Why Do CAT 988G and 988H Loaders Stop Hoisting When Curling Simultaneously

[MikePhua]

Legacy of the CAT 988 Series
The Caterpillar 988 series wheel loaders have long been a cornerstone of heavy-duty material handling in mining, quarrying, and large-scale construction. First introduced in 1963, the 988 quickly became a flagship model for CAT, known for its ruggedness and high breakout force. The 988G, launched in the late 1990s, and its successor the 988H, released in the mid-2000s, brought electronic controls, improved hydraulics, and emissions-compliant engines to the platform. Powered by the CAT 3406E and later the C18 ACERT engine, these loaders deliver over 500 horsepower and operate with a bucket capacity ranging from 7.5 to 12 cubic yards.
Caterpillar Inc., founded in 1925, has sold tens of thousands of 988 units globally, with the G and H variants dominating markets in North America, Australia, and China. Their reputation for reliability is matched by their complexity, especially in hydraulic systems that integrate load-sensing pumps, proportional valves, and electronic feedback loops.
The Problem of Hydraulic Interruption
Operators of two 988H units reported a puzzling issue: the machines could raise the boom or curl the bucket independently, but not simultaneously. When the boom was lifting and the operator attempted to curl the bucket, the boom would abruptly stop. Releasing the tilt lever allowed the boom to resume its motion. This behavior suggested a hydraulic prioritization conflict or flow starvation.
Initial diagnostics showed that the pump was maintaining high pressure—up to 4800 psi at the top of the boom stroke—but pressure dropped sharply to 1100 psi when the tilt function was engaged. This indicated that while pressure was available, flow might be insufficient to support dual-function operation.
Terminology Annotation

• Boom Hoist: The hydraulic function responsible for raising and lowering the loader arms.
  
• Bucket Curl: The action of rotating the bucket to scoop or dump material.
  
• Proportional Solenoid Valve: An electronically controlled valve that modulates hydraulic flow based on input signals.
  
• Cylinder Drift: Unintended movement of hydraulic cylinders due to internal leakage or valve failure.
  
• Ride Control: A system that dampens loader movement during travel to reduce bounce and improve operator comfort.
  

Sensor Calibration and Valve Logic
Both machines had undergone sensor calibration, including linkage sensors and control lever position sensors. These calibrations are critical for the electronic control module (ECM) to interpret operator input and modulate hydraulic flow accordingly. If a cylinder drifts during calibration, the system aborts the process and flags a fault. In this case, calibration completed successfully, ruling out drift or sensor misalignment.
However, the issue persisted. Technicians speculated that the hydraulic schematic routed flow first to the tilt function, then to the hoist. If the tilt circuit demanded high flow, it could starve the hoist circuit, especially if the pump was not fully upstroking.
Pump Performance and Flow Testing
The CAT 988H uses a variable displacement piston pump, which adjusts output based on demand. A pump performance test was recommended to verify whether the pump was achieving full stroke. According to factory specs:

• Boom lift from fully lowered to kickout should take approximately 8.5 seconds at high idle.
  
• Bucket rackback to full dump should complete in about 3.0 seconds.
  

If these benchmarks are not met, the pump may be underperforming due to wear, contamination, or electronic miscommand.
In one case from Victoria, Australia, a technician discovered that a loader’s pump was only partially stroking due to a faulty pressure sensor. Replacing the sensor restored full flow and resolved simultaneous function issues.
Pressure Relief and System Balance
The relief valve was tested at 4800 psi, slightly above the factory spec of 4500 ± 90 psi. While high pressure is not inherently problematic, it can mask flow deficiencies. Using a manual gauge, especially one not recently calibrated, introduces error. A better method is to power down the hydraulics and test static pressure, reducing stress on cylinder head bolts.
The sharp pressure drop when engaging tilt suggests that the tilt circuit may be consuming disproportionate flow. This could be due to:

• Internal leakage in tilt cylinders
  
• Malfunctioning flow divider or priority valve
  
• Solenoid valve not modulating correctly
  

Cylinder Rebuilds and Timing
Both tilt and hoist cylinders had been rebuilt, but the issue began prior to this work. This rules out assembly errors or seal failures as root causes. However, rebuilt cylinders should always be pressure-tested and verified for internal bypass before installation.
Ride Control and Hydraulic Isolation
Neither machine was equipped with ride control, eliminating one potential interference. In other cases, ride control systems have been known to introduce hydraulic anomalies, especially if accumulators are leaking or control valves are stuck.
Oil Analysis and Contamination
Oil samples from both machines showed acceptable particle counts and viscosity, though one sample had elevated silicon levels (18 ppm), which could indicate dirt ingress or seal degradation. While not immediately alarming, such contamination can affect valve performance and pump wear over time.
In a Nevada copper mine, a 988H suffered similar symptoms. After extensive troubleshooting, technicians discovered that a contaminated proportional valve was sticking intermittently, causing erratic flow prioritization. Flushing the system and replacing the valve resolved the issue.
Recommendations and Solutions
To address simultaneous function failure in CAT 988G and 988H loaders, the following steps are advised:

• Perform a pump performance test using factory timing benchmarks
  
• Verify relief valve pressure using calibrated equipment with hydraulics powered down
  
• Inspect proportional solenoid valves for contamination or wear
  
• Confirm full calibration of control levers and linkage sensors
  
• Check for internal leakage in tilt cylinders using bypass testing
  
• Review hydraulic schematic for flow prioritization logic
  
• Consider replacing pressure sensors if readings are inconsistent
  
• Conduct oil analysis quarterly to monitor contamination trends
  

Final Thoughts
The CAT 988G and 988H are complex machines with integrated hydraulic and electronic systems. When simultaneous functions fail, the issue often lies in flow management rather than pressure availability. By combining sensor calibration, pump testing, and contamination control, technicians can restore full functionality and maintain the legendary performance these loaders are known for.
In the end, solving hydraulic puzzles like this is not just about parts—it’s about understanding the dance between pressure, flow, and control. And for those who master it, the reward is a machine that lifts, curls, and conquers with precision.