CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Braking Problem 2002 BMW 525i

I have a 2002 BMW 525i with 100k miles. Since I bought it at 30,000 miles there has been a braking problem. If I am applying the brakes and hit a bump or rough pavement, the brakes start to “pulse”, they make a “rattley, clattering” kind of sound, and it feels as though they will fail to stop the car. Removing my foot from the brake for a breif moment then reapplying the brake past the bump or rough pavement stops this from happening. Most mechanics claim they can’t reproduce it, but I can. At about 50K miles I decided to self diagnos the problem as a bad sensor or “motor” for the antilocking system. I had it replaced which appeared to resolve the issue for a while. But it is now back.

Anyone have thoughts about what this may be?

That is definately your ABS brakes kicking in. Going over rough pavement when braking will cause the ABS to read a slip since one tire is temporarily off the pavement and that says slipping to the ABS. Same thing happens to cars with traction control when they are accelerating on broken pavement, the system detects wheel spin.

Other variables to consider if this just normal ABS action. Your suspension, struts specifically, might not be as healthy as when new and allowing less contact between the tire and road. Your tire(s) might have less grippy tread or the tire rubber could be harder as the tire(s) age. If you have a switch perhaps you can turn your ABS and trac control on and off when you are operating in areas of broken pavememt.

If the ABS is malfunctioning then you could have another bad sensor, or the same problem your corrected once before.

Thanks! I have both new tires and new struts. I concur that this is probably a sensor. I have not looked into it yet, but I suppose there is one sensor per wheel?

You are correct that there is one sensor per wheel. The rear sensors may be up close to the differential rather than out at the wheel. Look carefully at the wires to the sensors. Wheel sensor problems are often traced to wires that are blowing in the wind and rubbing their insulation off, or pinched somewhere between two pieces of metal.