Electrical Faults in the Liebherr 631B Track Loader and How to Resolve Them

[MikePhua]

The Liebherr 631B and Its Engineering Legacy
The Liebherr 631B track loader was produced in the early 1990s by Liebherr Group, a German company renowned for its precision engineering and innovation in heavy equipment. Founded in 1949, Liebherr has grown into a global leader in construction machinery, with its track loaders serving in mining, forestry, and infrastructure projects across Europe and North America.
The 631B was equipped with a Mercedes-Benz diesel engine and featured hydrostatic drive, robust undercarriage components, and a fully enclosed cab. Though not mass-produced in the same volume as Caterpillar or Komatsu equivalents, the 631B earned a reputation for durability and smooth hydraulic response. However, its electrical system—designed with European logic and safety interlocks—can be challenging to troubleshoot, especially decades after production.
Symptoms of Electrical Malfunction
Operators have reported a range of electrical issues on the 631B, including:

• Engine failing to start unless the starter relay terminals are manually jumped
  
• Electrical systems remaining active even when the ignition key is turned off
  
• Dashboard gauges, fans, and warning lights behaving as if the machine is in “run” mode despite the key being in the “off” position
  
• Audible low battery alarms triggered without ignition
  

These symptoms suggest a failure in the ignition circuit, possibly involving a stuck relay, faulty safety switch, or alternator feedback loop.
Terminology Explained

• Starter Relay: An electrically operated switch that controls high current to the starter motor.
  
• Safety Interlock: A switch or sensor that prevents engine start unless certain conditions are met (e.g., parking brake engaged).
  
• Master Disconnect Relay: A relay that isolates the battery from the electrical system, typically on the ground side.
  
• Ignition Switch: The operator-controlled switch that energizes the electrical system and allows engine start.
  

Root Causes and Diagnostic Path
The most common culprit in this scenario is a failed or misbehaving relay. On the 631B, relays are located inside the instrument cluster and near the battery compartment. One relay in particular—often a Bosch unit—controls both the starter circuit and the cab electrical system. If this relay sticks or fails to disengage, it can keep the system energized even when the key is off.
Another possibility is a faulty ignition switch. If the switch fails internally, it may allow current to flow regardless of key position. This can be tested by disconnecting the ignition wire and observing whether the system powers down.
Additionally, Liebherr’s design allows the alternator regulator to complete the electrical circuit once charging begins. If the regulator fails to disengage when the engine stops, the cab may remain powered until the alternator fully spins down.
Field Anecdote and Practical Insight
A technician in North Carolina encountered this issue on a 1992 631B. After replacing the starter relay with no improvement, he discovered that the ignition switch was allowing current to bypass its internal contacts. Disconnecting the switch wire immediately shut down the system, confirming the diagnosis.
He sourced a replacement Bosch relay from a local electric motor shop for $300—half the price quoted by the dealer. The relay matched the original part number and resolved the starter issue. However, the persistent power-on condition required ignition switch replacement.
Another operator noted that his machine behaved normally, powering down when the key was turned off. This contrast helped isolate the fault to the ignition circuit rather than a systemic design flaw.
Recommended Diagnostic Steps

• Inspect the starter relay for signs of heat damage or sticking
  
• Disconnect the ignition switch wire and observe system behavior
  
• Test the safety switch under the parking brake lever for continuity
  
• Check the alternator regulator for proper disengagement
  
• Verify ground connections and master disconnect relay function
  

Preventive Maintenance and Long-Term Reliability

• Replace relays every 2,000 hours or during major electrical service
  
• Clean and inspect ignition switch annually
  
• Use dielectric grease on connectors to prevent corrosion
  
• Monitor battery voltage and alternator output monthly
  
• Label and document wiring modifications for future troubleshooting
  

Cab Access and Hydraulic Line Repairs
In a related issue, operators attempting to repair hydraulic lines near the oil cooler found cab access difficult. To lift the cab:

• Remove neutral safety lever and air breather
  
• Center throttle and shutoff levers
  
• Remove cab mount bolts on both sides
  
• Use the factory-installed hydraulic jack (located in the right engine compartment)
  
• Insert the safety pin beside the fuel tank once raised
  

This procedure allows access to hydraulic components and simplifies line replacement. Operators should ensure the safety pin is properly seated before working under the cab.
Parts Sourcing and Cost Considerations
Liebherr parts can be expensive due to limited distribution. For example:

• OEM relay: $600+
  
• Aftermarket Bosch relay: $250–$350
  
• Ignition switch: $80–$150
  
• Safety switch: $40–$90
  

Used parts may be available through salvage yards or European suppliers. Operators are advised to cross-reference part numbers and avoid generic replacements that may not meet voltage or amperage requirements.
Conclusion
Electrical issues in the Liebherr 631B track loader often stem from aging relays, faulty ignition switches, or alternator feedback loops. By methodically testing each component and understanding the machine’s unique electrical logic, operators can restore functionality without excessive cost. The 631B remains a capable and rugged machine—provided its electrical heart is kept in check. With proper maintenance and a bit of diagnostic patience, it continues to serve reliably in demanding environments.