valve faulty | Replace safety valve | |
| Separator element completely clogged | Replace separator element | |
| Suction control valve faulty | Inspect and replace faulty parts | |
| Minimum pressure valve or pressure transmitter faulty | Inspect and replace if faulty | |
| Frequent load/unload cycling | Pressure band between upper and lower setpoints too narrow | Widen the pressure differential on the controller |
| Air leaks in installation | Locate and repair all leaks | |
| Minimum pressure valve faulty | Inspect and replace if faulty | |
| Separator element clogged (high differential pressure) | Check differential pressure and replace if elevated | |
| Suction control valve faulty | Inspect and replace if faulty | |
| Compressor runs unloaded but will not switch to load | Thermometer temperature sensor faulty | Inspect and replace if faulty |
| Suction control valve system fault | Inspect and replace faulty parts | |
| Suction control valve piston faulty | Inspect valve internals | |
| Pressure transmitter faulty | Inspect and replace if faulty | |
| Air control hose leaks | Inspect hoses and replace if faulty | |
| Oil discharges from air intake filter on sudden stop | Suction control valve control system faulty | Inspect and replace faulty parts |
| Suction control valve check plug O-ring faulty | Replace O-ring | |
| Compressor starts but stops after running | Oil level low (high temperature shutdown) | Check oil level; investigate cause of loss; refill |
| Oil filter or oil cooler clogged | Replace oil filter; clean or service oil cooler | |
| Thermostatic valve sensor faulty | Inspect and replace if faulty | |
| Cooler surfaces dirty | Clean cooler cartridge surfaces | |
| Cooling air insufficient or too hot | Check and replace air filter and cabin suction filter if dirty; verify room ventilation | |
| Cooling water insufficient or too hot (water-cooled units) | Adjust water flow or temperature | |
| Fan motor thermal protector fault | Inspect and replace if faulty | |
| Main motor thermal protector or thermistor relay fault | Inspect and replace if faulty | |
| Cooling water control valve faulty (water-cooled units) | Inspect and replace if faulty | |
| Controller parameter values incorrect | Verify controller parameter settings | |
| Electrical control circuit shorted, loose, or disconnected | Inspect the control wiring; repair or reconnect | |
| Airend (screw assembly) fault | Have the airend inspected by qualified service |
Component Reference
Use this diagram to locate the components named in the table above. The numbers match the legend.
IMAGE — FIG-01: Compressor flow / component diagram (items 1–17)
| # | Component | # | Component |
|---|---|---|---|
| 1 | Air filter element | 10 | Minimum pressure valve |
| 2 | Suction control valve | 11 | Pressure transmitter (outside pressure) |
| 3 | Airend | 12 | Air cooler |
| 4 | Temperature sensor (NTC) | 13 | Oil cooler |
| 5 | Separator tank | 14 | Thermostatic valve |
| 6 | Pressure transmitter (inside pressure) | 15 | Oil filter element |
| 7 | Safety valve | 16 | Venting valve |
| 8 | Separator element | 17 | Pressure regulator |
| 9 | Check valve (scavenge line) |
▲ Attention If a fault is active, the AirMASTER FORM controller will display the fault code on screen. Do not reset maintenance warnings on the controller until the associated maintenance or inspection has been physically completed. Resetting without performing service voids warranty coverage for resulting damage.
Related Articles
- Compressor Operation - Start, Load/Unload & Stop
- AirMASTER FORM Controller - Overview & Navigation
- Oil & Oil Filter Replacement
- Air Filter & Separator Element Replacement
- Safety Precautions & Symbols
Need help? If the fault cannot be resolved using this guide, contact South-Tek Systems Service — Phone: (910) 415-1880 · Open a support ticket at support@southteksystems911.zendesk.com or through the Help Center.