Brake & Hydraulic Problems


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Problems and Diagnostics

Brake & Hydraulic Problems

Anti-lock Braking System

Hydraulic Pressure Accumulator

Hydraulic Servo

Hydraulic Pump

Inoperative Handbrake

Caliper Rebuild (sedan)


(Scott Justusson)
A new sensor should be $5-$20 (allowing for Audi prices it is worth $1.50). If a sensor is bad, the light will stay off when the car is started, and then come on after driving the car from 10 feet to 2 miles. If the problem is elsewhere, it may turn the light on before moving the car. 

To find the bad sensor, listen carefully after starting your car for unusual noises from one wheel. That's the one with the bad sensor. The noise is the ABS kicking in and out, until it decides to shut down. You can hook up a DMM to the sensor outputs, raise the car and spin the wheel. You get a square wave (A/C) from the sensor. The actual value is irrelevant, matching them is. Basically, you want to get the same reading from all sensors spinning the wheel approximately the same speed by hand. 

Try removing the sensor, cleaning it, and reinstalling it until it rubs the hub. DON'T drive it like this -- you may damage it!! You must back it out slightly. It should not rub, but be very close. You can get a little plastic cap from the dealer for ~$0.80 that will set the proper distance for you and keep the sensors form getting eaten.

The sensors too far from the driveshaft will give you pedal "boing" -- this is a groaning/spring sound just as you come up to a stop. The sensors too close will give instant ABS pedal kick in. Too far out tends to shut off ABS quicker than too close. If you can't get the light to go out at all, you might have a fault in the center differential lock switch, or one of the sensors is just bad. Worn or crudded driveshaft splines can do it too. There is an inspection/water drain at the bottom of each strut, you can sometimes get a brass bottle brush in there, though pulling the driveshaft out is easier, and about a 20 minute job.

First check for proper sensor output, then check the differential switch, then the driveshafts. Rarely does that ABS box go bad. Sensors out of alignment usually allow the light to come on till the first brake app or 20-30 seconds, whichever comes first.

BOMB! (Hydraulic Pressure Accumulator)
The hydraulic pressure accumulator holds pressure in the system for emergency assist if the hydraulic pump fails or the engine quits altogether. It's a nitrogen vessel that holds several thousand PSI. It is believed that over time the nitrogen leaks out of the system (diffused through the diaphragm), because almost all accumulators start to go bad after about seven years of service. The unit has been given the nickname of "bomb" because of the shape of older versions -- rather like a cartoon bomb seen on the Rocky and Bullwinkle Show.

The standard tests:

A thorough test:
  1. Engine running at idle, steering to right or left full lock 20 seconds, at the same time pump brakes hard 10 to 15 consecutive times. Does brake light come on? Yes = bad belt, pump, or bomb (check valve). Try a pentosin bleed before replacing the parts!
  2. If light doesn't come on: stop engine, pump brake 10 to 15 times (short quick strokes). Brake light on? Yes = bad bomb (pressure accumulator).
  3. If light doesn't come on: Drive the car in a very safe place at 65 mph then apply full stop pressure to brakes (emergency stop pressure!). Does ABS actuate several times? Yes = good bomb! Brake light on? Yes = probably bad ABS sensor.
Above assumes booster cylinder (hydraulic servo) has been tested for leaks!

(Pablo Carselle, Peter Schulz)
Before replacing the bomb, do a complete pentosin bleed/refill with fresh fluid. If your fluid is old and/or dirty, it could be causing one of the valves to stick. If you must replace it, try Linda at Carlsen Audi (800-523-2408), GPR (800-321-5432), or Blaufergnugen (800-683-AUDI). The P/N is 893 612 061 C for older cars (CoupeQ up to #6900) and P/N 4A0 612 061 D for newer '90 cars and all '91s. You should probably try to look at the one on your car to make sure you're getting the right one, especially if your car was built around the time of the switch (10/89). Bentley says that the switch was "gradual," which is a nice way of saying that you can't be sure just by looking at your VIN. If you can look at the bomb (from underneath LF wheel, just above subframe), check the large banjo connection near the top. If it has copper washers it's the old-style unit, if it has rubber o-rings it's the newer one.

Make sure you replace the 2 copper washers or rubber o-rings when you replace the bomb. I recommend you use this opportunity to replace hydraulic fluid (Pentosin) for brake booster and steering rack. During the bomb replacement, you will have to disconnect hose from reservoir to pressure accumulator (bomb). At this time you can drain the Pentosin, assuming it has not been replaced recently.

