Fourth-Gear Flare on a Powershift Haul Truck
A 9,200-hour articulated dump truck with a six-speed powershift transmission flares about 200 rpm and loses ground speed in fourth gear under load on a grade; first through third feel normal. There are no active fault codes, but a stored event reads 'clutch 4 fill time out of range'. Oil has run 620 h against a 500 h interval, smells burnt, and the sump screen holds fine friction material while the pressure filter is clean. Main line pressure is 1,820 kPa at high idle in neutral (specification 1,750–1,850 kPa). Clutch pressure at the fourth clutch test port under load is 1,430 kPa (specification 1,700–1,850 kPa); first, second and third clutches sit at 1,760–1,790 kPa. Converter stall speed is 1,980 rpm against a 2,100–2,250 rpm specification taken at 85 °C oil temperature with the throttle against the stop. The customer wants the transmission pulled today.
1. Interpret the evidence. Rank the causes and list what you would check before authorizing removal.
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The evidence converges on clutch 4 itself, not on supply pressure.
- Main line pressure is in spec (1,820 kPa): pump, regulator and main relief are delivering. That rules out the whole low-supply-pressure family of causes for the flare.
- Only clutch 4 is low under load (1,430 vs 1,700–1,850 kPa): a common-supply fault would drop every gear. A fault confined to one circuit points downstream of the valve body - piston seal, shaft sealing rings, a cracked piston, or a pack worn to the point that piston stroke is excessive and pressure cannot build within the fill window.
- The stored 'clutch 4 fill time out of range' corroborates it: the ECU is seeing the clutch take too long to reach pressure, exactly what a leak or excessive clearance produces.
- Friction material in the sump screen with a clean pressure filter is significant. The screen is upstream of the pump and filter, so material there means a large volume of debris was generated. With 620 h on a 500 h oil change and a burnt smell, the oil has been degraded and the clutch has been slipping and overheating.
Checks worth an hour before you pull it:
- Verify with a known-good gauge at the test port - failed gauges are common.
- Check the clutch 4 solenoid connector and current draw; a chafed wire or partly commanded solenoid can look like a pressure fault.
- Air-test the clutch 4 circuit at the test port with the engine off - listen for leakage into the sump and housing.
- Confirm the ECU calibration is current.
If those hold, the transmission comes out - the customer gets it today, but with a reason rather than a hunch.
2. Does the low stall speed change your decision? How do you decide between engine and converter?
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It matters, but it does not change the removal decision - it changes what you verify afterwards.
Stall speed is a system test: engine at full fuel against a locked converter. A low stall (1,980 vs 2,100–2,250 rpm) can come from:
- Engine side: boost or charge-air leak, restricted air filter, low fuel rail pressure, a derate, altitude, or the throttle not actually reaching the stop.
- Converter side: a slipping stator one-way clutch, which reduces torque multiplication and pulls stall speed down.
- Test conditions: oil not at 85 °C, tachometer reading from the wrong source, or the test cut short.
How to decide: first confirm the test was valid - oil at temperature, throttle against the stop, calibrated tach. Then check engine basics: air filter restriction, boost pressure against spec at full load, fuel pressure. If the engine is healthy and stall is still low, the converter is suspect.
Why the decision stands: a weak engine flares in every gear under load, not only in fourth. First through third holding at 1,760–1,790 kPa with normal ground speed is strong evidence the engine can deliver. The 4th-gear flare is a clutch-capacity problem and the material in the screen is physical evidence of it.
Why you still settle it: rebuild the transmission, hand it back, and if the engine was also down on power the customer will see the same flare and blame the rebuild. Document the stall result and engine checks on the same work order before disassembly so the post-repair comparison is meaningful.
3. Describe commissioning and the follow-up program after the repair.
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- Set pressures, do not just accept them. After rebuild: main line pressure in spec at high idle and at stall; clutch pressure at every test port in every gear at 1,700–1,850 kPa. Record them - this is the baseline for the next diagnosis.
- Fill times and codes. Use the diagnostic software to read clutch fill times in each gear; confirm the stored clutch 4 event clears and does not return after a loaded road test.
- Loaded road test. The failure only appeared in 4th under load, so a yard test is not verification. Load the truck, pull a grade and monitor slip (engine rpm versus output speed) for the flare.
- Contamination control. Flush the cooler and lines, replace or clean the sump screen, verify the breather, and fill with the specified oil. Residual friction material will find the new clutch packs.
- Oil analysis and filter inspection. Sample at 50 h then 250 h; cut the filter open at the first service. Rising silicon, iron or friction material is the early warning.
- Root cause on the oil interval. 620 h on a 500 h interval with a burnt smell is a maintenance finding - check whether the machine runs hot (cooler, fan, oil level), whether the correct oil was used, and whether the operator holds gears under load. Fixing the clutch without addressing oil and heat produces the same failure at a similar hour count.