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Overview of the Problem
A Caterpillar 955L track loader can sometimes present a baffling issue where the machine still drives normally, but all bucket and boom functions stop working at once. The travel drivetrain is fine, yet the loader arms and bucket are completely dead. This kind of total loss of implement hydraulics points to a failure in the common hydraulic supply for the loader system rather than a problem in a single valve or cylinder.
On the 955L, the implement hydraulics are powered by a front-mounted pump driven from the torque converter housing. When that pump stops producing pressure, every loader function disappears together. Understanding how this pump is driven, what can fail inside it, and how to diagnose the system step by step is the key to getting such a machine back to work.
The 955L Track Loader and Its Hydraulic System
The Cat 955 series dates back to the 1960s and 1970s as Caterpillar’s mid-size track loader line, sitting between smaller 951 models and heavier 977 loaders. The 955L variant was an evolution with improved power, better operator comfort, and refined hydraulics. These machines saw extensive use in road building, quarry work, and general earthmoving. Tens of thousands of 955-series loaders were produced across different sub-models, and many are still working around the world, especially in markets where older machines are kept in service for decades.
On a typical 955L with a 3304 engine and a torque converter transmission, the core hydraulic elements include:
Initial Symptoms and Field Clues
In the case discussed, the operator reported:
Key Diagnostic Steps for a Dead Implement System
When a 955L suddenly loses all boom and bucket controls, a systematic diagnostic approach keeps you from blindly replacing expensive parts.
Recommended sequence:
Verifying Oil Supply to the Pump
The suction side of the pump is critical. If the pump ingests air or is starved of oil, it will not build pressure, and cavitation can quickly damage internal components. Typical checks include:
In this case, the suction pipe was confirmed to be supplying oil, which shifts suspicion away from the tank and suction circuit and toward the pump itself.
Confirming Pump Drive and External Coupling
On the 955L, the implement pump is bolted to the front right side of the torque converter housing and is driven via splines or a gear off the converter. Common external drive issues include:
Internal Pump Failures in Gear and Vane Designs
Once suction flow, reservoir level, and external drive are confirmed, the likely culprit becomes an internal pump failure. Two broad pump styles are commonly used on machines of this era:
Typical internal failure modes:
Case Example Pump Inspection and Likely Outcome
In the scenario that inspired this discussion, flow at the pump outlet was just a small trickle when a pressure line was loosened. Suction was confirmed, drive splines were good, and there were no obvious external leaks or strange noises. The strongly suspected root cause was a broken shaft or a severely damaged internal cartridge.
A mechanic on a similar machine reported a vane pump shaft that looked as if it had been cut with a saw, broken just inside the housing. Another technician noted that Vickers-style vane cartridges and shafts are known to fail this way on aging excavators and loaders, especially when they see dirty oil, repeated cavitation, or high shock loads.
One reported parts price comparison showed:
Relief Valve and Venting Considerations
Although pump damage was the most likely culprit in this case, it is still important to rule out simpler causes when diagnosing a similar failure:
The Cost and Logistics Challenge for Remote Operations
In this story, the loader was working roughly 200 kilometers from the workshop, in a remote bush area with poor phone coverage. That reality shapes how a problem is tackled:
Even though some failures are sudden, several practices can reduce the chance of catastrophic pump damage on older loaders like the 955L:
A Short Field Story
One contractor running older track loaders in a remote quarry adopted a simple rule: every time a machine came into the yard for more than a day, the crew checked suction hoses, cleaned tank vents, and pulled hydraulic filters for inspection. On one occasion they found a suction hose on a loader starting to collapse internally. Replacing it cost relatively little. The filter showed signs of cavitation damage, and if they had ignored it, the pump likely would have failed in the next season, taking thousands of dollars and weeks of downtime with it.
Stories like this parallel the experience with the 955L: what looks like an expensive failure in the middle of nowhere often has roots in small, easily missed items—tank vents, hoses, contaminated oil—that quietly created stress on the pump for months.
Practical Checklist for a 955L with No Loader Controls
For owners and mechanics facing a similar problem on a Cat 955L, the following checklist can serve as a quick reference:
Conclusion
A Cat 955L that suddenly loses all bucket and boom functions while still driving normally almost always points to a problem in the implement hydraulic pump or its supply. Once suction flow, reservoir level, and tank venting are confirmed, attention should turn to the pump drive and internal components. Broken shafts, worn or cracked vane cartridges, and occasionally stuck relief valves are the main suspects.
Given the age of most remaining 955L loaders, owners must balance repair cost against machine value, often relying on aftermarket pump parts or remanufactured units. With systematic diagnostics and sensible maintenance, however, many of these track loaders can continue working productively, even in remote and demanding environments.
A Caterpillar 955L track loader can sometimes present a baffling issue where the machine still drives normally, but all bucket and boom functions stop working at once. The travel drivetrain is fine, yet the loader arms and bucket are completely dead. This kind of total loss of implement hydraulics points to a failure in the common hydraulic supply for the loader system rather than a problem in a single valve or cylinder.
