CAT 908M Loader Electrical Shutdown Traced to Unlabeled Main Relay

[MikePhua]

Quick answer
A 2019 CAT 908M compact wheel loader experiencing random electrical shutdowns and crank-no-start behavior was ultimately repaired by replacing an unlabeled main relay. The relay was not listed in the fuse panel diagram or manual, making diagnosis difficult until the fault became permanent.
CAT 908M background and electrical system architecture
The CAT 908M is a compact wheel loader designed for urban construction, landscaping, and utility work. It features:

• A 74–100 hp Tier 4 Final engine
  
• Enclosed cab with password-protected ignition
  
• CAN-based electronic control system with multiple ECMs
  
• Integrated fuse and relay panel behind the operator seat
  
• Diagnostic interface compatible with CAT Electronic Technician (ET)
  

The machine relies on a network of electronic control modules (ECMs) to manage engine, transmission, and auxiliary functions. Power distribution is routed through relays and fuses, with startup logic gated by password entry and ECM handshake.
Symptoms and diagnostic challenges
The loader began exhibiting intermittent electrical loss, including:

• Crank-no-start condition after password entry
  
• Red flashing exclamation mark on the dash
  
• No glow plug activation
  
• Asterisks in the upper right corner of the display
  
• No power to the right-side switch panel
  
• Random shutdowns during operation
  

Sometimes the issue resolved by cycling the key. Other times, the machine would start hours later without intervention. The problem escalated from monthly to multiple times per day.
Initial diagnostics using CAT ET showed no active fault codes or events. This made root cause identification difficult, especially since the issue was intermittent.
Permanent failure and relay identification
Eventually, the fault became permanent, allowing for systematic testing. The technician discovered one relay in the fuse panel that lacked a CAT part number and was not labeled in the manual or fuse door diagram.
After replacing this relay, the machine returned to normal operation. CAT parts personnel identified it as the “Main Relay A,” one of two identical relays that control ECM power distribution. The other is “Main Relay B.” These relays are critical for energizing the ECMs and enabling startup logic.
Lessons learned and repair strategy

• Intermittent electrical faults often stem from relays, connectors, or grounding issues
  
• Unlabeled components may still be critical—visual inspection and part cross-reference are essential
  
• Permanent failure can simplify diagnosis by eliminating variables
  
• CAT ET diagnostics require full ECM scan with “All ECMs” box checked to capture hidden events
  

Technicians should avoid erasing logged codes until a full Product Status Report is downloaded. This preserves historical data for troubleshooting.
Recommendations for future reliability

• Replace both Main Relay A and B if one fails
  
• Label all relays manually for future reference
  
• Keep a spare relay in the cab for field replacement
  
• Use dielectric grease on relay terminals to prevent corrosion
  
• Perform ECM scans quarterly to catch early faults
  

One operator in Massachusetts noted that his loader had been reliable for years until the relay began failing. After the fix, the machine ran continuously for a week without issue.
Conclusion
Electrical shutdowns and crank-no-start behavior on the CAT 908M were caused by a failed, unlabeled main relay. With no fault codes and intermittent symptoms, diagnosis was difficult until the failure became permanent. Replacing the relay restored full function, highlighting the importance of relay integrity and thorough ECM scanning.