Classic Mercedes Benz diagnostics, consultations, inspections
This page is dedicated to Mercedes S-
In my life I owned and worked on many W140 S-
The pump is located under the rear seat on passenger side. It creates vacuum and pressure. Trunk lid soft close pull is assisted by another pump located in the trunk, but door lock pump has to release lock latch. If the door lock pump doesn’t work the trunk lock latch stays in the closed position and soft close feature will not work even if the doors close assist works fine. PSE pump also feeds vacuum to climate control flaps when the engine is not running. REST feature relies on pump to open center vents.
There might be few issues with the pump when it fails: internal pressure or vacuum leak, pump vane failure, air hose connectors failures, internal electronics failure, external components failure. But in most cases the internal pump motor fails. In some cases only lumbar supports stop working, but door locks still work. If pump senses that any seat lumbar support has a large pressure leak it simply shuts down lumbar circuit. But locks and trunk still work fine. If the lumbar pressure leak is small PSE pump does not detect it and keeps activating. Thus the motor overheats resulting in premature wear and eventual failure.
One can replace the complete PSE pump with used pump, but motors in used pumps are already worn and usually do not last long. Therefore the best option is to repair existing PSE pump and to replace internal pump motor.
I replaced many motors in different PSE pumps. Back in the days original motor was available. Nowadays the only option is aftermarket motor. They need slight modifications, but work fine. After motor replacement PSE pump needs to be bench tested to make sure it functions properly. Some pressure/vacuum adjustments to be made. And it is also recommended to shut down some circuits inside the pump depending on engine type. Fell free to contact me for more information.
Below is late W140 PSE pump bench test video: