BMW 530d: The innocent gearbox

BMW 530d: The innocent gearbox

The BMW 530d was first registered in 1999, in the last millennium. Despite its advanced age, the premium station wagon with a 184 hp six-cylinder diesel engine is still a faithful everyday companion today and has been getting its owner through 270,000 kilometers without any problems. One day the warning light on the dashboard becomes noticeable and the corresponding message on the display reads “Gearbox malfunction – go to a workshop”. The driver finds it strange that the message only appears when cornering and disappears again on a straight road. A visit to the workshop is arranged and AGW Kfz-Technik KG in Vienna-Liesing is chosen. Dejan Krleza, one of the best transmission specialists in the country, works here - he takes on the case.

● The automotive technician connects the BMW to a diagnostic device - a wheel speed anomaly is stored in the error memory.

● A test drive shows no abnormalities in the shifting processes of the 5-speed automatic and confirms the information given by the handlebars: Everything is okay when driving straight ahead, but the ABS indicator light lights up when cornering.

● After turning off the vehicle and restarting it, all errors are deleted. However, on the next curve the error message appears again.

● The BMW is picked up with the platform in the workshop and inspected from below. The automotive technician turns a front wheel by hand and at the same time reads the signal from the wheel speed sensor. Dropouts in the signal transmission become noticeable.

● A more detailed analysis shows that the speed sensor sender wheel attached to the brake disc has a crack. This causes a signal interruption, which in turn irritates the control unit. The following warning message incorrectly indicates a gearbox defect.

● The encoder wheel is replaced, the error is fixed, the customer is happy. Instead of an expensive transmission repair, he got away with around a tenth of the cost. Case solved.

The wheel speed sensor of the vehicle described works according to the Hall principle. On the encoder wheel, permanent magnets are arranged alternately in the direction of the north pole and south pole. The active wheel speed sensor is supplied with a defined voltage and generates a square-wave signal in the form of a data protocol that is sent to the control unit. The system works very precisely, so that a speed measurement can be carried out from just 0.1 km/h. The signal is therefore also used by navigation systems, rollback locks and parking systems. To check for correct operation, the vehicle is lifted and an oscilloscope is connected to the ABS wheel speed sensor connector. As you rotate the wheel by hand, the pulse waveform is checked. Distortion or noise in the signal indicates a defect. If the visual inspection of the mechanical parts does not reveal any abnormalities, it is recommended to replace the speed sensor.