  1. You may want to spray the connections that you will be removing with Liquid Wrench or WD-40 for a few days before beginning this job.
  2. Decide first wether you will reuse "pentosin" (hydraulic fluid) or replace. If you want to reuse, have available a very clean 1 quart container, or one not as clean if you intend to replace. Have some old rags ready for the inevitable stream of Pentosin that runs up your arm.
  3. After car is in position, pump brake pedal (with engine off) 30 to 40 times to relieve all pressure from the "bomb".
  4. Put car on jackstands and remove left front tire.
  5. Siphon out all the Pentosin from the reservoir (turkey baster works well). Disconnect hose from hydraulic reservoir at the top of the "bomb" (hose clamp), remove hose by rotating (twisting by hand) and quickly insert into recovery reservoir. Allow for full drain.
  6. Loosen the 17mm banjo bolt from high pressure hose (from hydraulic pump) and remove by hand retrieving two copper or rubber washers (both need to be replaced) located above and under the hose connection. There should be minimal fluid presence at this time.
  7. Loosen 11mm connector of line from "bomb" to booster cylinder at "bomb" side.
  8. Loosen nut at the front side of the "bomb" and carefully lift the bomb up and diagonally towards the passenger side. This will allow the front screw to move free from the short slot in the retaining bracket.
  9. Allow the bomb to drop a little and twist it (rotate) in both directions while pulling towards the front of the vehicle, until the back rubber bushing becomes free from the back hanging bracket. The bomb is now free....
  10. Install new "bomb", tighten forward retaining nut, attach line to booster cylinder and high pressure hose (2 new copper or rubber washers).
  11. Reattach hose to hydraulic fluid container. The reservoir has a center fine mesh screen that pulls out and can be cleaned or replaced. Since filtered fluid is in the center (and drawn out through the bottom), if you take the filter out, you might as well clean the canister and get the dirt out of there.
  12. Start the car and fill the reservoir to about halfway between MAX and MIN marks. The PA will now accumulate fluid and it will take about a pint of additional fluid, or close to a liter if you drained it completely. It will take some time for the pump to refill the bomb to proper pressure, usually a good 5 minutes or so.
  13. When no more fluid is needed, do NOT fill the fluid reservoir to the MAX line. If you do this, the reservoir will not hold all the fluid that the PA returns to it when you de-pressurize it.
  14. With the front of the car off the ground, turn the steering lock to lock 5 times to bleed the circuit.
  15. Check for leaks.
Hydraulic Servo
There is a relatively easy test for hydraulic servo (booster amplifier ???) leakage:
Run the engine at abt. 2000 RPM's for a few seconds, then shut it off. Carefully disconnect the return line from servo to Pentosin reservoir at the servo end (have rag handy) and observe for a couple of minutes. Only a few drops of Pentosin should come out from the servo. If a more definite flow is present, the servo is leaking. If the servo leaks during this test, there is a high probability that pressure accumulator is O.K.

Hydraulic Pump
This is an expensive little unit. Rod at The Parts Connection (800-517-6060) has ZF rebuilds at a good price. Your car will have one of three different types of pump.

Here is a simplified installation procedure. I imagine that the pump needs to be pre-filled with Pentosin before installation. Check Bentley before proceeding.
  1. Remove the 3 hydraulic hoses.
  2. Remove v-belt pulley.
  3. Remove old pump and attach new one.
  4. Reconnect pulley and hydraulic hoses.
Rebuild (Gary Hoyne)
  1. Remove pump from car. You have to remove the plastic air shroud that covers from the front of the car to the top of the radiator, but not the plastic engine compartment-to-radiator shield, which is quite a PITA to remove. Loosen the three hose bolts, loosen the two belt tension adjustment bolts, remove the hose bolts, remove the bolt mounting the brake hose to the rear of the pump, and remove the two bolts connecting the mounting bracket to the block.
  2. Remove the belt pulley from the front of the pump. You can wrap a used belt around the pulley and put the remaining portion of the belt in a vise, similar to a strap-type pipe wrench. Three socket head set screws are accessible through the pulley. Mark the orientation of the pulley to the mounting flange for reassembly location. You may have to gently tap the pulley off the shaft.
  3. Remove the two bolts attaching the pump to its mounting bracket.
  4. Four "small" bolts (probably 6mm) attach the brake booster pump to the front of the steering booster pump. Removing these splits the two pumps apart.
  5. Four larger bolts (about 8mm) hold the halves of the steering booster pump together.
  6. Finally, the access plugs for the pistons of the brake booster pump (the large, cross-slotted plugs) can be loosened and removed from the top and bottom of the brake booster pump. A conventional slotted screwdriver is too small to provide decent purchase in the slots. Get an 11/16 inch or 3/4 inch drag-link socket, and then grind down its width to fit into the slot lengthwise.
  7. You're now at maximum disassembly - unless you want or need to replace the shaft seal on the very front of the brake booster pump. Mine wasn't leaking (fortunately!) so I left it alone. To replace this seal, you need to press the pulley mounting flange off the main shaft, which would require a hydraulic or mechanical press. The shaft could then be removed from the pump, and the seal could be renewed. The flange would then need to be repressed onto the shaft.
  8. The rebuild kit from Audi consists of about 6 O-rings, one "rubber seal from hell" (RSFH), and one holder for the "rubber seal from hell". Two O-rings seal the access plugs for the pistons of the brake booster pump. One O-ring seals the halves of the steering booster pump. One O-ring seals the brake booster pump to steering booster pump mating surfaces.
  9. And then there's the RSFH - a very complex, molded rubber seal that seals the working portion of the steering booster pump impeller to the back of its case. STUDY how the old one is laid into the back of the impeller, then *gently* remove (pry) it out of place.
  10. Clean everything, especially the mating surfaces that are sealed by O-rings.
  11. The new RSFH and its holder should be positioned on the back of the impeller, and gently pressed into place. The seal has a "top" and "bottom" (i.e., it is asymmetric in cross section), but is symmetric rotationally (i.e., you can't install it 180 degrees out of phase).
  12. Reassembly is the reverse of the above. I used clean Pentosin as an assembly lubricant.
  13. Once the pump is bolted back to the engine block, reattach the low pressure supply hose from the Pentosin fluid reservoir. Add Pentosin slowly and rotate the pulley by hand to prime the pump. You're primed once you get fluid out of the steering pump high pressure port.
  14. Attach remaining hoses, belt, etc.
  15. Bleed system per Bentley. I needed about 1/2 litre of Pentosin.
Time: about 4.5 hours including removal / reinstallation from vehicle. (A second one could be done in about 3 hours, once you know the routine.)

