Torque converter overheating on a loaded grade
An articulated dump truck (40 t payload, 12,400 h) is hauling 36 t of wet clay up a 12% grade 1.8 km long. After six minutes on the grade the torque converter oil temperature reaches 122 °C and the dash warning illuminates; on the flat it stabilises at 95 °C. Coolant is 92 °C, transmission oil level is correct, the cooler matrix is clean, and there is no external leak. Transmission main pressure is 1.9 MPa at 2,000 rpm in second gear (specification 1.8–2.2 MPa). The operator says the lock-up clutch ‘feels’ like it is not engaging because the engine flares 250 rpm at the top of the grade. The site has a 110 °C stop limit and a 125 °C shutdown. The haul road cannot be regraded this week. The supervisor proposes reducing payload to 30 t and continuing. Evaluate the likely causes and advise.
1. Rank the likely causes of the high converter oil temperature and state what data would confirm or eliminate each.
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Most likely is the lock-up clutch not engaging, because the 250 rpm flare at the top of the grade means converter slip is still generating heat when it should be locked. Confirm with commanded lock-up current, engine versus turbine speed, and converter-out oil pressure; if turbine speed is below engine speed by about the flare amount, lock-up is not holding. Second is a stuck-open converter relief valve or low converter pressure, which keeps the converter in a high-slip condition; confirm with a gauge at the converter-out port. Third is restricted cooler or oil flow, confirmed by cooler inlet/outlet temperature difference and a flow check. Fourth is sensor error, checked against a calibrated gauge. Oil aeration from a suction leak would show foaming and a rising sump level. The 1.9 MPa main pressure is within specification, so it does not explain the heat.
2. Is reducing payload to 30 t an acceptable control? Justify with the heat balance and operational risk.
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Reducing payload to 30 t lowers heat input, but it is not an acceptable sole control. The warning already appears at 122 °C, which is above the site’s 110 °C stop limit and only 3 °C below shutdown, so the machine should not continue normal hauling. Payload reduction also masks a lock-up fault: if the clutch is not engaging, the converter still slips on the grade and temperature can climb quickly even at 30 t, especially if the operator uses a higher gear. The right control is to stop on the grade, cool down, and either repair the lock-up/converter or restrict the machine to flat ground with a documented temperature monitor. A small payload reduction with no fault diagnosis gives a false margin and risks a shutdown on the grade, where a loaded truck can run away or block the haul road.
3. What test would you run before condemning the converter?
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Before condemning the converter, run a stall test and a lock-up command test. Bring the oil to normal temperature, chock the machine, apply the service brake, select the specified gear and hold full throttle against the stall for no more than the manufacturer’s limit, typically 10–15 seconds, recording engine speed and converter-out pressure. A low stall speed points to the engine or converter; a high stall speed points to a slipping converter or lock-up clutch. Then command lock-up in the workshop with a diagnostic tool and compare engine and turbine speed; a persistent slip above about 50 rpm under load indicates a failed lock-up clutch or control. Check solenoid current and valve spool movement. Only after these tests should the converter be removed; otherwise a control or sensor fault could be misdiagnosed as a mechanical converter failure.