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The Bobcat 943 skid steer loader belongs to a generation of machines produced during the late 1980s and early 1990s, a period when Bobcat was rapidly expanding its product line and global market share. Bobcat, founded in North Dakota in the 1950s, became one of the world’s most recognized compact equipment manufacturers, with millions of skid steers sold worldwide. The 943 model was built for heavy lifting and demanding job‑site work, featuring a robust hydrostatic drive system that allowed precise control and strong pushing power.
As these machines age, however, hydrostatic performance issues become increasingly common. One recurring problem involves the left drive becoming weak after the hydraulic oil warms up. This article explores the symptoms, diagnostic logic, underlying causes, and practical solutions for this issue, while adding technical explanations, industry context, and real‑world stories.
Symptoms of a Weak Left Drive
According to the retrieved information, the machine operates normally for about ten minutes—until the hydraulic oil reaches operating temperature. Once warm, the left drive becomes noticeably weak, especially when pushing into a dirt pile. Under load, the left side may stop moving entirely, while reverse still functions but produces a squealing noise from the drive motor.
These symptoms point toward a hydraulic issue that worsens with heat, suggesting internal leakage or loss of pressure in the left‑side hydrostatic circuit.
Understanding the Hydrostatic Drive System
A skid steer like the Bobcat 943 uses a closed‑loop hydrostatic drive system, meaning hydraulic fluid circulates continuously between the pump and drive motor.
Terminology notes:
Initial Diagnosis and Motor Testing
A technician suggested monitoring charge pressure to determine whether the issue originated in the pump or the motor. The logic is straightforward:
Blocking Hydraulic Lines for Further Testing
To isolate the problem, the owner blocked off all hydraulic lines to the left side. With the lines capped, they tested forward and reverse movement using the control stick.
The results were revealing:
Possible Cause Internal Servo Leakage
A technician proposed that the servo mechanism inside the pump might be leaking internally, preventing the pump from reaching full stroke on the left side.
Terminology note:
Additional Clues From Control Arm Behavior
The owner observed that the right‑side actuator arm rotated easily, while the left side felt stiff, as if lacking hydraulic fluid on one side of the pump control mechanism.
This reinforces the servo‑leak theory: uneven servo pressure can cause one direction to respond sluggishly or fail under load.
A Story From the Field
A contractor once described a similar issue on a Case skid steer. The machine would drive strongly when cold but lose power on one side after warming up. After months of troubleshooting, the culprit turned out to be a worn servo piston seal inside the pump. Once replaced, the machine regained full power. This story highlights how internal pump components—often overlooked—can dramatically affect performance.
Why Heat Makes the Problem Worse
Hydraulic oil viscosity decreases as temperature rises. When oil becomes thinner:
Recommended Diagnostic Steps
Owners facing similar issues should consider the following:
Long‑Term Solutions
Depending on the severity of wear, solutions include:
The Bobcat 943 Legacy
The Bobcat 943 remains a respected model among skid steer enthusiasts. Its strong frame, reliable engine, and powerful hydraulics made it a workhorse in construction and agriculture. Even decades later, many units remain in service, a testament to Bobcat’s engineering. However, hydrostatic pumps are wear‑prone components, and issues like left‑drive weakness are common in machines of this age.
Conclusion
A weak left drive on a Bobcat 943—especially one that appears only after the hydraulic oil warms up—is typically caused by internal leakage within the hydrostatic pump, often involving the servo mechanism. By systematically testing charge pressure, isolating hydraulic lines, and evaluating actuator behavior, owners can accurately diagnose the issue. With proper repair, these machines can continue operating reliably for years to come.
As these machines age, however, hydrostatic performance issues become increasingly common. One recurring problem involves the left drive becoming weak after the hydraulic oil warms up. This article explores the symptoms, diagnostic logic, underlying causes, and practical solutions for this issue, while adding technical explanations, industry context, and real‑world stories.
Symptoms of a Weak Left Drive
According to the retrieved information, the machine operates normally for about ten minutes—until the hydraulic oil reaches operating temperature. Once warm, the left drive becomes noticeably weak, especially when pushing into a dirt pile. Under load, the left side may stop moving entirely, while reverse still functions but produces a squealing noise from the drive motor.
