Car - Brakes
Collection of Reference Material for Brakes (Pads & Rotors)
The sensors for the brake lining light on a BMW are located on the inboard pads of the left front and right rear wheels. When the sensor's insulation is cut through, the lining light will begin to come on when you touch the brake pedal, this is because the exposed wire is grounded against the rotor. If you leave the brakes a little longer, the wire will be cut completely and this will turn the light on constantly. BMW brake sensors tend to give premature warning of low pads, when the sensor is cut, there is often 3-5 thousand miles left in that particular pad. However, as only 2 pads out of 8 are monitored, it is possible that one of the other 6 is much lower, and will cause damage to rotors if left unchanged.
How to change pads and rotors
and support your vehicle, even if only removing one wheel at a time, do
not risk your life by leaving the vehicle supported by the jack alone.
If you are changing the rotors, you will need to remove the caliper bracket, on most models this is held on by 19 mm bolts, some use 17 or 15 mm bolts. Next , locate the mounting screw in the rotor, this will be a 5 or 6 mm Allen bolt, depending on model. The bolt is recessed into the rotor, take a hammer and give the rotor several hard hits directly above this bolt, this will help loosen the bolt. An impact driver is a good tool to use now, but an Allen key or socket and ratchet will usually work just as well.
If you strip the bolt head and cannot remove it, as you are changing the rotors anyway, you can turn the rotor so the bolt is on the opposite side of the caliper mount, and hit the rotor from behind until it breaks away from the bolt. This will leave the bolt in the hub, with a small section of cast steel from the rotor, you will now be able to get some vice grips on the bolt and remove it that way. Worst case scenario, you can cut the head off the bolt and not worry about it, It is not an essential part of the assembly. Its purpose is to hold the rotor in position during assembly. Leaving this bolt out causes no problem other than some slight difficulty in aligning the wheel and rotor with the hub to install the wheel bolts.
While the rotor is off, use a wire brush to clean the hub thoroughly, get rid of any rust or alloy residue. Coat the centre of the hub with some anti seize lubricant and then install the new rotor. If you are replacing or reusing the retaining bolt, use the anti seize on it also.
If you are doing one wheel at a time, you can now remove the old pads and squeeze the piston back into the caliper. If you are doing 2 or more wheels at once, you may wish to drain some brake fluid from the reservoir. The fluid system in BMW's should not need to be topped up as the reservoir holds enough fluid to keep the system supplied until all the brake pads are very low, however, if you have had a recent brake fluid flush, the reservoir will be full, and as you press the piston back into the caliper, you will displace fluid back into the reservoir, if the reservoir over flows, you will have a nice mess to clean up. Better to siphon some fluid out first. Also , as you do each wheel, its a good idea to pump the brake pedal a couple of times to move some fluid back into the caliper. DO NOT pump the brake pedal with a caliper off the rotor, you may force the piston right out of the caliper, a potential disaster for a home mechanic.
If you are at the front left or right rear wheel, install a new sensor in the inboard pad, this is the one with the metal clips on the back. The thicker part of the sensor goes to the front of the pad. Thread the sensor wire through the cutout in the caliper and press the pad's clips into the piston. Place some specific brake pad lube on each end of the caliper bracket where the funny shape of the pad will rest, install the bracket over the rotor, and place the outboard pad into the bracket. Make sure this pad is in the right way.
Press the caliper bolts into the bushings and slide the caliper over the rotor. Tighten both caliper bolts and replace the caps on the bushings. Replace the clip on the front of the caliper and route the sensor out of harms way. Pump the brake pedal and install the wheel. Repeat the procedure on the rest of the wheels.