Électrovanne de verrouillage de convertisseur défectueuse

The solenoid that applies and modulates the lock-up clutch inside the torque converter.

A torque converter drives through fluid, which always slips slightly — that is what makes an automatic smooth from rest and a permanent efficiency loss once moving. Above a certain speed the converter is locked mechanically to remove it, and this solenoid is what applies that lock: not as a switch, but as a ramp, and often holding a deliberate controlled slip of a few tens of rpm to absorb engine pulses.

The symptom people actually notice is a shudder, at light throttle, most often between about 60 and 80 km/h, described as driving over rumble strips. It is regularly misdiagnosed as tyres, wheel balance or a driveline vibration. The distinguishing test is free: it appears only under gentle acceleration at steady speed and vanishes the instant the throttle is lifted or pressed harder.

And the commonest cause of that shudder is not the solenoid. Controlled slip depends on the fluid's friction modifiers, which degrade with heat and age — so a fluid service is the cheap first move, and it resolves a good proportion of these before anything is replaced.

Stuck applied is the serious version: a lock-up clutch that cannot release stalls the engine when the car comes to a stop, exactly as braking in gear without the clutch would.