On the 955L, the implement hydraulics are powered by a front-mounted pump driven from the torque converter housing. When that pump stops producing pressure, every loader function disappears together. Understanding how this pump is driven, what can fail inside it, and how to diagnose the system step by step is the key to getting such a machine back to work.
The 955L Track Loader and Its Hydraulic System
The Cat 955 series dates back to the 1960s and 1970s as Caterpillar’s mid-size track loader line, sitting between smaller 951 models and heavier 977 loaders. The 955L variant was an evolution with improved power, better operator comfort, and refined hydraulics. These machines saw extensive use in road building, quarry work, and general earthmoving. Tens of thousands of 955-series loaders were produced across different sub-models, and many are still working around the world, especially in markets where older machines are kept in service for decades.
On a typical 955L with a 3304 engine and a torque converter transmission, the core hydraulic elements include:
- A front-mounted gear or vane pump driven from the torque converter housing
- A suction line from the hydraulic reservoir (tank) to the pump
- A pressure line from the pump to the control valve bank
- Main relief valves controlling maximum system pressure
- Loader boom and bucket cylinders supplied by directional valves
- A return line and filters back to the reservoir
Initial Symptoms and Field Clues
In the case discussed, the operator reported:
- Machine still moves and drives properly
- No response from boom or bucket controls
- Machine had been operating normally just before the failure
- No major leaks observed
- Hydraulic oil level appears normal
- No clearly audible strange noises from the pump
- Since travel is fine, the torque converter and engine are functioning.
- Total loss of all implement functions suggests a supply problem (pump or suction), not just a single valve or cylinder fault.
- Absence of major leaks and normal reservoir level indicate the oil is still in the system rather than dumping out through a burst hose.
Key Diagnostic Steps for a Dead Implement System
When a 955L suddenly loses all boom and bucket controls, a systematic diagnostic approach keeps you from blindly replacing expensive parts.
Recommended sequence:
- Check hydraulic reservoir level
- Inspect for obvious leaks and damaged lines
- Confirm suction line integrity
- Verify the pump is being driven and actually turning
- Check for blocked intake or faulty tank vent
- Test for oil flow and pressure at the pump outlet
- Inspect the pump internally for shaft or cartridge failure
- Consider relief valve stuck fully open only after confirming pump output
Verifying Oil Supply to the Pump
The suction side of the pump is critical. If the pump ingests air or is starved of oil, it will not build pressure, and cavitation can quickly damage internal components. Typical checks include:
- Ensure reservoir is filled to the recommended level on the sight gauge or dipstick.
- Crack open the suction pipe connection at the pump and see if oil flows freely by gravity.
- Inspect the suction hose for collapse, delamination, or severe kinks.
- Check internal suction strainers or screens for blockage.
- Inspect the tank cap and vent: if the vent is plugged, a vacuum can develop in the reservoir and prevent oil from feeding the pump.
In this case, the suction pipe was confirmed to be supplying oil, which shifts suspicion away from the tank and suction circuit and toward the pump itself.
Confirming Pump Drive and External Coupling
On the 955L, the implement pump is bolted to the front right side of the torque converter housing and is driven via splines or a gear off the converter. Common external drive issues include:
- Worn or stripped splines in the pump shaft
- Damaged drive gear or coupling pucks
- Misalignment between pump and torque converter housing
- Inspect the splines on the pump shaft for rounding or stripping.
- Inspect the mating drive inside the housing.
- Rotate the engine and observe whether the pump shaft turns.
Internal Pump Failures in Gear and Vane Designs
Once suction flow, reservoir level, and external drive are confirmed, the likely culprit becomes an internal pump failure. Two broad pump styles are commonly used on machines of this era:
- Gear pumps
- Vane pumps (such as Vickers cartridge-type units)
Typical internal failure modes:
- Broken pump shaft inside the housing
The shaft can shear just inside the pump body, leaving the external stub looking perfect. Externally it appears to be driven, but internally the gears or vanes have stopped turning.
- Severely worn gear teeth or vane cartridge
High internal leakage allows oil to recirculate inside the pump instead of being pushed into the pressure line. This often shows up as low pressure and poor performance rather than a sudden total failure, but a catastrophic mechanical failure can be abrupt.
- Cracked vane housing or damaged rotor
In vane pumps, the cartridge assembly (rotor, vanes, and ring) can break, destroying pump efficiency and sometimes jamming tolerances enough that flow nearly disappears.
- Relief valve stuck fully open
While often suspected, a wide-open main relief valve usually still allows some measurable pressure at the pump outlet. When the outlet shows only a trickle and no real pressure, it is more likely the pump itself is failing to move oil.
Case Example Pump Inspection and Likely Outcome
In the scenario that inspired this discussion, flow at the pump outlet was just a small trickle when a pressure line was loosened. Suction was confirmed, drive splines were good, and there were no obvious external leaks or strange noises. The strongly suspected root cause was a broken shaft or a severely damaged internal cartridge.
