Mitsubishi Legnum: F5A51 Torque Converter Shudder at Light Throttle

The F5A51 5-speed automatic transmission fitted to Legnum EC models uses a lock-up torque converter clutch that engages during light-throttle cruising between approximately 60-100 km/h to eliminate converter slip and improve fuel efficiency. The converter lock-up clutch friction disc degrades over time and with use of incorrect ATF fluid — Mitsubishi specifies SP-III or equivalent, but many NZ-imported examples have had Dexron II or generic ATF substituted at service intervals. As the friction material wears, smooth clutch engagement becomes impossible, producing a characteristic 10-20 Hz low-frequency vibration transmitted through the floor and drivetrain under steady light throttle. The shudder disappears immediately when throttle is increased to open the converter or when decelerating. Multiple ATF flushes with correct fluid may temporarily improve symptoms but worn friction material ultimately requires torque converter rebuild or replacement.

• Low-frequency vibration felt through floor and seats at 60-100 km/h under light throttle
• Shudder disappears when accelerating more firmly or when coasting
• Vibration is distinct from road vibration — regular frequency around 10-20 Hz
• Symptom temporarily improves after an ATF flush but returns within months

Road test at 65-85 km/h with light steady throttle and hold the speed constant — feel for rhythmic floor vibration characteristic of torque converter shudder versus driveline vibration. Check ATF colour by wiping on white cloth — brown or black colour with burnt smell indicates degraded fluid and friction material. If shudder persists after a full drain-and-fill with correct SP-III ATF, torque converter reconditioning is required.

Parts involved: Torque converter.

$180 to $3,500 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Has the automatic transmission fluid ever been changed and what fluid type was used?”
• “Does the car vibrate at highway speeds when cruising at light throttle?”
• “Has any transmission work been done since the car was imported?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.