Brake booster Vacuum Pump

[IVIUSTANG]

I heard a new noise on my car today, an electric motor whine under the hood I could hear while the car was running. I isolated it to the brake booster vacuum pump near the air filter. My question is, does this pump run all the time or does it cut in and out once it hits a certain pressure? I think I have heard this motor for a few seconds during startups before, but now it is running non stop and there is a slight audible "hiss" coming from the same area. The brakes and everything else are working perfectly. I suspect a leak in the hose; but don't know much about this pump.

I mainly want to know if I can put this off a few days, or if the pump running nonstop while driving will cause it to burn out.

Thanks,
Jesse

[Antherzoll]

It's possible that it can burn out if it runs constantly. The vacuum assist pump is supposed to maintain brake circuit vacuum when the engine is off or when engine vacuum is low (i.e. Cold starts and on throttle). There is a vacuum switch up top located between the air box and the DS headlight that activates the pump when vacuum is low. If there is a leak it will run constantly, but if no leak is found then it maybe the switch.

Vacuum pump cycling is normal when pushing the brakes with the engine off, key in position II. There's an issues if the vacuum pump runs on and on.

http://www.volvoxc.com/0/resources/how-to/pdf/power-brake-booster-replacing.pdf

Checking the power brake booster
Depress the brake pedal several times until it feels stiff.
Hold the pedal down. Start the engine. The pedal should travel and feel softer.
Release the brake. switch off the engine. Wait 1 minute. Then depress the pedal. The pedal should still feel soft but should become stiffer when it is depressed several times.
There should be no leakage between the vacuum hose and the terminal at the power brake booster.
There should not be any leakage between the master cylinder and the power brake booster.
Check that the stop lamp functions.
If the stop lamp switch has been modified or changed after installation of the master cylinder and power brake booster it must be readjusted.

Similar test with the vacuum pump. Instead of starting the engine, you turn the key to position II.

[hoonk]

if the pump running nonstop while driving will cause it to burn out.



You probably have a broken control switch, might be a 31400692. Pull the fuse or disconnect the control switch to avoid melting the pump! (until you get a new switch - without a pump the car might be hard to stop when first started)

[IVIUSTANG]

Interesting, I just took a hose off the "switch" you were talking off and gave it a slightly hard jerk... since then the pump has turned itself off; mighty suspicious. In the meantime I will stick to short trips until I determine what is going on. There is no air leak from the location I hear the hissing from, so I presume the sound is normal for the pump.

Thanks,
Jesse

[Scopeman]

I had to replace the switch on my 2005. It was sticky and would not always shut off.