Brake Pressure warning

[SergeBMW]

Hello everyone,

On my way to my job interview, the car decided to remind me it was thinking of me... It began throwing brake pressure warnings at me at random. Sometimes it would dissappear and reeapear. It is the 89 750. Pedal feels good, except maybe at nearly full depression, you hit a stop, but if you hold it there eventually the pedal will go down a bit further to the next stop. This kind of pressure though is a lot even for emergency braking. I tried while driving and never managed to hit the end which is good.

I disconnected both sensors, one at a time on the regulator, the valve on the side seems to be the one sending the warning. Is this the over pressure or under pressure sensor?
I read on Johan and Sean's E32 page that when he disconnected the bad sensor, the warning dissapeared. He said he knew it needed replacement. Isn't this diagnosis a bit flawed though? I'd the sensor is not electronically connected, you'll never get a warning regardless if it's a sensor fault or an actual over/under pressure issue.
This is the link to their page
http://bmwe32.masscom.net/sean750/br...kePressure.htm


Wondering how I could confirm if it's the sensor or an actual fault in the system



Regards,
Sergio S.

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[shogun]

02 Oil pressure switch M10X1 ZYL. 12/1990 34351182270 $40.63
03 Brake pressure differential switch 12/1990 34331150922 $43.02
http://www.realoem.com/bmw/enUS/show...diagId=34_0277

Looks like the switches are internally bleeding, that is the typical picture, the plastic between the pins disintegrates and then the switches do not work anymore.
There are aftermarket switches, search with the BMW p/n and you will find them. Autohausaz has them for example
61311362977 ATE Brake Pressure Warning Switch; Fine Thread; 20 Bar, Mounts horizontally on side of DS regulator. 1 per car. $29.66
34331150922 ATE Brake Pressure Warning Switch; DS Regulator Sensor; Coarse Thread 1.5 Bar,Vertical mount at top of regulator. 1 per car. $29.11

Have seen some cars using the brake pressure regulator without the switches connected. mostly modified ones like an E34 with an M70, or E30 with M70. Of course, then never a warning comes, I have my ones connected and replaced them already when they started leaking.
But also check the wiring from the switches, they are quite thin and could be broken.
Usually the fault is just the sensor or the wiring, the DS regulator itself is quite bullet proof

[SergeBMW]

Good to know, guess as long as the pedal feels ok we're good.
Is it good to run it with the sensor disconnected until I get a new sensor in? I noted your comment on E30s and E34s, but wondering if in the E32 it controls the pump pressurization or is just a warning to the driver. I don't want to ruin anything by doing so.

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[shogun]

I know a Racing Dynamics E34 550 with the M70 and the H31 brake booster where I made a trans service for the 4HP24, that car has non of the 2 sensors attached to a wire loom. But better do it, see below explanations.
Here some more info on the system https://www.bimmerforums.com/forum/s...+brake+booster
https://www.bimmerforums.com/forum/s...+brake+booster

34331159752 is the brake proportioning valve on the pressure Regulator/DS regulator.
34331150922 is a pressure warning switch/Brake pressure differential switch
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. So if the brake proportioning valve is not functioning as per spec., the brake bomb has a different load.

Download this, German technical desciption of the H31 system, including testing of the single components. http://www.bimmerboard.com/members/s..._H31_00000.pdf

function, how it works, download it http://www.bimmerboard.com/members/s...mskraftverst_D[1].pdf
Hydraulische ATE Bremskraftverstärkungsanlage / hydraulic brake booster system H31
1. Vorratsbehälter / storage container
2. Tandemhauptzylinder mit Ausgleichsbehälter / tandem main cylinder with expansion tank
3. Hydraulischer Verstärker/ hydraulic booster
4. Druckgesteuerter Stromregler / pressure controlled flow regulator
5. Hydrospeicher / hydraulic storage
6. Lenkungspumpe / power steering pump
7. Servolenkung

Ausgangsstellung = normal position/when not breaking
Teilbremsstellung = partial braking
Vollbremsung = full braking

this is the important point: Druckgesteuerter Stromregler mit Hydrospeicher = pressure controlled flow regulator with hydro storage
Speicherladestellung = brake bomb loading position
Umlaufstellung = circulation position

[SergeBMW]

Thank you very much, some great info. I had disconnected it just to get the car moving without it chiming every 10 seconds. I will be ordering a new sensor and seeing if that fixes my issue. If not then it seems its safe to assume I have a bad regulator valve.

Ill update once I install the new sensor.

[CamE32]

Pedal feels good, except maybe at nearly full depression, you hit a stop, but if you hold it there eventually the pedal will go down a bit further to the next stop. This kind of pressure though is a lot even for emergency braking. I tried while driving and never managed to hit the end which is good.

A couple months ago I started getting the same brake warning when it started getting cold. I knew since I got the car that my brake accumulator was shot because of the exact symptoms you describe. Seems like mine might have been a little worse though, it felt like hitting a brick wall on the occasion you need to panic-brake and then it would go down a bit further before applying braking force... with a scary half-second delay.

I bought a new accumulator and tried to install it myself... but I gave up and paid a mechanic to do it. Since then I've had no more brake pressure warnings and the brake feel is drastically improved. I actually skidded a few times at yellow lights because I was so used to the old amount of pressure required to stop the car in those situations. I've had to relearn the brakes but they are better and smoother than ever.

All this to say I'd suspect the accumulator itself as the cause of the warning and the other symptoms you describe over a faulty sensor.