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The PC35MR-2 and Its Role in Compact Excavation
The Komatsu PC35MR-2 is a compact hydraulic excavator designed for tight workspaces, urban construction, and utility trenching. Introduced in the early 2000s, it features a zero-tail swing design, allowing full rotation within the machine’s footprint. Powered by a Komatsu 3D88E-5 diesel engine producing around 29 horsepower, the PC35MR-2 balances fuel efficiency with hydraulic responsiveness.
Komatsu, founded in 1921 in Japan, has long been a leader in construction equipment innovation. The MR series—short for “Minimum Radius”—was developed to meet growing demand for maneuverable machines in congested job sites. By 2010, the PC35MR-2 had become one of the most popular compact excavators in its class, with thousands sold across North America, Europe, and Asia.
Symptoms of the No-Start Condition
An intermittent no-start issue on the PC35MR-2 typically presents as follows:
Initial Troubleshooting and False Positives
In one case, the operator replaced the starter after bench testing revealed sluggish operation. A new starter temporarily resolved the issue, but the no-start condition returned days later. Cleaning the key switch terminals and nearby electrical connections restored function briefly, but the problem reappeared after another work cycle.
This pattern—temporary fixes followed by recurrence—often points to a deeper electrical fault, such as:
Understanding Alternator Diode Failure
The alternator on the PC35MR-2 includes a built-in diode to prevent reverse current flow when the engine is off. If this diode fails, voltage can leak into the control circuit, confusing the ECU and preventing proper starter engagement.
In this case, corrosion inside the three-wire alternator plug masked the issue during initial testing. Once the plug was cleaned and the faulty diode identified, disconnecting a single wire allowed the machine to start reliably. Replacing the alternator resolved the problem permanently.
Key terms:
Compact excavators like the PC35MR-2 often operate in muddy, wet, and vibration-prone environments. Over time, electrical connectors can corrode, especially in canopy models without enclosed cabs. Moisture ingress, dust accumulation, and thermal cycling degrade insulation and contact surfaces.
Preventive measures include:
A semi-retired field technician in Louisiana encountered this issue while rebuilding the stick and replacing hydraulic hoses. The machine sat idle for extended periods, and the intermittent no-start condition emerged during repositioning. After extensive testing and part replacement, the root cause was traced to the alternator diode—a reminder that even seasoned mechanics can be misled by surface symptoms.
In another case, a contractor in British Columbia experienced similar issues with a PC40MR-2. After replacing the starter and key switch without success, they discovered a frayed ground wire under the operator’s platform. Once repaired, the machine started consistently.
Recommendations for Diagnosing No-Start Issues
When facing an intermittent no-start on a compact excavator:
Conclusion
Intermittent no-start issues on the Komatsu PC35MR-2 can be frustrating and elusive. While starters and key switches are common culprits, deeper electrical faults—such as alternator diode failure and connector corrosion—often lie at the heart of the problem. By combining methodical testing with field intuition, operators and technicians can restore reliability and avoid unnecessary part replacement. In compact machines, where every wire matters, attention to detail is the key to consistent performance.
The Komatsu PC35MR-2 is a compact hydraulic excavator designed for tight workspaces, urban construction, and utility trenching. Introduced in the early 2000s, it features a zero-tail swing design, allowing full rotation within the machine’s footprint. Powered by a Komatsu 3D88E-5 diesel engine producing around 29 horsepower, the PC35MR-2 balances fuel efficiency with hydraulic responsiveness.
Komatsu, founded in 1921 in Japan, has long been a leader in construction equipment innovation. The MR series—short for “Minimum Radius”—was developed to meet growing demand for maneuverable machines in congested job sites. By 2010, the PC35MR-2 had become one of the most popular compact excavators in its class, with thousands sold across North America, Europe, and Asia.
Symptoms of the No-Start Condition
An intermittent no-start issue on the PC35MR-2 typically presents as follows:
- No crank when the key is turned to the start position
- No click from the starter solenoid
- Starter bench tests show sluggish performance
- Battery voltage appears normal
- Starter receives power, but the “kicker wire” (signal wire) shows no voltage
Initial Troubleshooting and False Positives
In one case, the operator replaced the starter after bench testing revealed sluggish operation. A new starter temporarily resolved the issue, but the no-start condition returned days later. Cleaning the key switch terminals and nearby electrical connections restored function briefly, but the problem reappeared after another work cycle.
This pattern—temporary fixes followed by recurrence—often points to a deeper electrical fault, such as:
- Corroded connectors
- Faulty relays
- Voltage bleed from a failed diode
- Ground loop interference
Understanding Alternator Diode Failure
The alternator on the PC35MR-2 includes a built-in diode to prevent reverse current flow when the engine is off. If this diode fails, voltage can leak into the control circuit, confusing the ECU and preventing proper starter engagement.
In this case, corrosion inside the three-wire alternator plug masked the issue during initial testing. Once the plug was cleaned and the faulty diode identified, disconnecting a single wire allowed the machine to start reliably. Replacing the alternator resolved the problem permanently.
Key terms:
- Diode: An electrical component that allows current to flow in one direction only. Used to isolate circuits and prevent backfeed.
- Solenoid: An electromechanical switch that engages the starter motor when energized.
- Kicker Wire: The low-voltage signal wire that activates the starter solenoid.
Compact excavators like the PC35MR-2 often operate in muddy, wet, and vibration-prone environments. Over time, electrical connectors can corrode, especially in canopy models without enclosed cabs. Moisture ingress, dust accumulation, and thermal cycling degrade insulation and contact surfaces.
Preventive measures include:
- Applying dielectric grease to connectors
- Using sealed weather-pack connectors for critical circuits
- Periodic inspection of ground straps and battery terminals
- Installing a battery disconnect switch to eliminate parasitic draw during storage
A semi-retired field technician in Louisiana encountered this issue while rebuilding the stick and replacing hydraulic hoses. The machine sat idle for extended periods, and the intermittent no-start condition emerged during repositioning. After extensive testing and part replacement, the root cause was traced to the alternator diode—a reminder that even seasoned mechanics can be misled by surface symptoms.
In another case, a contractor in British Columbia experienced similar issues with a PC40MR-2. After replacing the starter and key switch without success, they discovered a frayed ground wire under the operator’s platform. Once repaired, the machine started consistently.
Recommendations for Diagnosing No-Start Issues
When facing an intermittent no-start on a compact excavator:
- Verify battery voltage under load (≥12.4V)
- Check voltage at the starter’s kicker wire during key-on
- Inspect key switch terminals for corrosion or looseness
- Test relays and fuses with a multimeter, not just visually
- Examine alternator output and diode integrity
- Clean and reseal all electrical connectors in the engine bay
Conclusion
Intermittent no-start issues on the Komatsu PC35MR-2 can be frustrating and elusive. While starters and key switches are common culprits, deeper electrical faults—such as alternator diode failure and connector corrosion—often lie at the heart of the problem. By combining methodical testing with field intuition, operators and technicians can restore reliability and avoid unnecessary part replacement. In compact machines, where every wire matters, attention to detail is the key to consistent performance.
