No vacuum assist and no servo assist?

Audi Nut Joined: Sep 26, 2012 Posts: 76

Hi all, put my 20v coupe in for MOT yesterday and all was fine apart from there was no vacuum build up is too slow and no servo assist, how can I diagnose and fix this and could it be something I have/haven't done when removing and replacing pas pump?
TIA
Cadan

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

Hi,

Sorry to say it is a hydraulic system, so no vacuum. It should be producing flow from the pump, but leakage in the hydraulic booster/servo could be the problem and checked first.
It is more likely to be a fault with the hyd accumulator or 'bomb' which should be tested after the servo.
You should be getting a warning for low hyd pressure?

http://20v.org/brakprob.htm

Audi Nut Joined: Sep 26, 2012 Posts: 76

Cheers Scotty, so where should I look first? Accumulator or servo test? I'm not getting any warnings at all but I'm almost certain the bomb needs replacing Sad

if this is the case will this affect the brakes during normal driving?
As for the vacuum build up is that just worded wrong on the failure sheet then?
Sorry if the questions seem a bit tardy but this is my first experience with this braking/pas system Confused

Cheers

Posted: Sun Jun 02, 2013 4:56 am

I've had these problems with all the 20v's I've had in the past. On average the bomb in these cars only last about ten or so years before the nitrogen inside them leaks out. The way to test the bomb is to pump the brakes with the engine off and to see how many pumps it takes before the brake pedal goes hard. With a good bomb it should take about 20 strokes of the pedal before it does this, any less than 10 and the bomb is on the way out. If it only take one or two or none at all then your bomb has bombed. A good source for a replacement is Pete Reeves at Quattro Corner in Birmingham. (0121 476 0034). Cheers Steve

Audi Nut Joined: Sep 26, 2012 Posts: 76

I was kinda hoping it wouldn't be but did expect it to be honest.
Thanks for your help
Cadan

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

I agree it probably is the bomb, but a leak in the servo gives the same symptoms, would be foolish imo to not rule this out before buying a bomb?

The wording on the fail is inaccurate, but he basically means you have no servo assist, does not matter whether vac or hyd, it needs to work for a pass?

Audi Nut Joined: Sep 26, 2012 Posts: 76

Right so after doing the tests described on 20v.org I'm pretty sure it's the bomb not the servo, however I'm still not getting any warning lights on the dash, I took the instrument cluster out and couldn't see any bulbs missing.
Also does anyone know the exact pressure inside a charged accumulator? Only reason I ask is my dad has a friend who can recharge accumulators but would need to know the pressure to do so.
Cheers
Cadan

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

The pressure sensor is on the underside of the hydraulic servo, have the wires been shorted together? = no light.
I think the alarm point is around 116bar so the system pressure should be a little above this. nitrogen precharge is usually about 2/3 system pressure I think, guess your guy will know?

Audi Nut Joined: Sep 26, 2012 Posts: 76

Hi Scotty, not yet checked the switch but will do tomorrow, I found that the working pressure is 140 bar so can hopefully Work out what the nitrogen pressure should be.
Thanks for your help guys I'll keep you updated as I progress.
Cadan

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

This link says the nitrogen precharge is 90psi, it's not specifically for the 20v, but I think the parts are fairly generic across the range at that time.

http://www.sjmautotechnik.com/trouble_shooting/brake.html#bomb

Worth looking at the steering one:

http://www.sjmautotechnik.com/trouble_shooting/susp.html#hydoil

Audi Nut Joined: Sep 26, 2012 Posts: 76

90psi? That's only 6 bar? Seems very low for a 140 bar system?

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

Good call, my blonde moment for the day!

Should be 90 Bar

Inexperienced Joined: May 16, 2013 Posts: 31

got the same problem w8ith my 2.0 coupe mate. Brake warning light has come on and got a rock solid pedal. brakes work but just work very badly and doesnt matter how hard you press the pedal it doesnt make any difference. Bought a second hand bomb off ebay and got a mate to fit it at the weekend and its not made a scrap of difference. either the second hand bomb is knackered or (and more likely opinion) its the servo. Cant seem to find a servo at the minute though

Site Moderator Joined: Jan 31, 2003 Posts: 9144

Always the problem with 2nd hand parts.

I wouldn't fit a 2nd hand bomb to any car.

The 2.0 - is it a 16v ? Standard 8v cars didn't have a bomb and run a normal vacuum servo. The 16v has a hydraulic servo. It sits between the master cylinder and the brake pedal.

Inexperienced Joined: May 16, 2013 Posts: 31

Mines a 16V. Yeah your completly right, second hand brake parts are not worth bothering with. A friend of mine thought he was doing me a favour by picking me one up free of charge so couldnt say no really but a new one is the way to go. I think I will get a Servo and try that next as I think its the more likely problem

Audi Nut Joined: Sep 26, 2012 Posts: 76

Well after getting prices and weighing up options I have started installing a vacuum servo into my coupe. It's all bolted up and am just trying to work out where the larger black hose should go. Any thoughts anyone? When I took it off of a 1z diseaseal it looked to go to a crank case breather? If anyone could shed some light on this I would be uber grateful.
Cheers
Cadan

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

There is a vacuum pump on the back end of the cam on my passats, wonder id the 1Z is the same? My guess is it would need to go to a big 'nipple' on the inlet manifold for it's vac feed?

