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I have just had an 80,000 mile service on my 2003 A4 Avant. I have an extended warranty and was told the diagnostic check had revealed a faulty brake vacuum pump and that it needed replacing. This was duly done under warranty. But after a little think, I realised that I do not know anything about a brake vacuum pump. I have heard of servos but not a vacuum pump. Also. whereas I never heard the old one I now hear a whirring noise when I put my foot on the brake that comes from the new part fitted under the bonnet. Any Backroomer enlighten me as to a brake vacuum pump and should I hear it? Thanks.
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Presumably your A4 is a diesel. Diesel engines do not have a throttle butterfly in the engine inlet tract to control engine output - this is done purely by means of fuel delivery control, with boost as a secondary influence. The downside of omitting this dreadful butterfly device is that the inlet mainfold is never far below atmospheric pressure, even at idle and is of course above atmospheric pressure when the engine is working under boost conditions. The consequence of all of this is that a conventional vacuum booster for the brake system needs a separate engine driven pump in a diesel installation.
Your vac pump will be working when you use the brakes to replenish the vacuum in the servo. If you can hear it doing this, there is a problem. Back to the dealer asap as this is a safety related problem.
659.
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Thanks for your reply 659FBE. Should have said, A4 is 2 litre petrol.
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There was an antilock braking system (Teves I think) with an electric pump. I wonder if this is the cause of the noise on your vehicle. In any event, if work has been recently done by a dealer on a brake-related problem and there is spurious noise, it should go back asap.
659.
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The 2003 A4 with automatic transmission and 'otto engine' uses a vacuum brake servo with an electric vacuum pump - part number 8E0 927 317 for the techy.
Ian.
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Update. Went back to dealer. Discovered vacuum pipe from inlet manifold collapsing, pump therefore cutting in to provide vacuum to servo. Booked in for fitting of new pipe on Monday. Question; why should pipe collapse? Wear and tear or another reason? Thaks for help everyone.
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Update2. Dealer fitted new pipe. However new pipe also collapsed (squashed in for the technical) Vacuum too strong, too much vacuum? Some sort of sensor / valve not working? Any Backroomers with ideas? Car booked in for a day next week at dealers for further investigations!
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I can't help with the Audi pump, but we do have a few problems with the pumps fitted to the VW LT vans I work on. These are camshaft driven via a pushrod, something similar to and old cam driven fuel pump. Like the old fuel pumps, the valving on these 'disconnects' the pump when enough vacuum is created. The pump then sort of 'freewheels'.
On the VW's the first sign of a problem is a knocking noise from the pump when it's 'freewheeling', applying the brake a couple of times loads the pump, and the noise stops.
With a good pump (or exhauster as they used to be called), the vacuum felt by finger at the end of the pipe hardly feels worthwhile, certainly not as good as a petrol engined manifold vacuum, but it does work - just takes a bit longer.
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