These symptoms point toward a hydraulic issue that worsens with heat, suggesting internal leakage or loss of pressure in the left‑side hydrostatic circuit.
Understanding the Hydrostatic Drive System
A skid steer like the Bobcat 943 uses a closed‑loop hydrostatic drive system, meaning hydraulic fluid circulates continuously between the pump and drive motor.
Terminology notes:
- Hydrostatic pump: A variable‑displacement pump that controls speed and direction by changing the angle of its swash plate.
- Drive motor: Converts hydraulic pressure into rotational motion to turn the wheels.
- Charge pressure: Low‑pressure oil supplied to keep the closed loop full and prevent cavitation.
- Servo: A hydraulic control mechanism that adjusts the pump swash plate angle.
Initial Diagnosis and Motor Testing
A technician suggested monitoring charge pressure to determine whether the issue originated in the pump or the motor. The logic is straightforward:
- If charge pressure drops when the left drive weakens, the pump is likely at fault.
- If charge pressure remains stable, the motor is more likely the problem.
Blocking Hydraulic Lines for Further Testing
To isolate the problem, the owner blocked off all hydraulic lines to the left side. With the lines capped, they tested forward and reverse movement using the control stick.
The results were revealing:
- Forward motion caused only minimal engine load.
- Reverse motion caused significantly more engine load, even with the lines blocked.
Possible Cause Internal Servo Leakage
A technician proposed that the servo mechanism inside the pump might be leaking internally, preventing the pump from reaching full stroke on the left side.
Terminology note:
- Pump stroke refers to the maximum displacement of the pump. If the pump cannot reach full stroke, it cannot deliver full hydraulic flow or pressure.
Additional Clues From Control Arm Behavior
The owner observed that the right‑side actuator arm rotated easily, while the left side felt stiff, as if lacking hydraulic fluid on one side of the pump control mechanism.
This reinforces the servo‑leak theory: uneven servo pressure can cause one direction to respond sluggishly or fail under load.
A Story From the Field
A contractor once described a similar issue on a Case skid steer. The machine would drive strongly when cold but lose power on one side after warming up. After months of troubleshooting, the culprit turned out to be a worn servo piston seal inside the pump. Once replaced, the machine regained full power. This story highlights how internal pump components—often overlooked—can dramatically affect performance.
Why Heat Makes the Problem Worse
Hydraulic oil viscosity decreases as temperature rises. When oil becomes thinner:
- Internal leakage increases
- Pressure drops
- Pump efficiency declines
- Weakness appears under load
Recommended Diagnostic Steps
Owners facing similar issues should consider the following:
- Monitor charge pressure during operation.
- Compare forward and reverse engine load with lines capped.
- Inspect servo pistons and seals for wear.
- Check actuator arm movement for stiffness or imbalance.
- Evaluate pump case drain flow (excessive flow indicates internal leakage).
- Inspect hydraulic oil condition and temperature.
Long‑Term Solutions
Depending on the severity of wear, solutions include:
- Rebuilding the hydrostatic pump
- Replacing servo seals and pistons
- Replacing worn actuator components
- Flushing the hydraulic system
- Upgrading to higher‑quality hydraulic oil
- Installing an auxiliary cooler if overheating is an issue
The Bobcat 943 Legacy
The Bobcat 943 remains a respected model among skid steer enthusiasts. Its strong frame, reliable engine, and powerful hydraulics made it a workhorse in construction and agriculture. Even decades later, many units remain in service, a testament to Bobcat’s engineering. However, hydrostatic pumps are wear‑prone components, and issues like left‑drive weakness are common in machines of this age.
Conclusion
A weak left drive on a Bobcat 943—especially one that appears only after the hydraulic oil warms up—is typically caused by internal leakage within the hydrostatic pump, often involving the servo mechanism. By systematically testing charge pressure, isolating hydraulic lines, and evaluating actuator behavior, owners can accurately diagnose the issue. With proper repair, these machines can continue operating reliably for years to come.
