BMW E39 ABS Control Unit Repair
Typical fault description:
The diagnostic lights for ABS and ASC/DSC are sometimes flashing in my BMW E39.
Is it a control unit or ABS sensor fault?
How do I know whether my control unit is broken or it’s only the sensor?
BWM ABS ecu 5.7
If the diagnostic lights are on, you will usually find a fault code in the fault memory of your control unit. (Exception: The control unit does not communicate anymore)
BMW ABS control units have two types of faults:
- electric circuit fault
- signal plausibility fault
Signal plausibility faults:
- fault code 4: Sensor rear left no/wrong signal
- fault code 5: Sensor rear right no/wrong signal
- fault code 6: Sensor front right no/wrong signal
- fault code 7: Sensor front left no/wrong signal
Electric circuit fault:
- fault code 30: Sensor rear left interruption
- fault code 31: Sensor rear right interruption
- fault code 32: Sensor front right interruption
- fault code 33: Sensor front left interruption
Each diagnostic device may show slightly different diagnose texts, thus the error code itself is the decisive factor.
Electric circuit faults
Causes of electric circuit faults:
- wheel sensor
- connection cable between the control unit and the sensor (interrupted or short-circuit e.g. with chassis)
- ABS control unit
In case of electric circuit faults the control unit cannot determine whether the fault occured inside or outside
This can be diagnosed easily by the symptoms.
Usually the car starts normally on the first drive of the day, diagnostic lights signal only after a few minutes and fade after the engine is stopped for a while and has cooled down.
In the case of the BMW 5 E39, a quite unfavourable spot was chosen for the ABS control unit to be mounted. Due to its proximity to the engine manifold, the control unit is constantly exposed to heavy heat-fluctuations, as the waste heat is leaving the engine.
In case the fault occurs only when the engine is running hot, the control unit probably needs repair
Toggling ignition state (switch engine on an off) does not help.
Signal plausibility fault
In rare cases signal plausibility is the fault source.
The reason is, as the name suggests in the signal reception and usually originates from the combination of sensor and signal encoder rim (Geberkranz, wheel bearing).
Symptoms that hint to signal plausibility issues are for example pulsating brake pedals upon gentle braking, even though no wheels are blocked.
This results from an ABS interference and suggests that the control unit is itself functional. In this case the control unit does not receive a velocity signal from the wheel which is interpreted as blocked wheel, thus the control unit tries to unblock the wheel.
The sensor is marked defunct only after a longer plausibility test, which is signaled by the diagnostic light.
Toggling the ignition (turning the car on and off) turns off the diagnostic lights and initiates a new run of this process.
Sources for signal plausibility faults
- dirt on ABS sensor or wheel bearing
- defunct wheel bearing
- too high distance between sensor and wheel bearing
- defunct sensor
- cheap sensor from 3rd party suppliers
Based on our long experience in repairing this ABS control units we suggest, that in most of the cases, the sensor itself is the issue.
Siemens-VDO is the original equipment manufacturer of those sensors, we suggest replacing it with a functional Siemens-VDO sensor. We strongly advice against the usage of 3rd party components!
Solving the issue step by step
- compare the symptoms of your ABS with the above described issues
- try reading the fault codes with a diagnostic device to narrow down the problem
- fixing
Do not hesitate to contact us on further questions.
ECU.de stands for quality ecu repairs at a fair price.
Packing and shipping
Please fix the magnet coils with tape before packing the control unit. Magnet coils tend to snap off due to rough shipping.
