1. Background & Initial Symptom

A welding machine, driven by a Perkins 3-cylinder turbocharged engine (SAE 300), would start reliably and idle steadily—but when welding (i.e., under load), the engine RPM would drop significantly and then stall.

2. Systematic Diagnostic Approach

• RPM Confirmation

  I confirmed RPM behavior using a digital tachometer, verifying that RPM dropped precisely under welding load.
  

• Fuel System and Injection Pump Inspection

  The engine’s fuel supply was tested and found adequate. Injection pump settings were reviewed and calibrated per OEM guidelines—no issues detected.
  

• ECG Controller & Magnetic Pickup Check

  Diagnostics via the ECG (electronic engine control) controller revealed correct system behavior. A/C voltage from the magnetic pickup (speed sensor) was measured and within expected values—indicating the ECU was receiving accurate engine speed feedback.
  

• Manual Operation Test

  Operating the machine manually (without welding load), the engine performed flawlessly—strong and stable RPM, ruling out mechanical or fundamental engine failures.
  

• Speed Actuator Examination

  I then focused on the speed actuator (governor actuator) controlling engine RPM. An ohmmeter test revealed the actuator was not passing a circuit—indicating electrical or mechanical failure.
  

• Replacement & Final Testing

  I replaced the faulty actuator with a new OEM component, recalibrated and installed it per specifications. A full test under welding load confirmed stable RPM and reliable performance—no further issues.
  

3. Technical Context & Industry Insight

Perkins—and similar OEM diagnostic manuals—highlight that modern engines utilize electronic governors with actuators or wastegate assemblies that directly influence engine speed under varying load conditions. When these components fail, engine RPM control is compromised, especially during load changes such as welding. For instance, wastegate actuators are serviceable parts, while the turbocharger core often is not; similar logic applies to speed actuators on generator or welding engines.

Furthermore, diagnostic procedures in Perkins manuals emphasize testing PWM speed-control circuits and actuator behavior electrically (i.e., resistance checks, connector inspections) before condemning mechanical systems. This approach aligns precisely with the method used in this case.

4. Summary of Findings & Results