Engine Speed Signal Lost, Genset Shutdown

Common Symptoms

• Genset cranks normally but shuts down within 2 to 5 seconds of starting, before it reaches full speed
• Panel flashes fault code 19 (nineteen blinks) after shutdown
• Engine starts briefly, runs rough for a moment, then dies as if fuel was cut
• No RPM reading appears on the service display panel during or after crank
• Genset attempts multiple crank cycles, fails all of them, then locks out
• No output power is delivered even if the engine briefly fires
• STOP lamp illuminates or fault indicator LED is active after the failed start attempt

Probable Causes (Ranked by Likelihood)

• Magnetic pickup (speed sensor) has failed, drifted out of gap spec, or is physically damaged Very Likely
• Magnetic pickup wiring is broken, chafed, shorted, or has a loose connector at the sensor or controller Very Likely
• Flywheel ring gear teeth are stripped, cracked, or coated in debris, preventing the sensor from reading pulses Likely
• Magnetic pickup air gap is too large due to vibration loosening the sensor over time Likely
• Controller board input circuit for the speed sense signal has failed internally Possible
• Battery voltage is marginal, causing crank speed to be so low that the controller cannot read a valid RPM signal Possible
• Stator speed sense winding (on units without a magnetic pickup, using a stator signal for RPM) has an open or shorted winding Less Likely

Step-by-Step Diagnostic Procedure

1. Step 1: Locate the magnetic pickup sensor. On most Onan RV gensets it threads into the bellhousing or timing gear cover near the flywheel or gear. It looks like a brass or steel cylinder about the size of a thumb with a two-wire connector.

2. Step 2: With the genset OFF and the battery disconnected, unplug the two-wire connector at the magnetic pickup. Set your multimeter to resistance (ohms). Measure across the two sensor terminals. A healthy Onan magnetic pickup typically reads between 130 and 200 ohms. A reading of zero (short) or OL (open/infinite) means the sensor has failed. Replace it.

3. Step 3: Inspect the sensor tip for physical damage, cracks, or heavy oil fouling. Clean the tip with a rag. Spin the flywheel by hand (engine off) and look for metal shavings clinging to the sensor tip. Shavings are normal in small amounts but a large buildup means the ring gear teeth are wearing abnormally.

4. Step 4: Check the air gap. The magnetic pickup should be threaded in until it just touches the ring gear tooth, then backed out 1/2 to 3/4 turn and locked with the jam nut. This gives roughly 0.030 to 0.050 inch clearance. A sensor that has backed out more than one full turn from finger-tight will lose signal. Adjust and retest.

5. Step 5: Trace the two sensor wires back to the controller harness connector. Inspect for chafing against exhaust components, pinched sections at the bellhousing edge, corrosion at the connector pins, or a broken wire that looks intact on the outside. Wiggle the harness while watching for continuity changes on your multimeter set to continuity mode.

6. Step 6: With the harness reconnected and the genset in a safe test condition, use the Onan service display panel or InPower diagnostic software if available. Attempt a start and watch whether an RPM reading registers at all during cranking. Zero RPM throughout the entire crank cycle confirms the signal never reached the controller. A brief RPM reading that drops to zero mid-crank points to an intermittent wire or a borderline sensor.

7. Step 7: Check battery voltage at the genset terminals during cranking. Connect your multimeter to the battery posts and watch voltage while a helper attempts a start. Voltage should stay above 9.6V during crank on a 12V system. If voltage drops below 9V, crank speed may be too low to generate a readable signal, and you have a battery or charging problem causing a false code 19.

8. Step 8: If the sensor, wiring, and battery all check out normal, the controller input circuit itself may be at fault. This requires bench testing or replacement of the controller board. Call a certified Cummins Onan service technician for this step, as controller programming is involved.

Frequently Asked Questions