A mechanic on a similar machine reported a vane pump shaft that looked as if it had been cut with a saw, broken just inside the housing. Another technician noted that Vickers-style vane cartridges and shafts are known to fail this way on aging excavators and loaders, especially when they see dirty oil, repeated cavitation, or high shock loads.
One reported parts price comparison showed:
- Aftermarket cartridge (for a similar vane pump) around 200 USD
- OEM cartridge from a dealer quoted around 1000 USD
Relief Valve and Venting Considerations
Although pump damage was the most likely culprit in this case, it is still important to rule out simpler causes when diagnosing a similar failure:
- Main relief valve stuck open
Contamination can wedge a relief valve open, dumping oil directly back to tank. You will usually still see a healthy flow at the pump outlet, but little system pressure. Installing a pressure gauge on the main pressure line is the fastest way to confirm.
- Defective tank cap vent
As mentioned, a plugged vent can prevent oil from feeding the pump. If breaking the vacuum by loosening the cap restores function, the cap or vent needs replacement or cleaning.
The Cost and Logistics Challenge for Remote Operations
In this story, the loader was working roughly 200 kilometers from the workshop, in a remote bush area with poor phone coverage. That reality shapes how a problem is tackled:
- Mechanics may need to travel long distances with limited tools and spare parts.
- Removing the pump and transporting it back to the yard is often more practical than trying to fully disassemble it in the field.
- Parts pricing and availability from the local dealer may be significantly higher than from international aftermarket suppliers, prompting careful cost-benefit analysis.
- Remove the suspected pump
- Tear it down in a controlled shop environment
- Decide whether to:
- Rebuild with a new shaft or cartridge
- Replace with a remanufactured or aftermarket pump
- Replace the entire machine if repair cost exceeds machine value
- Rebuild with a new shaft or cartridge
Even though some failures are sudden, several practices can reduce the chance of catastrophic pump damage on older loaders like the 955L:
- Maintain clean hydraulic oil
- Change oil and filters on schedule.
- Sample oil periodically and check for particles and water contamination.
- Change oil and filters on schedule.
- Inspect suction hoses and clamps regularly
- Replace softened, kinked, or delaminated hoses.
- Make sure clamps are tight but not crushing the hose.
- Replace softened, kinked, or delaminated hoses.
- Ensure proper tank venting
- Clean or replace vented caps as part of routine service.
- Clean or replace vented caps as part of routine service.
- Avoid extended cavitation
- Do not run implements when the reservoir is low.
- Address any whine or growl from the pump promptly.
- Do not run implements when the reservoir is low.
- Monitor system pressure
- Occasionally check main pressure with a gauge to detect a dropping trend before total failure.
- Occasionally check main pressure with a gauge to detect a dropping trend before total failure.
- Use appropriate oil
- Follow Caterpillar’s viscosity and specification recommendations for ambient conditions to maintain lubrication and pump life.
- Follow Caterpillar’s viscosity and specification recommendations for ambient conditions to maintain lubrication and pump life.
A Short Field Story
One contractor running older track loaders in a remote quarry adopted a simple rule: every time a machine came into the yard for more than a day, the crew checked suction hoses, cleaned tank vents, and pulled hydraulic filters for inspection. On one occasion they found a suction hose on a loader starting to collapse internally. Replacing it cost relatively little. The filter showed signs of cavitation damage, and if they had ignored it, the pump likely would have failed in the next season, taking thousands of dollars and weeks of downtime with it.
Stories like this parallel the experience with the 955L: what looks like an expensive failure in the middle of nowhere often has roots in small, easily missed items—tank vents, hoses, contaminated oil—that quietly created stress on the pump for months.
Practical Checklist for a 955L with No Loader Controls
For owners and mechanics facing a similar problem on a Cat 955L, the following checklist can serve as a quick reference:
- Machine drives normally but no bucket or boom movement
- Verify hydraulic oil level in reservoir
- Look carefully for leaks or burst hoses
- Confirm reservoir vent is open; test briefly with cap loosened
- Loosen suction line at pump and confirm free oil flow
- Inspect suction hose condition along its full length
- Confirm pump is being driven and the external shaft turns with the engine
- Loosen a pressure line at the pump outlet and check for strong flow
- Install a gauge and measure system pressure if possible
- If flow is only a trickle and pressure is near zero:
- Remove pump
- Inspect shaft inside housing for breakage
- Inspect gears or vane cartridge for severe damage
- Remove pump
- Evaluate repair vs. replacement using OEM vs. aftermarket parts
Conclusion
A Cat 955L that suddenly loses all bucket and boom functions while still driving normally almost always points to a problem in the implement hydraulic pump or its supply. Once suction flow, reservoir level, and tank venting are confirmed, attention should turn to the pump drive and internal components. Broken shafts, worn or cracked vane cartridges, and occasionally stuck relief valves are the main suspects.
Given the age of most remaining 955L loaders, owners must balance repair cost against machine value, often relying on aftermarket pump parts or remanufactured units. With systematic diagnostics and sensible maintenance, however, many of these track loaders can continue working productively, even in remote and demanding environments.