Cost: about $40 for the seal kit, $5 for 4 crush washers and 2 O-rings that seal the banjo bolt hose connections, and $9 worth of Pentosin. A lot better than $300 for a rebuilt ZF pump (aftermarket) or $950 for a new pump (Audi).

Inoperative Handbrake
(Peter Schulz)
An inoperative or weak handbrake can be a disastrous thing. It might just need to be adjusted, but a common problem problem is the cams on the rear calipers get frozen with rust. To check/fix the operation of the cams:

  1. Handbrake off, car in gear, front wheel blocked by a wheel chock, rear end lifted SECURELY by jackstands.
  2. Under the car, follow the handbrake cable to the rear calipers--they attach to a cam which also has a circular spring held in place by a slot in the cam and a 10mm bolt. The cam is attached to a shaft internal to the caliper. This shaft, in turn, applies mechanical pressure on the rear caliper piston.
  3. That shaft is not protected externally by a boot and tends to collect rust. The rust prevents the spring from tensioning the cam, which in turn gives little or no feedback to the handbrake--hence the "out of adjustment" feel.
  4. Carefully compress the spring and removed it, then pull the handbrake brake cable out of the cam slot. If the cam is difficult to move back and forth, then you've found your problem.
  5. Liberally spray lubricant/rust remover around the cam shaft and work the cam back and forth to it's limit stops repeatedly. Let the lubricant soak in for a while, then repeat several times. The cam should now move easily, allowing the spring to apply tension and feedback to the handbrake.
  6. If the cam is badly rusted, you can try taking it all the way out to clean it up. You'll have to remove the piston from the caliper first, though. If refuses to move, you'll have to get new calipers.
  7. When you feel like the cams are freed up, apply some brake grease to the exposed part of the cam shaft, then put it all back together.
  8. It is rumored that stronger springs from some Ford model will help keep this problem from happening. The Ford P/Ns are: 6141147 & 6141148.
  9. Please check out this link for useful tips from an URS4 ebrake mod .
Caliper Rebuild for 90Q20V
(Dave Kempa)
If you need to rebuild the original front brake calipers on the 90Q20V (single piston ATE), you may find that your Audi dealer says there is no rebuild kit. They want you to buy new calipers for $450+. However, this caliper is also used on some VW Vanagons as well as some 3 series BMWs, so rebuild kits are available. Three options for the ATE caliper are (there is also a Girling version, which will likely have different P/Ns):
  1. Automotive Imports

  2. Clawson, MI
    Rebuild kit is P/N 15154 or they will rebuild your calipers for about $50 each if you ship them.
  3. NAPA Autoparts

  4. P/N 251698471B
  5. Pep Boys offers rebuilt calipers at about $95 each.
The Audi parts fiche shows a set of seals for the caliper piston is P/N 443 698 471 for the ATE, or 431 698 471 B for the Girling version, which might be all you need (even though the dealer says there is no rebuild kit).

(J Tomislav)
Simple Check for it is the following

  1. Remove footwell cover (driver's side ... this is actually the whole shelf).
  2. Remove ABS relay (should be middle row all the way to left and it has a red fuse in it which is 10 amp I believe).
  3. Look at fuse ... is it blown? If so, then here is your problem. Don't bother replacing fuse ... it will blow again. Replace the relay. If you still don't believe me, go ahead and put in another fuse and I bet it too will blow. This will prove the relay is toast.
The relay is found on all the 1990 - 1992 audi 90's and coupes ... regardless of drivetrain. Best way to make sure is to look at the part number on relay.
What happens is that there is this mother-of-a-diode inside that I can only assume shorts out over time. This diode is put there to 'absorb', if you will, any counter-EMF that occurs from turning off the ABS pump, so that the rest of the car's electronics doesn't go berserk.

Lucas Rebuild with Pics by Peter Schulz

BIG pictures and scans of the procedure

See also: Brake Maintenance, Brake Upgrades, Pentosin

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