Pompe à huile de boîte auxiliaire défectueuse

The electrically driven pump that holds transmission line pressure while the engine is stopped has failed. This is a separate part from the engine-driven main pump, and confusing the two matters: the main pump wearing out gives a car that slips everywhere, while this one failing gives a car that is perfect on the move and awkward from a standstill.

The failures are ordinary for a small brushless motor. The winding goes open or shorts, a bearing stiffens until the motor draws more current than its driver allows, or the integrated control electronics fail. Debris circulating in old fluid finds this pump readily, because it is smaller and tighter than the main one.

Two things are worth establishing before it is replaced. First, whether it is actually being fed — a pump with no supply is indistinguishable from a failed pump, and a fuse, relay or connector is a fraction of the price. Second, whether it is failing or simply losing a race: a pump asked to hold pressure against an internal leak runs far longer and harder than it was designed to, so it eventually fails because of the leak. Fitted to a transmission that still leaks, the new one goes the same way.

On many designs the pump and its control module are one assembly, so "replace the module" and "replace the pump" turn out to be the same job — worth asking before comparing quotes.