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Thread: Brake Pressure sensors

  1. #1
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    Brake Pressure sensors

    Both my pressure sensors, placed on the pressure distributor, are leaking. First I thought that it was the brake cylinder but accidentally I touched the pressure sensor place right above the cylinder and got oil on my hand. It is the hydraulic oil that the sensors are monitoring and it can explain my loss of Pentosin and also air in the system. Checking the other sensor as well it also showed oil between the plastic insert and the metal case. Have ordered new once. Is this a common failure or is my 850i -92 somewhat unique.

  2. #2
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    Yes

  3. #3
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    After changing the 20 bar brake pressure sensor I got a warning in the instrument cluster, NOT the MID. The warning light (exclamation mark within a circle) flickers when I brake. After changing the brake pressure sensor back to the old leaky one the error disappeared. Can not find any information about this in the repair manual. The new sensor is most likely defect!

  4. #4
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    How far have you driven it since changing the sensor? Its possible that the system needs to bleed air out that was introduced when installing the sensor. Some cars have a bleed port on the pressure manifold. This hydraulic system controls braking, steering, and traction control.
    Regards,

    Brian
    Cave Creek, AZ

  5. #5
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    The new sensor has been driven for just a couple of miles. Bleeding was done when the new sensor was mounted.

  6. #6
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    Changed back to my old sensor and now everything is OK again! Have ordered a replacement sensor for the new one from my dealer.

  7. #7
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    Got a replacement sensor today for the new one that issued the warning light. Did the change and did the bleeding and everything look good. Did take my 850 for a short ride of some 64 km (40 miles) when I did brake to park and the warning light came on again. No error codes to be found in the ASC+T/ABS system. The warning light is now also on even when standing still and I touch the brake pedal slightly. The hydraulic oil level is 10mm below the top edge of the tank when the pressure tank is emptied. I don't know what to do?

  8. #8
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    Are you absolutely sure you changed the pressure sensor and not the DIFFERENTIAL pressure sensor?
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  9. #9
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    I have changed both pressure sensors or switches, (5) and (6) are changed. The devices are described as "circulating pressure warning switch (5)" and "hydraulic pressure switch (6)" in the repair manual section 34 33 100 and picture 31 34 192. I hope that we are talking about the same parts here. You make me uncertain, Timm. Sorry if I have used the wrong description, the sensor is of course a switch! I did hook up a my old analog multimeter across the switch (6), started the engine and could see that the switch was activated when I pushed the brake pedal lightly. The switch (6) was closed briefly as the red warning light lighted up for a short moment. I then changed switch (6) back to the old one and did perform the same operation. The old one did not react to the brake pedal being pushed.

  10. #10
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    It seems strange that you started with leaking switches - but apart from that a fully working brake system, and now you have warning lights! I can't say I fully understand the H31 brake system (who does), but more info can be found here:

    https://www.bimmerforums.com/forum/s...teering-system

    ....and hopefully Shogun will be around, he's fiddled with it to a much greater extent.....
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  11. #11
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    do you mean (p/n's from my E32 750 VIN):
    34331159752 is the brake proportioning valve on the pressure Regulator/DS regulator, that is # 18 in the drawing, horizontal mounted
    34331150922 is a pressure warning switch/Brake pressure differential switch , that is # 20 in the drawing, vertical mounted
    34331159752 controls a small flow of approx. 0.7 liter/minute for the hydraulic brake accumulator (DS) and loads/keeps it at a pressure of 35 - 57 bar.

    http://www.bmw-etk.info/parts-catalo...70/34331159752

    when I use Authaus with a 1991 850 they have
    ATE Brake Pressure Warning Switch; Fine Thread; 20 Bar P/N: 61311362977 ATE Cross References: 34351182270, 857919457, W0133-1627160, 340029, Mounts horizontally on side of DS regulator (has M10x1 thread)
    34331150922 ATE Brake Pressure Warning Switch; DS Regulator Sensor; Coarse Thread 1.5 Bar Vertical mount at top of regulator. 1 per car. Has M12x1.5 thread

    So the threads are different and could not be mixed up. Have you checked that?

    That Pentosin is coming out on the old switches between the terminals is usual when they get old. Provided you got the correct part numbers and put them in the right place , they should work. On some cars I found broken wires in the loom to the switches, that you should also check.
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  12. #12
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    Thank you both, Timm and Shogun, for your input. The switches are in their correct places, it's been checked twice. But what I can understand from your inputs and from what my checking the the high pressure switch (#19 in Shoguns picture), it is that the old switch is actually broken and always is open which is the normal state for it. Now when I changed it to a new one it is actually working as it should and showing, with a flickering warning light, that I likely have a brake bomb error, and it probably have been there for quite some time.

    From Timm's thread:

    "The common symptoms of a failed accumulator can be an intermittent brake warning light that can come on when the brakes are applied, or a too soft brake pedal that does not improve after bleeding the brake hydraulics. A good check for the brake hydraulics is to pump the pedal till all boost is gone, then if the pedal is still spongy then the problem is in the brakes. Other issues common to the booster system are leaking pressure switches at the DS regulator and leaks at the hydraulic booster. The brake booster leaking will vent power steering fluid (either ATF or Pentosin depending on the system) at a drain hole between the booster and the master cylinder.

    Paul Dzimian
    BMW CCA TSA
    Shop Forman
    Motorwerks BMW"

    This looks like a case of "..who's guarding the guardian..". The EKM computer has no way to check if the switches (both of them are in parallel with each other and is normally open) are working as they should. Which in my case is the problem, one of them did not work.

