Hitachi 120‑2 Excavator: Tackling Throttle Control Glitches

[MikePhua]

Decoding the Throttle System
The Hitachi 120‑2 employs an electronic throttle mechanism—commonly called the "EC motor"—which adjusts engine speed via a controller, relay, wiring, and a mechanical linkage to the injector pump. This system responds to ignition position and control panel input, calibrating throttle smoothly. When misaligned, corroded, or malfunctioning, this network can manifest serious throttle irregularities.
Typical Symptoms of Faulty Throttles

• Engine starts but stalls when switching to idle.
  
• Throttle motor runs constantly or “jumps” between on/off.
  
• Power inconsistencies when switching between modes like "P" and "idle."
  
• Clicking relays, skewed voltages, or throttles that freeze at high RPM despite input.
  

Technical Terms to Know

• EC Motor: Electric actuator that sets throttle via linkage control.
  
• Relay Legs: Electrical terminals in a relay; misvoltages here indicate grounding or controller issues.
  
• Pump Valve Controller (PVC): A controller module managing throttle or pump-related circuitry.
  
• Engine Learning Procedure: A setup routine enabling throttle systems to recalibrate after repairs or replacements; typically involves toggling the key in “P” mode, waiting, then powering system off.
  

Insights from Field Experience
One technician shared a scenario where a greymarket Hitachi 120‑2 had a problematic stop motor due to faulty wiring—once fixed, the EC motor relay displayed around 1 V in the wrong relay leg (leg 2). According to the manual, this should never be energized. Plugging the relay back in changed voltage readings to 24 V, but the throttle refused to work. When probing the corresponding circuit (pin 104 on the PVC), the voltage shifted from 24 V (key off) to 1.5 V (key on). Such anomalies pointed to a likely issue in the engine controller logic, not the motor or wiring alone.
In another case, a greymarket excavator’s throttle control kept strobing due to incorrect wiring in the pump access area—essentially two connectors had been plugged into wrong positions. Once corrected and followed by the engine learning cycle (key on → P mode → wait → key off), the throttle became functional again.
Older user reports have also pointed toward intermittent ground faults or deteriorated harnesses—especially where harnesses run near hydraulic pumps or cab floors, getting rubbed, exposed to oil, or prone to deterioration.
Practical Diagnostic Checklist

• Measure voltage at EC motor relay legs—expect proper grounding behavior (key-off high, key-on grounded).
  
• Inspect wiring and connectors near the pump access—ensure no mismatched plug connections.
  
• Watch EC motor behavior with a camera or visual—stuck or growling movement suggests control rather than mechanical failure.
  
• Run the engine learning cycle after any repairs to recalibrate throttle control system.
  
• Evaluate overall harness health—especially near hot, oily, or abrasive areas.
  

A Real‑World Scenario
Picture this: a workshop receives a 120‑2 that revs uncontrollably, with nonexistent idle response. No fault codes appear on the screen. Techs bring in digital diagnostics, but everything seems within spec. Tracing errors through the wiring harness, they eventually identify two wires tucked behind the pump that were plugged into the wrong terminals after a prior repair. Swapping them back, followed by the learning procedure, restores normal throttle behavior. Operator control is regained—the machine idles, revs, and shuts off precisely when expected.
Why It Matters

• Safety and Control: Unpredictable throttle behavior is a serious hazard for operators and nearby personnel.
  
• Efficiency: Small wiring mistakes or failed calibrations can result in costly downtime.
  
• Diagnostics Savvy: Understanding throttle systems—motor, controller, harness, learning cycle—empowers faster, more reliable repair outcomes.