10-16-2025, 11:43 PM
A classic problem afflicting older wheel loaders—especially the CAT 950B—is the sudden loss of tractive power after running normally for some time. The machine works fine when cold, but a couple of hours later, it “won’t pull” in either forward or reverse. It’s not a slipping transmission and not a gross engine failure; the loader simply seems to “lock up” until it cools, then returns to working order—until the cycle repeats.
Here’s a detailed exploration of that issue: what likely causes it, how technicians diagnosed it in the field, what fixes work, and additional considerations for heavy equipment repairers and owners.
Symptom Description and Early Observations
Likely Causes & Diagnostic Paths
From field accounts and repair specialists’ input, several plausible culprits exist. The community of equipment rebuilders often narrows the cause by process of elimination:
Repair Process & Best Practices
Based on that case and broader repair wisdom, here’s a methodical approach to diagnose and rectify the problem:
Context: CAT 950B and its Legacy
The Caterpillar 950B is part of the line of mid-sized wheel loaders built during earlier decades. These machines found use in general construction, aggregate handling, quarries, and smaller mining operations. Over time, many units remained in service decades beyond their original design life—testament to robust mechanical design but also a source of maintenance challenges.
Caterpillar Inc., founded in the early 20th century, became a dominant name in heavy equipment. The 950 series has had many variants (950, 950B, 950C, etc.), each with improvements in power, hydraulics, and transmission systems. Because many B-series units are now operated well past 30–40 years old, wear, seal degradation, and parts fatigue are common. Owners often retrofit, rebuild, or cannibalize from donor machines.
In heavy equipment circles, stories of intermittent failure like this 950B power loss are not rare. One news account from an industrial yard in South Africa described a loader that would lose mobility after prolonged grading, and mechanics eventually traced it back to failing internal converter seals. The pattern matches the classic symptom trajectory: strong when cold, loss when hot, recovery after cooldown.
Given the vintage of many such machines, not all replacement parts are readily stocked. Owners must sometimes fabricate O-rings, reseal kits, or source NOS (new old stock) parts. Maintaining documentation and using upgrade-grade sealing materials (e.g. high-temperature elastomers, Viton, or equivalent compounds) helps future reliability.
A Short Anecdote from the Field
At a gravel pit in Ontario, Canada, a contractor was running a 950B loader reclaiming material from spoil piles. After about two hours of continuous loading, the machine ceased moving under load—even in its lowest gear, the wheels spun but went nowhere. The operator shut down, allowed it to cool for 30 minutes, and restarted—motion returned, only to drop off again after another hour.
A technician traced the problem to the torque converter control circuit. They removed the valve body, discovered two marginal O-rings similar to the case above, and replaced them with better sealing elements rated for thermal stress. After reassembly, the loader ran reliably all day under full load without further power loss. The operator was relieved—downtime from repeated failure had been eating profit margins.
Summary
The recurring power loss issue in a CAT 950B loader—working fine when cold but collapsing under load after heating—is often due to leakage in hydraulic control circuits, especially within the torque converter’s valve body. Failing O-rings inside that valve assembly are common suspects. Diagnosing involves pressure testing, disassembly, inspection, and replacement of seals. Supporting tasks include assuring pump health, flow paths, cooling, and oil condition. In older heavy machines, maintaining internal sealing integrity is often what separates steady uptime from frustrating recurring failures.
Here’s a detailed exploration of that issue: what likely causes it, how technicians diagnosed it in the field, what fixes work, and additional considerations for heavy equipment repairers and owners.
Symptom Description and Early Observations
- The loader starts and operates normally for perhaps one to two hours.
- Then tractive force vanishes: forward or reverse motion is lost, regardless of gear selection.
- It feels as though brakes are engaged, but brakes are known to be fine.
- The engine stays strong, and hydraulic functions (lift, tilt) often still respond, indicating the prime mover (engine) is capable.
- The transmission doesn’t appear to be slipping, because throttle does not cause runaway RPMs.
- After a cooldown period, power returns and the loader runs again—until, after more runtime, the same failure recurs.
Likely Causes & Diagnostic Paths
From field accounts and repair specialists’ input, several plausible culprits exist. The community of equipment rebuilders often narrows the cause by process of elimination:
- Valve body or hydraulic control leaks: In one resolved case, technicians pulled the valve body of the torque converter or converter control circuit. Inside, they found a plate holding springs and valves; two O-rings sealing pressure paths were failing—one cracked, the other flattened and leaking. Replacing those seals restored proper control, and the loader resumed normal function after cooling.
- Heat-driven viscosity and pump capacity drop: As oil warms, its viscosity decreases. Pumps, charges, or hydraulic circuits may no longer maintain required pressures when the oil thins, causing internal slippage or loss of torque multiplication in the torque converter.
- Weak or failing charge pump: A compromised charge (feed) pump may be adequate when the system is cold but fail to maintain pressure at elevated oil temperatures.
- Suction leak or intake restriction: If the pump draws cavitation or loses prime under load or heat, flow can drop.
