Diagnosing Electrical and Starting Issues in the Volvo L120E Wheel Loader

[MikePhua]

Overview of the Volvo L120E
The Volvo L120E is a mid-sized wheel loader known for its reliability, fuel efficiency, and comfort. Built to operate in a range of industries—from construction to aggregates—it features an electronically controlled diesel engine, load-sensing hydraulics, and a transmission management system. However, like any complex machine, it can suffer from electrical and starting issues that require a methodical approach to diagnose and repair.
Common Starting Symptoms and Their Implications
Operators occasionally encounter a scenario where the machine fails to start or starts intermittently. In the case of the L120E, reports include:

• No crank when turning the key
  
• Starter clicks but does not engage
  
• Warning lights on the monitor cluster but no engine activity
  
• Machine starts when bypassing starter solenoid manually
  

These symptoms indicate a likely failure somewhere in the starting circuit or electrical control path. Such behavior is typical in equipment with aging wiring harnesses or relays that are affected by vibration, moisture, and corrosion.
Key Components in the Starting System
To understand where failures can occur, it's important to break down the major elements of the L120E's starting and electrical system:

• Start Switch (Key Ignition): Sends signal to the starter relay when turned to the start position.
  
• Starter Relay: Acts as a bridge between the key switch and the starter solenoid. A weak relay can intermittently fail.
  
• Starter Solenoid: Receives high-amperage power from the battery to crank the engine.
  
• ECU (Electronic Control Unit): Controls permission to crank based on safety and operational conditions.
  
• Fuses and Wiring Harness: Essential for signal integrity between components.
  
• Battery Cables and Grounds: Must be clean and secure for proper current flow.
  

Electrical Terminology Explained

• Continuity: An unbroken electrical path allowing current to flow. Testing continuity ensures wiring or a switch isn’t open.
  
• Voltage Drop: Loss of voltage between two points due to resistance; often a sign of corroded terminals or undersized wiring.
  
• Backfeeding: When current flows the wrong way through a circuit, often due to failed diodes or miswiring.
  
• Load Testing: Applying resistance to a component like a battery to see how it performs under demand.
  

Step-by-Step Troubleshooting Approach
When faced with a no-start condition on a machine like the L120E, mechanics follow a proven diagnostic path:

• Check Battery Voltage: Ensure batteries are charged and producing 24V under load.
  
• Inspect Battery Connections: Clean corrosion from terminals, tighten cables, and inspect for frayed wires.
  
• Test Key Switch: Use a multimeter to ensure voltage exits the switch when turned to the start position.
  
• Verify Starter Relay Function: Swap with a known good relay or use a jumper wire to test.
  
• Bypass Starter Solenoid: Jump directly from battery to starter post—if the starter turns, the solenoid and motor are likely fine.
  
• Scan ECU for Codes: Some faults may inhibit starting for safety reasons.
  
• Trace Wiring Harness: Look for damaged insulation, crushed wires, or signs of heat exposure.
  

In one real case, a faulty relay behind the right-side panel was identified as the culprit. It intermittently failed to close the circuit to the starter solenoid. Replacing it resolved the issue immediately. Another case involved corroded wiring inside the starter’s control harness—a problem solved by splicing in new wire sections with heat-shrink tubing.
Field Case: Cold-Weather Starting Problems
In northern Alberta, a contractor found that their L120E would occasionally fail to start in sub-zero temperatures. After ruling out the starter and relay, the mechanic discovered the problem lay in a poorly grounded ECU. Moisture had accumulated in the connector, causing resistance. Cleaning the terminals and applying dielectric grease fixed the intermittent issue. As a preventative measure, the crew installed a battery blanket and block heater, significantly reducing future problems.
Pro Tips from Veteran Mechanics

• Use a test light with a long lead to check if current reaches the starter solenoid terminal when the key is turned.
  
• Always inspect grounds on the frame and engine block—rusted bolts and paint can inhibit proper grounding.
  
• Keep spare relays and fuses in the cab. Many issues can be resolved in minutes with a simple swap.
  
• If starting problems occur only after warm-up, suspect heat-related resistance in the starter motor windings or solenoid.
  

OEM vs Aftermarket: Component Considerations
Some operators report better longevity with OEM relays and switches due to tighter tolerances and better sealing. While aftermarket options can be cost-effective, they may lack the durability for harsh environments.
Preventative Measures to Avoid Starting Failures

• Conduct monthly visual inspections of all major wiring paths and connectors.
  
• Apply corrosion inhibitors to exposed electrical connections.
  
• Use dielectric grease on multi-pin connectors to reduce oxidation.
  
• Avoid pressure washing near electrical panels unless properly sealed.
  
• Train operators to report any odd behavior in the monitor cluster or crank delay immediately.
  

Conclusion: Chasing Voltage, Not Luck
Electrical problems in machines like the Volvo L120E often masquerade as intermittent ghosts. But with a systematic approach and basic tools like a multimeter, test light, and wiring diagram, even elusive faults can be tracked down. Machines that start when hotwired but not with the key are telling a story—one that can be deciphered by anyone willing to listen to the volts.