W124 vacuum question...
 

Hi All,

I understand that a running engine can provide vaccum, as well as the vacuum pump. Are these the only vacuum sources in my car?

For all the sources, which parts does it energize?

thanks,

The electric vacuum pump under the rear seat is for the door locks. Possibly for the orthopedic seats if you have that option.

Engine vacuum is used for everything else including the brakes, emission controls and ACC (maybe that's everything else). There is a check valve to isolate ACC leaks from the engine and an ACC vacuum reservoir (so to speak) to tide you over in low engine vacuum conditions.

I don't know if the rear headrests use vacuum and if so which source they use.

Sixto
95 S420
87 300SDL

thanks...
 

Great info Sixto...

One more thing though... Isn't there a vacuum pump in front of the engine block that's powered by the serpentine belt? Maybe that's what you meant as the "engine"...

ACC = Accessories?

thanks,

The vacuum pump driven by the engine pertains to diesel vehicles because they don't produce any vacuum in the intake manifold at idle.

On a gasser, the intake manifold is the primary source of vacuum.

Re: thanks...
 

Automatic climate control.

Sixto
95 S420
87 300SDL

One interesting vacuum interface occurs on cars with the "REST" function on their AC controls.

The rest function allows one to drive somewhere, shut the cars motor off and sit in the car with the climate control using the engine heat for heating the cabin as prescribed on the controls; 72deg, etc.

To do this function the auxilary water pump keeps moving coolant from the warm motor to the heater core and the VACUUM is used to control duct function as commanded. This wouldn't last long from a car sized reservoir so the PSE module (vacuum pump, etc), pulls down the system while checked from the engine vacuum system which is the source when running and open when not.

Thats amazing. Mercedes must have spent more time working on their ACC than any other mechanism in the car!

And some days when my nose and fingertips are cold I wonder if MB spent enough time on the SDL ACC.

Sixto
95 S420
87 300SDL