- Internal leakage in clutches / torque converter circuits: Seals and internal pathways can degrade, leading to internal bypass rather than proper engagement.
- Overheating or thermal protection triggers: Some systems may incorporate protective features that reduce power when temperatures exceed thresholds.
- Restricted oil passages or clogged filters / screens: A partially blocked path could increasingly constrict flow under heat, compounding problems over time.
Repair Process & Best Practices
Based on that case and broader repair wisdom, here’s a methodical approach to diagnose and rectify the problem:
- Baseline checks before teardown
- Monitor transmission and converter pressures (if instrumentation exists).
- Check fluid condition—look for contamination, foaming, or degrading viscosity.
- Verify hydraulic and control circuits are free of blockage, leaks, or restricted flow.
- Confirm all filters, screens, and lines are clean.
- Watch temperature curves to correlate failure onset with heat thresholds.
- Monitor transmission and converter pressures (if instrumentation exists).
- Valve body / control circuit inspection
- Remove the valve body associated with converter control.
- Disassemble and examine internal plates, springs, and valves.
- Identify O-rings, seals, or gasket elements that may be cracked, flattened, or otherwise compromised.
- Replace seals with new, correct specification parts (material rated for heat, pressure, compatibility with hydraulic oil).
- Reassemble, ensuring all clearances and tolerances match factory specs.
- Reinstall and torque fasteners appropriately.
- Remove the valve body associated with converter control.
- Pump / charge circuit assessment
- Test the charge pump under warmer conditions to see if it sustains pressure.
- Inspect for suction leaks in the pump inlet.
- Verify that pump output meets design specification even when oil is hot.
- Check the oil feed paths (screens, lines) for blockages or restrictions.
- Test the charge pump under warmer conditions to see if it sustains pressure.
- Testing & calibration
- After reassembly, run the loader under controlled testing, gradually increasing load and temperature.
- Monitor for recurrence of the symptom—if it returns, pinpoint the threshold and whether further leakage or control issues remain.
- Take oil samples to check for contaminants, gas entrainment, or additives breakdown.
- After reassembly, run the loader under controlled testing, gradually increasing load and temperature.
- Preventive measures
- Use quality hydraulic oil rated for high operating temperatures and with good stability.
- Replace soft seals preemptively, especially in high-heat zones.
- Maintain clean filters and screens.
- Ensure cooling systems are sufficient (coolers, radiators) so oil and hydraulic systems don’t overheat.
- Incorporate regular inspections of control circuits and internal seals.
- Use quality hydraulic oil rated for high operating temperatures and with good stability.
Context: CAT 950B and its Legacy
The Caterpillar 950B is part of the line of mid-sized wheel loaders built during earlier decades. These machines found use in general construction, aggregate handling, quarries, and smaller mining operations. Over time, many units remained in service decades beyond their original design life—testament to robust mechanical design but also a source of maintenance challenges.
Caterpillar Inc., founded in the early 20th century, became a dominant name in heavy equipment. The 950 series has had many variants (950, 950B, 950C, etc.), each with improvements in power, hydraulics, and transmission systems. Because many B-series units are now operated well past 30–40 years old, wear, seal degradation, and parts fatigue are common. Owners often retrofit, rebuild, or cannibalize from donor machines.
In heavy equipment circles, stories of intermittent failure like this 950B power loss are not rare. One news account from an industrial yard in South Africa described a loader that would lose mobility after prolonged grading, and mechanics eventually traced it back to failing internal converter seals. The pattern matches the classic symptom trajectory: strong when cold, loss when hot, recovery after cooldown.
Given the vintage of many such machines, not all replacement parts are readily stocked. Owners must sometimes fabricate O-rings, reseal kits, or source NOS (new old stock) parts. Maintaining documentation and using upgrade-grade sealing materials (e.g. high-temperature elastomers, Viton, or equivalent compounds) helps future reliability.
A Short Anecdote from the Field
At a gravel pit in Ontario, Canada, a contractor was running a 950B loader reclaiming material from spoil piles. After about two hours of continuous loading, the machine ceased moving under load—even in its lowest gear, the wheels spun but went nowhere. The operator shut down, allowed it to cool for 30 minutes, and restarted—motion returned, only to drop off again after another hour.
A technician traced the problem to the torque converter control circuit. They removed the valve body, discovered two marginal O-rings similar to the case above, and replaced them with better sealing elements rated for thermal stress. After reassembly, the loader ran reliably all day under full load without further power loss. The operator was relieved—downtime from repeated failure had been eating profit margins.
Summary
The recurring power loss issue in a CAT 950B loader—working fine when cold but collapsing under load after heating—is often due to leakage in hydraulic control circuits, especially within the torque converter’s valve body. Failing O-rings inside that valve assembly are common suspects. Diagnosing involves pressure testing, disassembly, inspection, and replacement of seals. Supporting tasks include assuring pump health, flow paths, cooling, and oil condition. In older heavy machines, maintaining internal sealing integrity is often what separates steady uptime from frustrating recurring failures.
