BMW owners often report the issue of “speed limit failure”, ranging from new owners who have not mentioned the car for 2 months to “old cars” that have been used for a few years. In general, owners of the 3 Series have reported the most. In principle, I will summarize a little maintenance experience today.
First understand BMW’s speed limit function:
The BMW speed limit system was first applied to the European F10 as a standard configuration system. The vehicle speed can be limited to 30~230km/h. The driver can control the speed through the accelerator pedal, but once the set speed limit is reached, the accelerator will fail and the vehicle Will only be able to travel at a maximum speed.
The BMW speed limit system uses the engine control system and automatic transmission control system to control the vehicle speed. The speed limit module is generally integrated in the chassis management control module. Once the speed limit module learns that the vehicle speed has reached the limit speed, it will communicate with DME (engine Control system) and EGS (Automatic Transmission Control System) communicate to tell DME and EGS not to increase the speed of the car.
Therefore, from the control principle, the main reasons for the failure of the speed limit are:
1. The wire harness of the speed-limiting module is damaged or the plug connector is loose;
2. The communication between the speed limit module → FlexRay bus → DME and EGS is faulty;
Realistic solution experience sharing:
We know that the switch button for the speed limit function is integrated in the button area on the left side of the steering wheel. How is the switch signal transmitted to the control module? This requires the wiring harness in the steering wheel and the steering column in the steering system. Switch Center (SZL), this is also the main fault point encountered by BMW owners in reality. There are three points in summary:
1. Steering wheel wiring harness assembly;
2. Steering wheel airbag hairspring;
3. Steering column switch center (SZL);
Most BMW owners solve the problem by replacing one of the above three components.
Tips: When going downhill or stepping on the accelerator deeply, the speed limit function will temporarily fail and the vehicle speed will exceed the speed limit. But this kind of short-term failure will not appear the above alarm prompt, but the speed limit warning light will keep flashing until the speed is lower than the speed limit, the speed limit function will be automatically activated .