Article by: Sean
Article applies to: all e32 and e34 models using hydraulic pressure for brake assistance (instead of vacuum assisted brake pressure).
Problem : Every now and then a ‘GOING’, warning ‘brake pressure’ (or ‘Bremsdruck’ depending on selected OBC language) and the red brake light comes on.
In my case, turning the steering wheel had some influence…sometimes the warnings disappeared and sometimes raised during driving and scaring the h*ll out of me.
First I had nightmares of a worn hydraulic pump ($$$), blown hydraulic pressure hoses and other sorts of problems causing stressful situations with high financial consequences, but it turned out to be a fairly cheap-to-fix problem.
Background : This has nothing to do with the level of brake fluid or worn brake pads (these are different warnings) but has something to do with the hydraulic system pressure.
The hydraulic system in your car operates at a certain system pressure for assisting the LAD rear suspension (if applicable), the power steering and the brake booster.
It exits out of a reservoir holding the hydraulic fluid (aka Pentosin), pump, hydraulic pressure regulator and a myriad of high pressure and return hoses.
Immediate brake pressure is delivered by an attached accumulator (hydraulic capacitor which is round like a bomb, aka ‘brake-bomb’, located underneath the pressure regulator (six bangers) or with an extended line behind left fog light (12 bangers).
This acts like a pressure reservoir, but has in fact nothing to do with the system pressure. If the bomb is blown you’ll feel a noticeable difference when applying the brakes, meaning a stiff pedal in emergency braking.
Some reported also the ‘brake pressure’ warning at that point, but again, only in combination with a stiff (rock-hard) pedal.
To keep an close eye to the system pressure, there is an over-pressure and under pressure switch mounted at the pressure regulator.
Let’s start unbolting parts:
When I popped the hood to check the hydraulic fluid level, I saw this:
This is the actual pressure regulator, with on top one pressure (the other just visible at the bottom of the picture).
So I pulled the cover of the upper one because of all fresh dirt and saw Pentosin seeping out of the sensor.
It shouldn’t be doing that (although it gives a nice rust protection to the surrounding environment).
I disconnected one of the wires and the warning immediately disappeared.
At that moment I knew the sensor was shot:
I bought a sensor from a salvaged 735i (identical to 750 sensors) because they are new pretty expensive. It’s a EXTREMELY good idea to relieve the system pressure first.
Engine off, depress brake pedal at least 15-20 times until the pedal feels rock hard. Get a 24mm wrench and unbolt sensor.
The actual sensor, unbolted:
Clean the area thoroughly and please, don’t let dirt fall into the bore.
You can use a new copper washer but, not surprising anybody, I used the old one and just tightened the ‘new’ sensor (it’s really a switch) just a bit more.
Total amount of parts and cost:
|BMW Part Number
|Brake Pressure Switch
Total amount of time:
Faster than the eye can see.
Skills needed/difficulty level:
I wish every repair was this simple, and found new confidence in myself after some serious LAD debacles.