Engine RPM Signal Missing or Out of Range

Common Symptoms

• Sullair WS Controller LCD displays E021 and the compressor shuts down or refuses to start
• Engine cranks normally but the compressor never comes up to load
• RUN light does not stay lit after the start sequence completes
• STOP fault indicator illuminates on the control panel
• No air pressure builds at the service valve even though the engine is turning
• On older units with Sullair II controller, the LED fault display flashes a pattern corresponding to a speed sensor fault
• Engine may hunt or stall shortly after reaching idle because the WS Controller cannot confirm RPM

Probable Causes (Ranked by Likelihood)

• Speed sensor (magnetic pickup) gap out of spec or sensor face fouled with debris or ferrous buildup Very Likely
• Speed sensor wiring harness damaged, chafed, or connector corroded, causing an open or intermittent signal Very Likely
• Speed sensor failed internally, no output signal produced Likely
• Reluctor ring (tone wheel) on crankshaft or flywheel damaged, missing teeth, or coated with debris Possible
• WS Controller input circuit fault, internal board damage causing the RPM signal not to be read correctly Possible
• Engine ECM or ECU (on electronically controlled engines) not broadcasting RPM over J1939 CAN bus, and the WS Controller relies on that signal Possible
• Incorrect speed sensor type installed after a previous repair, signal voltage or frequency outside WS Controller range Less Likely

Step-by-Step Diagnostic Procedure

1. Start at the WS Controller LCD. Confirm the code is E021 and note whether it trips during crank, immediately at start, or after the engine reaches idle. That timing tells you whether the sensor is producing any signal at all or just losing it under load.

2. Locate the magnetic pickup speed sensor. On John Deere 4045 and 6068 engines it is typically threaded into the bellhousing or timing gear cover. On CAT C4.4 and C6.6 and Cummins QSB engines, check the flywheel housing. Trace the two-wire shielded cable from the sensor back to the WS Controller harness.

3. Inspect the sensor connector and harness for corrosion, moisture, cracked insulation, or chafing against metal edges. Wiggle the harness with the engine off and verify continuity pin-to-pin with a multimeter. An open or intermittent reading here is the most common root cause on rental fleet units.

4. With the engine OFF and key power removed, unscrew the speed sensor from its bore. Clean the tip of any metal filings or debris with a clean rag. Check that the sensor tip is not cracked or physically damaged. Reinstall and set the air gap between the sensor tip and the reluctor ring teeth to the spec on the Sullair service label, typically 0.030 to 0.050 inch (0.76 to 1.27 mm). Use a feeler gauge. Too wide a gap is a very common cause of E021 on units that have been in rough rental service.

5. With a multimeter set to AC voltage, reconnect the sensor and crank the engine. You should read an AC voltage signal on the sensor output wires during cranking, typically 0.5 VAC or higher at low RPM rising with speed. No reading means the sensor is not producing a signal. A reading that disappears at higher RPM points to a reluctor ring problem or intermittent harness fault.

6. If the unit uses an electronically governed John Deere, CAT, or Cummins engine, connect the engine OEM diagnostic tool (JD Service Advisor for John Deere, CAT ET for Caterpillar, or Cummins INSITE for Cummins) and check whether the engine ECM or ECU is logging its own crank or cam sensor DTCs. If the engine has its own speed sensor fault, that will prevent the J1939 CAN RPM broadcast the WS Controller depends on, triggering E021 on the Sullair side even though the Sullair sensor itself is fine.

7. Check J1939 CAN bus wiring between the engine ECM or ECU and the WS Controller. Look for damaged 120-ohm termination resistors at each end of the CAN backbone. With both ends of the CAN bus disconnected from all nodes, measure resistance across the CAN High and CAN Low wires. You should read close to 60 ohms with both terminators in circuit. A reading of 120 ohms or open means one terminator is missing or the wiring is broken.

8. If all wiring and sensor checks pass and the fault persists, the WS Controller input board may be damaged. This requires Sullair factory support or a replacement controller. Contact your Sullair distributor and have the unit serial number and controller software version from the LCD menu ready before you call.

Frequently Asked Questions