Audi Nut Joined: Sep 26, 2012 Posts: 76

No vac pump on the 1z that I know of, although I could be wrong, times like this a manual would come in handy Mad

Site Moderator Joined: Jan 31, 2003 Posts: 9144

Don't forget you'll also need to consider the tandem pump which provides brake and steering assist.

If you change to a vacuum servo and don't blank off the brake assist side of the pump, you'll wreck the pump.

It may well be the pump which is the source of the problem

Inexperienced Joined: May 16, 2013 Posts: 31

Sorry dont mean to keep hijacking this thread but just to say I sorted mine out myself in the end. For some reason the second hand accumulator I bought had a different size banjo end coming on the hose coming from the power steering pump. The muppet friend that was fitting it for me was trying to bodge it on and of course the fluid was just leaking and it wasnt pressuring. I got pirtek to make up a new hose and put the accumulator on myself last night. Seems to have sorted the problem as I now have a brake pedal but the brake warning light is still on.

Inexperienced Joined: May 16, 2013 Posts: 31

Bit strange as the last post i put on here says probelm solved- where as actually I still have a brake problem on my car and have about had enough of it now! probably easier if I start from the beginning!

I bought a 1994 Audi 2.0 Coupe as a non runner. The main reason the car was off the road was because the owner at the time had the brake warning light come on the dash and the brake pedal had gone hard. They had taken it to one garage who had said they didnt have a clue and that it needed to go to an Audi garage. It then went to an audi garage who said it was acumulator and quoted £600 to supply and fit. At that point the owner gave up and sold the car to me.

I boughtthe car, fitted a second hand accumulator and the brakes seemed to be a bit better but the light was still on. After a about 10 miles worth of driving the brake drifted back to the same old solid pedal it had perviously and the brakes were back to useless again.

I then sent it to a local Garage who said its the most strange braker system they have ever seen- didnt have a clue. Told me the car has no servo so that cant be the problem. Howevee two of the calipers were knackered so I had new pads and discs all round and two new calipers.

I've now sent it to a mates garage who seemed to think he could sort it but is also now struggerling with it. He tried a new accumulator and that didnt work. He says that the accumulator on the car which is the second hand one i fitted is working fine as you get the correct response when pumping the pedal with the engine off. He has a theory that the brake system is working fine but I've just put cheap crap pads in it which is causing the lack of good brakes. I cant see its anything to do with pads myself especially as the brake warning light is on and has been on ever since i've had the car.

Overall I've now spent around £700 on this issue and still dont have brakes and still garages it goes to dont have a clue. Does anyone no who can sort this problem out or maybe someone on here can sort the problem out for me and earn themselves a few quid. Just frustrating as other than the brake problem the car is sound and its the only thing letting it down.

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

It does have a servo, it's hydraulic not vacuum which is what confuses everyone!
The warning lamp can come on due to 3 things: Low brake fluid level, low hydraulic fluid level (PAS reservoir), and low hydraulic system pressure. The fluid is hydraulic oil not PAS fluid, using PAS fluid causes problems. It should be fluid to VAG spec G002000 or the newer synthetic fluid G004000. Though some folk use Citroen LHM fluid which seems to work OK, guess it's the right type of fluid despite not meeting the spec.
Everything else I can think of has already been posted in this thread by myself or others, there is good info that is not too difficult to follow, in many of the links.
Your bomb may well be OK, the first test should be for bypass within the hydraulic servo/booster, details in the 20v.org link

Audi Nut Joined: Sep 26, 2012 Posts: 76

I sorted mine by ditching the expensive hydraulic assist unit and fitting a vacuum servo and PAS pump off a 1.9 80, super easy to do and I've had no problems since (did this in june) I dont however know how this will affect the auto check as mine doesn't have that.
If you fancy giving it a go then drop me a pm and I'll try and talk you through it, as for the auto check then someone on here should be able to help you out.
Cheers

Posted: Mon Nov 11, 2013 8:13 pm

there are only three things it can be, the bomb, the hydrualic servo or the pas/hydrualic system pump, assuming the calipers and hydro pipes are fine. maybe the switch/sensor for the brake/hydro pressure warning is broken, there must be a simple test for it, maybe info somewhere for the test but i guess its a simple open/ closed circit type, i think there may be two sensors one on the servo and one on the steering rack, heres a pic of the brake servo and you can see the sensor on the bottom, http://www.ebay.co.uk/itm/AUDI-80-AUDI-COUPE-2L-16V-2-6-V6-BRAKE-SERVO-MASTER-CYLINDER-CABRIOLET-/231092343543?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item35ce2d52f7 , if the power steering feels fine when turning the steering you can count that out.

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

I think connecting the two wires from the servo pressure switch together would satisfy the autocheck (assuming it is a normally closed switch, as others have indicated)