Faulty Brake Master Cylinder Cap... - BMW Forum - BimmerWerkz.com

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post #1 of 3 (permalink) Old 09-17-2004, 08:48 AM Thread Starter
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2001BMW330iRob is an unknown quantity at this point
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...Intermittent warning lights going off on my dash?
My emergency brake light, DSC, and ABS light go on some days and off others.
I topped up the brake fluid in the Master Cylinder and the lights went out for a while.
I didn't lose any fluid and the lights now are intermittent.
Now they come on one day and off the next?
Was wondering if I replace the Master Cylinder CAP if this might solve the problem.
I know it is nothing serious, just a sensor.
Unfortunately the CAP is $50 CDN. Still cheaper than the Stealership to work they're magic finding all kinds of problems when there are none.
There is a sensor than plugs into the Master Cylinder Cap and was wondering if that cap could be the problem.
Anyone else have this problem and what was the fix.
I hate the Stealership and would never go there, I don't want to be hit with a $500 bill saying there is something seriously wrong with the car when infact it was a sensor somewhere!
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post #2 of 3 (permalink) Old 09-27-2004, 09:10 PM
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Superfry is an unknown quantity at this point
I have this exact some problem on my 2000 323i. I bought the car in january with 75k, and around 90k those same lights starting coming on intermittently (now i have 99k). I had always assumed the cause was a flaky or dirty abs wheel speed sensor - haven't gotten around to taking all the wheels off and checking them all. I started noticing that the system seemed to work when it was wet or humid out, and be more likely to fail when it was dry. therefore, i assumed it was a sensor connection. of course there have been many exceptions to this rule, so it could just be my imagination. also - occaisionally during startup, i will only have the DSC and ABS lights on ('brake' light goes off with parking brake is released). in this case, the DSC and ABS lights always go off after i drive for a few seconds.

i checked my brake fluid reserviour after reading your post. the cap on my reserviour doesn't have a level sensor, but there appears to be one next to the cap - next time the system seems to work, i will unplug the level sensor and see if it causes the lights to come on.

does anyone know of any other sensors that i can check which may cause this problem? i really don't feel like going to a dealer for this one, but i really need DSC for the upcoming winter.

Rob - let me know if you make and progress and i will do the same

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post #3 of 3 (permalink) Old 09-28-2004, 03:51 PM Thread Starter
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2001BMW330iRob is an unknown quantity at this point
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I did flush my brakes and the lights have not come back?
Not sure if that did the trick cause they were off before I did it.
Figured it could not hurt.
With the right tools its easy.
Without them you could screw everything up.
Anyhow I dont drive my 330i all that much, my daily driver is a '97 318i.
I have a long commute and would rather rack up the kms on the old bimmer.
As a result haven't seen the lights again come on.
I know what you mean about the humidity or wet conditions seems to make the lights go out.
Once the lights were on for an extended period, then I washed my car and the lights went out.
For me the ABS, DSC, and parking brake light all come on or all go off together.
Check your fluid level. Even just a hint under the max line could set the sensor off.
I topped mine off twice and both times the lights went off.
Really weird.

I did find this interesting post and reply from a Mechanic

Question: 1994 BMW 840Ci 4000 mileage: 83,000. Hi, I'm the unfortunate owner of a BMW 840Ci, with an ABS problem. On starting the engine the ABS carries out its usual test and both the traction control and ABS lights go out. As the vehicle accelerates away and reaches above 5mph, both lights immediately illuminate and stay on. The fault can sometimes be cleared by switching off the engine, restarting the engine and then counting to 30 before engaging reverse, then reverse the car with full left hand lock on the steering wheel! The car will then drive away normally without either of the ABS lights illuminating. If however either of the rear wheels are spun, i.e. on gravel, then both lights will immediately illuminate and stay on. The car has been into the local BMW dealership for diagnostic checking, where they informed me that the pump relay was faulty. This has been replaced to no avail (and also the memory reset). They are now talking of requiring to test the car further for up to a day, at 85 per hour, just to find the fault! Prior to sending in the car initially, I removed all four wheel sensors and checked each of their resistances, which were all comparable. Although one on the left rear was found slightly bent due to stone damage, it was easily straightened. All four wheel segment rings were also cleaned up and in good condition. Any positive advice of tests that I could carry out would be greatly appreciated, before breaking the bank. Thanks in anticipation.

Answer: Your problem probably lies in one of the four wheel sensors for the ABS. There is a better test than the resistance check you tried. Unplug each of the wheel sensors. Spin the wheel segment rings and check for AC voltage at the sensors. You should have between 1 to 3 volts AC if the system is operating properly. Also check to see if you have a slightly loose wheel bearing that is allowing the segment ring to contact the wheel sensor

Good luck.
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