Vacuum pump cycles after car turned off?

Astro Bob · Post by **Astro Bob** » 22 Jul 2010, 22:27

Greetings,
I've got a 2001 S80 2.9 non-turbo that has started to do something odd over the last couple of months. After stopping the engine, following moderate drives of 15-20+ miles, I would hear the cycling of a pump (every 5-6 secs) coming from under the hood? First impression were the sounds of a windshield washer pump or perhaps an ABS brake pump refreshing. A closer look determined it to be a small pump located just below and rearward from the Airbox. It didn't appear to be connected to the fuel system, but instead lead into a plastic manifold, one hose of which eventually went to the brake booster- so it I'm fairly sure it's for maintaining vacuum for some reason? This cycling continues for about 5 mins or as the engine cools (BTW, it is not a radiator fan, nor the circulating fan in the processor box).
So a few questions would be:
1) What does this vacuum pump do and why is it trying to maintain vacuum (assumed) after the engine has stopped?
2) Assuming the pump is functioning properly, where is the sensor that's tells this pump to cycle located- perhaps it is bad?

There are no error messages stored in memory. It does seem to happen more often during hot weather. The battery is 6 months old/ voltages look good. No electrical problems. Have tried resetting/ disconnecting battery in hopes that a computer may be stuck "on" in regards to this one pump/ item- with no CPU problems that I'm aware of. The car is well maintained and has 75K. I'm hoping someone else has some experience with this and what the results may have been.
Other than this the car runs well and I really enjoy the ride?
Thanks, Bob

Post by **JDS60R** » 22 Jul 2010, 23:09

Could it be one of these parts?

eli · Post by **eli** » 16 Aug 2010, 01:36

Astro Bob, are you still having this issue?
I had the same problem with my Volvo, except my pump would always run with the key in the on position or engine running. It is an auxiliary vacuum pump for the brake booster. It is ONLY supposed to come on when the booster needs vacuum when applying the brakes (when the engine is not running, key in the on position). This way you can still safely pump the brakes and stop the car even if the engine is not running.
There is a small vacuum/electrical switch that controls this pump. That is the part that fails. On my Volvo, it is attached on the front of the air cleaner, near the aux. pump. It has an electrical connector, and a few vacuum hoses attached to it. One of those vacuum hoses goes to the auxiliary vacuum pump. It is very easy to replace, no need to disconnect the battery or download software to replace the part. It should only take a few minutes to replace.