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#4507832 - 09/05/17 07:37 AM booster design, why use aux vacuum pump?
meep Offline


Registered: 02/20/07
Posts: 3548
Loc: Southeast
I'm still a relative newbie into volvo ownership - I've had 3 s60's under my care, 2 NA and 1 turbo; so I'm still learning the tricks to living with them as they age. My daughter currently has my previous 2005 NA and I'm commuting in a high-mile 09 turbo. I had an 01 a while back but didn't hang on to it.

Both have standard vacuum-assisted power brakes. Both also have a brake vacuum boost pump located immediately behind the radiator. it's triggered when vacuum is low and runs intermittently. The primary vacuum source is the manifold, but neither is a simple direct connection to the manifold. In both boosted and NA variants, there looks to be a Bernoulli pump which runs from the high side to the low side of the throttle body. The resulting vacuum from that pump feeds the brake booster. That pump likely has a limited flow rate, so then you have the auxiliary electric pump to supplement.

WHY?

Perhaps with the turbo I get it - there's not a lot of manifold vacuum (though there should be vacuum when not under boost), but you don't need brakes when your foot's in the throttle. Even in a NA car, there's much-less-to-no vacuum when you're in the throttle. In a diesel, obviously you need a different source of boost (do diesels use vacuum?)... but in this case I have to be missing something.

The only thing I can come up with is they standardized for "cost savings" by employing an $80 vacuum switch and extra $265 pump across all of their models .. except that doesn't save any money.

So what am I missing? Remember, the NA cars have it too.

do all gas turbo cars/trucks do this?

(Background - car has 0 brake assist when cold, and that torquey motor will roll it right along during high (1200rpm) idle - I'm not sure my wife could stop it, my butt lifts off the seat myself until idle drops or I pull it from gear - i'm fixing it now)

-m


Edited by meep (09/05/17 07:38 AM)
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#4507856 - 09/05/17 08:24 AM Re: booster design, why use aux vacuum pump? [Re: meep]
WyrTwister Offline


Registered: 01/13/13
Posts: 1487
Loc: Texas
I know nothing about Volvo's , but your question is interesting .

I can understand the need for the vacuum pump on a turbo or a diesel . No idea on a NA gas engine ?

If you find an answer , please post back .

Best wishes , :-)
Wyr
God bless
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#4507863 - 09/05/17 08:37 AM Re: booster design, why use aux vacuum pump? [Re: meep]
Linctex Offline


Registered: 12/31/16
Posts: 6148
Loc: Waco, TX
Originally Posted By: meep

(Background - car has 0 brake assist when cold, and that torquey motor will roll it right along during high (1200rpm) idle - I'm not sure my wife could stop it, my butt lifts off the seat myself until idle drops or I pull it from gear - i'm fixing it now)


Good Lord! Even old manual brakes were better than that!
1) what does a vacuum gauge at the booster say?
2) if good vacuum, bad booster.

Honestly, though - is there ever a time these cars DON'T have "good vacuum"?
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#4507895 - 09/05/17 09:09 AM Re: booster design, why use aux vacuum pump? [Re: meep]
meep Offline


Registered: 02/20/07
Posts: 3548
Loc: Southeast
the booster works great when it's engaged. The failure on mine is the pressure switch which commands the relay to run it. A common failure mode is for the switch to fail closed and run the pump continuously until it burns up. Usually that blows a fuse which also supports some emissions circuits, resulting in a drop in mpg. That's how I figured out what was going on, tracing mpg woes. Anyway, that's $350-400 in parts, or a dealer repair of $650. When I power the pump manually, works great! Just seems like a lot of parts for something I've never seen in other makes.

manual brakes - agreed - having learned on a 50's chevy, I failed to truly understand why a 2,300 lb escort needed power brakes.

-m
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2015 crv (wifey!)
2009 s60 2.5T

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#4508064 - 09/05/17 12:29 PM Re: booster design, why use aux vacuum pump? [Re: meep]
Tman220 Offline


Registered: 09/20/16
Posts: 144
Loc: MN
I know my regal uses an auxiliary vacuum pump. The purpose for it in this case is because of variable valve timing. While cruising the PCM times the cams to keep vacuum low, this reduces pumping losses and increases efficiency. It's not unusual to only see 3-4 inches of vacuum while cruising at low loads. I'm not familiar with your Volvo engine though if it might be a similar scenario.
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#4508071 - 09/05/17 12:38 PM Re: booster design, why use aux vacuum pump? [Re: meep]
meep Offline


Registered: 02/20/07
Posts: 3548
Loc: Southeast
^^^ see, that's interesting. Both my NA and 2.4T models have VVT. Maybe that's part of it.
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#4508157 - 09/05/17 02:27 PM Re: booster design, why use aux vacuum pump? [Re: meep]
SHOZ Offline


Registered: 06/28/03
Posts: 5742
Loc: Illinois
The Hyundai genesis coupe 2L turbo has this with the automatic transmission but no the manual. There is on the manual a check valve to shut off the intake vacuum supply line when under boost. But there is also some other 4 way valve that I have seen on other cars that generates vacuum when the engine is in boost as it switches to the pre turbo intake when under boost.

I eliminated it on mine as the same line will reverse and feed air into the intake manifold under high vacuum. Caused a bit of throttle hang issues.
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#4509208 - 09/06/17 04:06 PM Re: booster design, why use aux vacuum pump? [Re: meep]
seanf Offline


Registered: 12/16/14
Posts: 167
Loc: Canada
my wifes 2002 Audi A4 automatic has the vac pump, I believe all the automatic 2002-2005 had it, most definitely the 1.8t and I'm pretty sure the 3.0.

Your symptoms describe a bad booster or a bad check valve. Thats what I'd look at first if the Audi had hard brake pedal on a cold start.

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#4509719 - 09/07/17 07:48 AM Re: booster design, why use aux vacuum pump? [Re: meep]
meep Offline


Registered: 02/20/07
Posts: 3548
Loc: Southeast
turns out for me the culprit was just the pressure switch. it works once the hood is warmed up but won't trigger the pump when it's cold -- probably gummed up inside.

so I guess these aux pumps are becoming much more common now that they used to be. wow. things are getting more complicated....

-m
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2018 F150
2015 crv (wifey!)
2009 s60 2.5T

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#4579516 - 11/19/17 09:15 PM Re: booster design, why use aux vacuum pump? [Re: meep]
lbhsbz Offline


Registered: 02/09/12
Posts: 30
Loc: Long Beach CA
Vacuum is a function of restriction (closed throttle plate) against the engine trying to pump air. At cold start, the IAC / throttle control is reducing that restriction to run the engine at high idle, which means lower vacuum. This is why the aux. vacuum pumps exist.

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#4581052 - 11/21/17 01:24 PM Re: booster design, why use aux vacuum pump? [Re: meep]
PandaBear Offline


Registered: 08/05/02
Posts: 14205
Loc: Silicon Valley
So the aux pump is there because you cannot guarantee a continuous flow of vacuum then? Seems like force induction, throttle-less, hybrid, or Atkinson style engine will need additional vacuum elsewhere than intake to work, make sense now.
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#4581091 - 11/21/17 02:24 PM Re: booster design, why use aux vacuum pump? [Re: meep]
SHOZ Offline


Registered: 06/28/03
Posts: 5742
Loc: Illinois
Happy Thanksgiving.....


Edited by SHOZ (11/21/17 02:26 PM)
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