08-26-2025, 06:04 PM
The Bobcat 743 and Its Role in Compact Equipment History
The Bobcat 743 skid steer loader was introduced in the early 1980s by Melroe Company, which later became Bobcat Company under Ingersoll Rand and eventually Doosan. As part of the 700 series, the 743 was a mid-frame machine powered by a Kubota V1702 diesel engine and equipped with a gear-type hydraulic pump. It quickly became a favorite among landscapers, contractors, and farmers for its reliability and maneuverability. With over 50,000 units sold globally, the 743 helped solidify Bobcat’s dominance in the compact loader market.
Its simple mechanical layout, open-loop hydraulic system, and rugged steel frame made it easy to maintain and modify. However, as these machines age, hydraulic issues—especially those triggered by heat—become increasingly common.
Symptoms of Hydraulic Failure After Warm-Up
A recurring issue with older Bobcat 743 units is a sudden loss of hydraulic function after 10–15 minutes of operation. The machine starts and runs smoothly, but once it reaches operating temperature, the hydraulics begin to squeal loudly, and functions like lift, tilt, and steering become sluggish or unresponsive.
Operators often describe the sound as a high-pitched squeal, similar to a belt slipping or a pump cavitating. The loader may stop lifting or tilting altogether, and steering becomes erratic. These symptoms typically point to hydraulic starvation, cavitation, or relief valve malfunction.
Terminology Clarification
Several factors can contribute to this failure pattern:
In Pennsylvania, a Bobcat 743 owner reported that his machine ran fine for ten minutes before squealing and losing lift. After replacing the hydraulic filter and switching to Rotella 15W-40 motor oil—once recommended by Bobcat for older models—the symptoms persisted. Upon inspection, the suction hose had shrunk, loosening the clamp and allowing air into the system. Replacing the hose and resealing the connection resolved the issue.
In Indiana, a contractor experienced similar symptoms after resealing the tilt cylinder. Air trapped in the system caused erratic movement and squealing. He performed a relief purge by holding the boom and tilt functions over relief repeatedly, which eventually bled the air and restored normal operation.
Recommended Diagnostic Steps
The Bobcat 743 remains a dependable workhorse, but its aging hydraulic system requires careful attention—especially when symptoms appear only after warm-up. Squealing noises and loss of function are often signs of cavitation, suction leaks, or trapped air. With proper diagnostics and a few targeted upgrades, these machines can continue serving reliably for decades. Understanding the quirks of vintage hydraulic systems is key to keeping them alive and productive.
The Bobcat 743 skid steer loader was introduced in the early 1980s by Melroe Company, which later became Bobcat Company under Ingersoll Rand and eventually Doosan. As part of the 700 series, the 743 was a mid-frame machine powered by a Kubota V1702 diesel engine and equipped with a gear-type hydraulic pump. It quickly became a favorite among landscapers, contractors, and farmers for its reliability and maneuverability. With over 50,000 units sold globally, the 743 helped solidify Bobcat’s dominance in the compact loader market.
Its simple mechanical layout, open-loop hydraulic system, and rugged steel frame made it easy to maintain and modify. However, as these machines age, hydraulic issues—especially those triggered by heat—become increasingly common.
Symptoms of Hydraulic Failure After Warm-Up
A recurring issue with older Bobcat 743 units is a sudden loss of hydraulic function after 10–15 minutes of operation. The machine starts and runs smoothly, but once it reaches operating temperature, the hydraulics begin to squeal loudly, and functions like lift, tilt, and steering become sluggish or unresponsive.
Operators often describe the sound as a high-pitched squeal, similar to a belt slipping or a pump cavitating. The loader may stop lifting or tilting altogether, and steering becomes erratic. These symptoms typically point to hydraulic starvation, cavitation, or relief valve malfunction.
Terminology Clarification
- Cavitation: A condition where air bubbles form in hydraulic fluid due to low pressure, causing noise and damage.
- Relief Valve: A safety valve that limits maximum pressure in the hydraulic system to prevent damage.
- Charge Pressure: The baseline pressure supplied to hydraulic pumps to ensure proper function.
- Suction Hose: The hose that feeds hydraulic fluid from the tank to the pump; critical for maintaining flow.
Several factors can contribute to this failure pattern:
- Clogged Hydraulic Filter: A restricted filter reduces flow and causes the pump to cavitate. Older filters may trap debris or degrade internally.
- Shrunken Suction Hose: Over time, the suction hose can shrink or harden, causing poor seal at the clamps and allowing air to enter the system.
- Foamy Hydraulic Fluid: Air contamination leads to frothy fluid, reducing lubrication and pressure.
- Stuck Relief Valve: A partially open relief valve bleeds off pressure, especially when hot fluid thins out.
- Incorrect Fluid Type: Using ISO 32 hydraulic fluid instead of recommended motor oil (e.g., Rotella 15W-40) can affect viscosity and pump performance in older machines.
In Pennsylvania, a Bobcat 743 owner reported that his machine ran fine for ten minutes before squealing and losing lift. After replacing the hydraulic filter and switching to Rotella 15W-40 motor oil—once recommended by Bobcat for older models—the symptoms persisted. Upon inspection, the suction hose had shrunk, loosening the clamp and allowing air into the system. Replacing the hose and resealing the connection resolved the issue.
In Indiana, a contractor experienced similar symptoms after resealing the tilt cylinder. Air trapped in the system caused erratic movement and squealing. He performed a relief purge by holding the boom and tilt functions over relief repeatedly, which eventually bled the air and restored normal operation.
Recommended Diagnostic Steps
- Check hydraulic fluid level and condition. Look for foam or discoloration.
- Replace the hydraulic filter and inspect the old one for metal or rubber debris.
- Inspect the suction hose for shrinkage, cracks, or loose clamps.
- Perform a relief purge: hold boom and tilt functions over relief for several seconds in both directions.
- Check charge pressure at the pump using a gauge. Low pressure may indicate suction leak or pump wear.
- Inspect relief valve for sticking or contamination. Clean or replace as needed.
- Use motor oil (15W-40) in older Bobcat 743s unless upgraded to modern hydraulic fluid.
- Replace suction hoses every 5 years or sooner if signs of hardening appear.
- Flush hydraulic system every 1,000 hours or after major component replacement.
- Keep radiator and oil cooler clean to prevent overheating.
- Monitor for seat-of-the-pants vibration, which may indicate pump cavitation or air ingress.
- Upgrade to modern hydraulic hose with reinforced lining and heat-resistant clamps.
- Install a transparent sight gauge on the hydraulic tank to monitor fluid clarity and level.
- Add a temperature sensor to the hydraulic reservoir to track overheating trends.
- Use a digital pressure gauge to monitor charge pressure during operation.
The Bobcat 743 remains a dependable workhorse, but its aging hydraulic system requires careful attention—especially when symptoms appear only after warm-up. Squealing noises and loss of function are often signs of cavitation, suction leaks, or trapped air. With proper diagnostics and a few targeted upgrades, these machines can continue serving reliably for decades. Understanding the quirks of vintage hydraulic systems is key to keeping them alive and productive.
