Troubleshooting Fault Code 115 on the Case CX210 Excavator

[MikePhua]

Understanding the Role of the Engine Speed Sensor
The Case CX210 hydraulic excavator, a mid-size model widely used in construction and utility work, relies on a network of electronic sensors to manage engine and hydraulic performance. One critical component is the engine speed sensor, which provides real-time crankshaft position data to the Electronic Control Module (ECM). This sensor reads a tone wheel mounted on the crankshaft, which has 35 teeth and a single missing tooth to indicate top dead center (TDC) for cylinders 1 and 6. The ECM uses this signal to control fuel injection timing, monitor engine speed, and manage cold-start behavior.
Terminology Note

• ECM (Electronic Control Module): The onboard computer that manages engine and hydraulic functions.
  
• Tone Wheel: A toothed wheel mounted to the crankshaft, used by the speed sensor to detect rotation.
  
• Fault Code 115: A diagnostic code indicating a loss or corruption of the engine speed signal.
  
• Derate: A condition where the ECM reduces engine power to protect components or prevent damage.
  
• Pin A/B/C: Electrical terminals on the speed sensor connector, typically for power, ground, and signal.
  

Symptoms and Diagnostic Observations
When Fault Code 115 is active, the machine may exhibit:

• Engine derate or reduced power output
  
• Difficulty starting, especially in cold weather
  
• Inaccurate or missing RPM readings on the monitor
  
• No other active fault codes, suggesting a localized issue
  

In one case, the speed sensor was replaced, but the fault persisted. Voltage checks showed 5V on both wire A and B with the key on, and a signal present on wire C while the engine was running. This indicated that the sensor was receiving power and generating a signal, yet the ECM still flagged a fault.
Root Causes and Electrical Considerations
The speed sensor circuit typically includes:

• Pin A: 5V supply from ECM
  
• Pin B: Ground (return)
  
• Pin C: Signal output to ECM
  

If both Pin A and B show 5V, it suggests a possible open ground circuit or shared voltage feedback. The 5V supply is often shared with other sensors, so a short or open in a different sensor circuit can affect the speed sensor’s operation.
Potential causes include:

• Corroded or loose ground at Pin B
  
• Damaged harness between sensor and ECM
  
• Faulty ECM input channel
  
• Interference from another sensor sharing the 5V line
  

Recommended Diagnostic Steps

• Confirm Pin B is a true ground using a multimeter (should show near-zero resistance to chassis ground)
  
• Check continuity from sensor to ECM connector
  
• Inspect the harness for abrasion, pinched wires, or corrosion
  
• Unplug other sensors sharing the 5V line to isolate interference
  
• Test the ECM input with a known-good signal generator if available
  

In Alberta, a technician resolved a similar issue by unplugging the engine position sensor—a separate but related component. This cleared the fault, suggesting that a conflicting signal from the second sensor was corrupting the ECM’s interpretation of the speed signal.
Component Location and Access
The engine speed sensor is located on the intake side of the engine block, between cylinders 4 and 5. It is mounted near the crankshaft tone wheel and secured with a single bolt. Access may require removal of intake piping or wiring harness brackets.
Conclusion
Fault Code 115 on the Case CX210 typically points to a disruption in the engine speed signal, often caused by grounding issues, harness faults, or interference from other sensors. Even after replacing the sensor, the fault may persist if the ECM receives a distorted or conflicting signal. A methodical approach—checking grounds, isolating circuits, and verifying continuity—can restore full engine performance and eliminate derate conditions.