John Deere 42 – ECU, PTI: Engine Overspeed

The system has detected that the engine is operating at a rotational speed (RPM) that exceeds the predefined limit. This condition triggers a critical alert that may lead to the engine derating or shutting down to prevent mechanical failure. It can indicate a malfunction in the component responsible for regulating engine speed, a faulty engine speed sensor sending erratic signals, or operator error such as driving downhill with a heavy load.

• 6020 Series
• 6920S Series
• 7630 Series
• 7730 Series
• 7830 Series
• 7930 Series
• 7720 Series
• 7820 Series
• 7920 Series
• 7810 Series
• 8120 Series
• 8220 Series
• 8320 Series
• 8420 Series
• 8520 Series
• 9320T Series
• 9420T Series
• 9520T Series
• 9620T Series
• 5M Series
• 6M Series
• 6R Series
• 7R Series
• 8R Series
• 9R Series
• 6650 Forage Harvester
• 6750 Forage Harvester
• 6850 Forage Harvester
• 6950 Forage Harvester
• backhoes
• dozers
• excavators
• wheel loaders

1. 1

   Manually reduce the engine speed

2. 2

   Shut down the engine, wait a moment, and restart the vehicle to see if a temporary communication glitch clears the active fault

3. 3

   Check for operator-induced overspeed (e.g., operating downhill with a heavy load pushing the engine beyond its set speed)

4. 4

   Check for related fault codes, such as Code 39 (Primary speed input error) or crankshaft position input noise/pattern error, which point to the speed sensing circuit

5. 5

   Visually inspect the engine speed sensor (magnetic pickup sensor) for physical damage

6. 6

   Inspect the wiring harness and connections leading to the speed sensor for fraying, corrosion, or loose connections

7. 7

   Test the engine speed sensor resistance (Ohms) and AC voltage output during cranking against technical manual specifications

8. 8

   Inspect the fuel system governor (on mechanically governed engines) for broken linkages or worn gears causing surging

9. 9

   Inspect the injection pump and control system (on electronically controlled engines) for improper fuel delivery

1. 1

   Address Simple Fixes First

   If any wiring is found to be damaged, repair or replace the affected section of the harness. Clean any corroded connectors.

2. 2

   Replace the Engine Speed Sensor

   If the sensor is tested and found to be faulty, it must be replaced. The location varies by machine but is often on the engine timing gear case or near the flywheel to read its rotational speed.

3. 3

   Consult a Dealer

   If the above steps do not resolve the issue, the problem may lie within the Engine Control Unit (ECU) or the fuel injection pump. These repairs typically require specialized diagnostic tools and expertise from a John Deere dealer.