  13. #13
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    Test the brake bomb
    The simple test of the bomb is to run the car for a minute to allow the pressures to normalize then shut off the engine and then pump the brake pedal until all power assist is lost and the pedal becomes hard. Typically a good accumulator will give you about eight pumps of the brake pedal before you loose assist. A bad one will be hard after one pump and the accumulator needs to be replaced. The second part of the test is to check the DS regulator. Again run the motor briefly to build pressure then shut it off. Now wait five minutes before applying the brakes. You should have at least half the pumps with assist that you had when you did the test the first time. If not the valves in the DS regulator are leaking down to quickly. The DS regulator cannot be serviced and would need to be replaced.

    From my experience a DS regulator breaks very, very seldom, actually I have never seen one.
    Here a pic how the system works , function diagram from the maker
    Hydr.Bremskraftverst_D[1].pdf

    For general function test
    Bei abgestelltem Motor Hydrospeicher durch ca. 20 Betaetigungen des Bremspedals mit dem einer Vollbremsung entsprechenden Kraftaufwand den Hydrospeicher entleeren
    = with engine off push the brake pedal with the same force like in emergency brake situation about 20 times to empty the brake bomb
    (the you should also see the level in the Pentosin container remarkably higher)

    Bremspedal geringfuegig belasten, Motor starten, dabei muss sich dass Bremspedal spuerbar tiefer senken. Sollte dieser Effekt nicht eintreten, kann die Lenkungspumpe, der DS-Regler, der Hydrospeicher oder der hydraulische Bremskraftverstaerker defekt sein
    = Push down the brake pedal slightly, start the engine, then the brake pedal must go down further. In case this does not happen, the flwg parts can be defective: power steering pump, the DS-regulator oder the hydraulic brake booster unit
    Last edited by shogun; 06-15-2018 at 08:27 AM. Reason: trying to fix the picture
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  14. #14
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    Interested to see the resolution to this one.

  15. #15
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    The link you gave, Shogun, does not work for me it only gives a blank page, sorry.

    I did the following tests:
    1/ Started the engine briefly and then checked the hydraulic level in my reservoir. It is just below the filter mesh. Pushed the pedal 11 times before the level stopped raising about 10mm below the top edge of the reservoir.
    2/ Did the same again but waited 5 minutes before the braking operation. Could brake 8 times before the level stopped raising.
    3/ The brake pedal gives when I start the engine and pushes the brake pedal at the same time.

    I assume that this shows that my hydraulic system is pretty OK!
    3/ Says, according to the repair manual, that my ABS/ASC+T is OK.
    2/ My pressure regulator is OK and does not leak too much.
    1/ That I can stomp 11 times on my brake pedal says that my pressure accumulator is not top-notch but still usable. Also when I brake to a screeching halt the warning light is flickering lightly showing that my pressure reservoir is not large enough. I better find a new one!

    I think I have found a way of checking if the high pressure switches are working. Both switches are connected in parallel with each other and are normally closed when pressure is below their rated pressure, a closed switch is shown with a red warning light. When I have emptied the bomb of any pressure and starts the engine the warning light (red) will light for about 5-8 second before going out. This is because of the time it takes to build up the pressure in the system. In my car this never occurred with the old high pressure switch since it was always in an open state.

    Thank you all, for your help.

  16. #16
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    Since the last thread I have read more about the hydraulic system in the 850. One question lingers in my head. How did the manufacturers (ATE, Febi etc) pressurize the accumulators from the very beginning? There ought to be some valve on the bottom side of the "bomb" where the nitrogen gas was first introduced. Also the Olaer company (https://olaer.ch/en/hydraulics/hydraulic-accumulator/) asks its customers what pressure they want in their accumulators when buying. Maybe there is a possibility to re-pressurize the "bomb" with new nitrogen?

  17. #17
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    The Nitrogen is in a rubber bladder - it is usually the failure of the rubber that stops them working
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  18. #18
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    Yes Timm, I know but I am asking if there is any valve in the bomb that can be used to fill it with nitrogen.

  19. #19
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    HAHAHA yep this is an interesting one, the solution is to replace your old brake bomb. I have seen several of these exact scenarios.

  20. #20
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    Yee, that's right or replace it with another not oem or buy a second hand on ebay or refill it to the factory standard with nitrogen gas.

  21. #21
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    There is no valve outside the brake bomb like on the ones for Citroen. I once cut a LAD bomb into 2 halves and made a youtube, there you can see the bladder and the plastic nipple on top https://www.youtube.com/watch?v=ZWmQ0pECJ10
    At the bottom you can see a welded piece, someone in Germany installed a valve there for refilling, but for that the bladder must be still in good condition.

    Citroen
    DIY How To Refill Citroen Xantia [ BX XM C5 ] Shock Absorber Spheres , Sphere Recharging https://www.youtube.com/watch?v=l8KaM9bpyu0
    Last edited by shogun; 07-19-2018 at 05:15 AM.
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  22. #22
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    This is fascinating, so how did the manufacturer got the nitrogen into the "bomb"?

  23. #23
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    probably from the bottom of the bomb, there is a small welded piece. Someone on the German forum removed this welded piece and added a connector for refill, see post #38 https://www.bimmerforums.com/forum/s...1#post30069861
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  24. #24
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    I finally found a new pressure accumulator or "bomb". It is dated 10 05 15. Changing was quite undramatic and a new filter in the small filter cup was changed as well. The filter cup was quite dirty as a matter of fact. Some new Pentosin went in, engined started and the steering wheel was moved from left to right a couple of times. I did bleed the hydraulic system as well. No more flickering of the warning light and the brakes are working better than ever. What was a big surprise was the fact that the steering suddenly was much more distinct and precise.

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