Electric/Auxiliary Transmission Fluid Pump

An electrically driven pump that maintains transmission line pressure while the engine is stopped. It is a different part from the transmission's main fluid pump, and the difference is the reason it exists. The main pump is turned by the engine through the torque converter, so it delivers pressure whenever the crankshaft turns — a design that quietly assumes the engine never stops with the car in gear. Hybrids and stop-start vehicles break that assumption at every set of traffic lights, and the clutches still need to be held.

So this pump covers the gaps: engine off at rest, engine off during electric driving, engine off on a coasting descent. It runs from its own motor and, because that motor is usually three-phase, from its own small control module rather than a relay.

Its failures have a signature worth knowing: they are invisible while the engine runs. With the crankshaft turning, the main pump supplies everything and the car behaves perfectly, so a road test proves nothing. The complaint is always about the moments the engine was off — a thump or a hesitation pulling away from a stop, stop-start that has quietly stopped working, electric driving that is no longer offered.

Also known as: auxiliary transmission pump, electric transmission pump, aux trans pump, auxiliary gearbox pump, electric gearbox oil pump, secondary transmission pump, stop-start transmission pump

Referenced by 67 codes

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Codes involving this component (67)

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Information, not instruction

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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.

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