Clark Ranger 666B Starting Problems and Electrical Workarounds

[MikePhua]

The 666B and Clark’s Skidder Legacy
The Clark Ranger 666B was part of a series of purpose-built logging skidders produced during the 1960s and 1970s by Clark Equipment Company. Known for their rugged frames, powerful Detroit Diesel engines, and straightforward mechanical systems, these machines were designed to thrive in harsh forestry environments. The 666B typically came equipped with a Detroit 4-71 two-stroke diesel, delivering reliable torque and a distinctive sound that became synonymous with logging operations across North America.
Clark Equipment, founded in 1903, was a major player in the development of off-road and industrial machinery. The Ranger series was especially popular in Canada and the northern United States, where cold starts and remote repairs were part of daily life. The 666B’s mechanical simplicity made it a favorite among operators who valued field-serviceable systems over electronic complexity.
Common Starting Issues and Electrical Symptoms
Owners of the 1968 Clark Ranger 666B often report the following starting problems:

• Clicking sound when pressing the starter button
  
• Starter engages only intermittently
  
• Batteries fully charged but no crank
  
• Starter works when solenoid terminals are manually bridged
  
• System resets temporarily after manual override
  

These symptoms point to a weak or failing starter solenoid, poor grounding, or degraded wiring. The original solenoid design used in the 666B is often non-standard and may include a remote relay mounted near the cab or firewall. Over time, corrosion, vibration, and thermal cycling can degrade contact surfaces and reduce current flow.
A logger in Northern Ontario described manually bridging the solenoid terminals while someone held the starter button—this workaround temporarily restored function, confirming that the starter motor itself was operational and the fault lay in the control circuit.
Electrical Layout and Solenoid Behavior
The 666B’s starting system includes:

• Dual 12V batteries wired in parallel
  
• Starter motor with integrated solenoid
  
• Remote relay or secondary solenoid near the cab
  
• Starter button wired through ignition circuit
  
• Grounding strap from engine block to frame
  

When the starter button is pressed, it energizes the remote solenoid, which in turn sends current to the starter solenoid. If either solenoid fails to engage fully, the starter motor receives insufficient current and produces only a click.
Key inspection points:

• Voltage drop across solenoid terminals during cranking
  
• Resistance in starter button circuit
  
• Ground continuity between engine and battery
  
• Corrosion or looseness at terminal lugs
  

A technician in Alberta found that the remote solenoid on a 666D had oxidized terminals and a weak coil. Replacing it with a modern 12V relay and upgrading the wire gauge restored reliable starting.
Cold Weather Starting and Block Heater Options
In subzero conditions, the Detroit 4-71 engine can be difficult to start without supplemental heat. Inline coolant heaters are commonly installed, but may not be sufficient to warm the block evenly. Options include:

• Freeze plug-style block heaters (if compatible with casting)
  
• Oil pan heaters to reduce viscosity
  
• Battery warmers to maintain cranking amperage
  
• Glow plug retrofit kits (rare for 2-stroke Detroits)
  

A forestry crew in Alaska installed a 1,000-watt lower radiator hose heater on their 666B. While it improved coolant temperature, air locks in the system prevented full circulation. After bleeding the system and adding a secondary heater near the oil pan, cold starts became more reliable.
Cooling System Air Locks and Thermostat Behavior
After coolant replacement, air pockets can form in the system, especially in machines with long hose runs and high-mounted radiators. Symptoms include:

• Engine stalls at idle despite normal temperature readings
  
• Radiator cap removed but no flow observed
  
• Thermostat fails to open due to trapped air
  
• Heater core remains cold
  

Solutions:

• Run engine with radiator cap off until thermostat opens
  
• Squeeze upper and lower hoses to dislodge bubbles
  
• Install bleed valve at high point in system
  
• Use vacuum fill tool to pre-load coolant without air
  

A contractor in Finland noted that his 666B stalled after 30 minutes of idling. The temperature gauge read 100°C, but the thermostat hadn’t opened. After manually bleeding the system and replacing the thermostat, the machine ran smoothly.
Workarounds and Field Modifications
Operators have developed creative solutions to bypass unreliable starting circuits:

• Install push-button starter directly wired to starter solenoid
  
• Use 8-gauge wire for reduced voltage drop
  
• Add relay between starter button and solenoid for stronger signal
  
• Replace original solenoids with modern sealed units
  

A logger in British Columbia added a dashboard-mounted push button wired directly to the starter solenoid. This bypassed the remote relay and eliminated intermittent starting issues. The modification was paired with a fuse and indicator light for safety.
Stories from the Field
In Michigan, a 666B used for hardwood skidding developed a no-start condition during winter. The operator traced the issue to a cracked solenoid housing. After replacing it with a marine-grade unit and adding a battery warmer, the machine started reliably even in –20°F.
In Sweden, a forestry team retrofitted their 666B with a dual-solenoid system and upgraded wiring harness. The modification improved cold-weather reliability and reduced starter wear. The machine continued operating in mountainous terrain with minimal downtime.
Conclusion
Starting issues in the 1968 Clark Ranger 666B are often rooted in aging solenoids, weak wiring, and cold-weather challenges. With targeted diagnostics and practical upgrades, operators can restore dependable performance and extend the life of this legendary skidder. In the woods, where downtime costs more than parts, a reliable start is the first step toward a productive day.