FAQ > ASC or DSC, ABS, Brake lights on. What’s up?

Very often I come across people who have the ASC or DSC ABS lights/brake on and sometimes with the gas gauge niddle going up and down and speed niddle staying put.
Often as well as all those scary symtoms the mileage does not move either. It is not counted anymore.
Christmas tree time like some like to say :-)

Here is a typical picture of what you might have. For the record, it is a pretty common fault on BMWs

If your car is unable to show the speed and the mileage is not moving anymore, most likely it is due to a faulty speed sensor. I have seen people taking a guess and changing all of them (Extremelly useless). I have also seen people sending their ASC/DSC control unit to some refurbishement company (There is a few who do a good job) or even buying a new module from the dealer just by taking a guess without running a computer diagnostic on their car first.
Many of them are reporting the problem is persistent because they have not done the right thing and replaced the good part.

Remember guys, advices from forums are fair enough to get some directions sometimes, but nothing is ever going to replace a proper diagnostic. And this is the only proper reflex you should always observe first. Stop buying parts for the sake of it if you do not know what is the problem is exactly.

If something like this is happening to you, here is what you must do to avoid buying irrelevant parts.

1. Get your car properly diagnosed (Yes that means no Carsoft, Made in China eBay crap for 65 quids etc…) with a BMW aware computer. A general OBD scanner will not do the job either. So that means Autozone will not work either.
Pay your dealer if need be, or consult your indy, as long as they provide you with a report from a proper diagnostic system (BMW GT1 or independant system such as Autologic).
Make sure they provide you with the report so you can make sure they are not trying to screw you.
Those systems cost well over 1000 quids, so usually specialised mechanics or dealers have these only.

Autologic

BMW GT1

The report will show exactly what is wrong with your car. My bet it is either a speed sensor or a ASC/DSC unit. If it is a speed sensor, it will also tell you which one from the 4 is not responding exactly.
My little finger is also telling me the speedometer data is actually pulled out from only one or two speed sensors out of the 4. But I cannot find an accurate technical source to back this up. If anyone knows, I am glad to be corrected…

Anyway bear in mind older BMW such as E36, E34 etc do not have 4 speed sensors. The speed sensor was on the differential as opposed to newer models where there is 1 speed sensor on each wheel such as e46 etc.

2. If the problem is down to a speed sensor, make sure you inspect it first. It might be disconnected, corroded, dirty. Give a clean before you change it. If the problem persists, just replace it.

3. Now of course if the diagnostic pointed out your ASC/DSC/ABS unit is faulty it should be fixed too.
BMW does not fix this unit, they can only provide you with a new one. They are not cheap usually (Over 700 USD).
Alternatively you can send your faulty unit to a specialist to get it repaired or buy a refurbished one. Either way it will need to be encoded by the dealer or independant specialist.

4. It might also be the case that your steering angle sensor is faulty. If a change is needed, bear in mind it should be encoded and calibrated by the dealer or independant specialist.

Note:
You can drive without a ASC or DSC/ABS unit.
Just bear in mind none of those function will be available to you.


So for safety reasons only drive your car slowly for short distances and under ideal weather conditions if you need to bring your car over a garage for example.

HTH guys

5 comments

  1. Takechan /

    Hi,

    I’ve been dealing with a ASC/DSC problem for a long time. My car is a BMW E39 520i from 1996 (December).

    I had my car scanned with INPA and it showed a fault on the left front speed sensor. I replaced it, but the problem persisted. I then measured the resistance through the speed sensor as well as the resistance through ASC/DSC unit. No problems.

    Now only two things could be wrong. I was 100% sure it was my ASC/DSC unit which was faulty, but getting a new (used) one would be quite expensive (I live in Europe). I instead got a new left wheel bearing and replaced that first. Solved the problem!

    The magnetic ring in the bearing had somehow become damaged. So the first sensor I replaced was actually working. It was just destroyed when it was removed, because it had never been removed before.

    So, I just wanted to say that remember that it’s also very probable that there might be something wrong with your wheel bearing. So check the resistance/wires also!

  2. Takechan /

    Admin,

    No, thank you! I managed to solve another electrical problem as well thanks to this great website (BMW WDS).

    The driver door didn’t work, or well only my window and side mirror did. I couldn’t control the passengerside window or mirror. The central lock didn’t work either (only on the other doors).

    So the door had no connection from the door to the general module, meaning a problem with the P-BUS. I managed to find the pins by looking through BMW WDS, so I could measure the resistance from the wire in the door – yeah it was broken, so I installed a new wire and now everything works again!

  3. patrick s /

    mostly there are two types of abs(asc, dsc whatever)systems installed in e38 and e39
    pre 9.98 and after
    pre 9.98 has only hydraulics in engine compartment while after 9.98 has all electronics and hydraulics there.
    when pre 9.98 systems control units will not fail at all, after 9.98 control units FILL FAIL WHEN DRIVING WITH ABS LIGHT ON.
    when your system will stop working and you have car produced after 9.98 it is essential to remove main connector from abs/dsc unit to protect it. then you can safely drive to car diagnostic and diagnose your problem.
    if you have pre 9.98 and having acs only not abs lamp lit then you have to check cable that connects asc control to throttle body (petrol engine). later models will have eml lamp added to system, then its probably electronic throttle body or connecting wires that are failing (petrol engine)
    asc or dsc control units have to be coded when changing engine type (petrol-diesel)
    dsc control units need to have steering angle sensor and acceleration sensors adjusted

    btw. left rear abs sensor will transmit speed signal to speedo

    e46 abs will not fail from broken(shorted)abs sensor.
    they have other common problems but its another story already
    patrick s

  4. i installed a rebuilt abs module of the same model, but the fault code did not clear and instead telling to calibrate module for detection, how do i calibrate this?

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