CHANGING BRAKE PADS AND  ROTORS ON YOUR BMW


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.

If you own a car that displays brake linings in the information display, this warning will appear from the first time the sensor is grounded. It will not be reliant on brake use to remind you. Also, if you own an E36 or E38, these cars have buffers built in to the system that will not turn the warnings off even after the sensors have been replaced. These buffers have to be bled down. The method varies between year models on the E36, and could be time related or ignition cycle related. The E38 is an unknown entity, even BMW does not have a method for turning the warning off, as a result, dealers often have to give customers their $60K + cars back after a brake job, with the lining warning still present. Regardless, the warning will go away after a short time, so don't kill yourself trying to find what you did wrong.

First you must identify what pads need to be replaced, front or rear, and if the rotors need to be changed with them.

BMW is renowned for soft brake rotors, they give much better stopping performance, but tend to wear out fairly quickly. Despite this, it is not usually necessary to replace the rotors with every brake pad change.

The major reason for replacing rotors is due to excessive lip on the outer circumference of the contact area. A large lip will cause the pads to sit on an angle, reducing contact between pad and rotor to less than half of the pads width. This results in very low pedals, and diminished braking capacity.

If the lip is small, say less than 1mm, you can notch the pads where they would contact the lip, this will avoid the problem. If the lip is larger, or there is scoring on the contact surface, the rotor should be replaced.

Brake pads should always be changed when rotors are replaced, and pads or rotors should also always be replaced as an axle set. Don't just replace one front rotor, it will result in unbalanced braking.

It is not recommended that you have BMW rotors cut, this almost always results in rotors that are either below minimum thickness or that will warp within weeks.

How to change pads and rotors
Raise 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.

With the wheel off, use a screwdriver to gently pry the clip on the outside of the caliper. You need to pry the clip from the centre of the rotor, towards the caliper, this will release the clip. Keep one hand over the clip while doing this, as the clip is a spring and it could fly off.

Locate the two rubber bushings at the rear of the caliper, pop off the caps and you will find the caliper bolts, which are 7 mm Allen head bolts. Remove these bolts, pull the caliper towards you so as to force the piston back slightly, then slide the caliper off the bracket and rotor.

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, don't laugh, I have seen many instances of mechanics, both home and professional, who, in a moment of inattention, managed to install this pad backwards. this will cause some nasty noises when you first drive off :-)

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.

Some BMW models use 4 piston calipers on the front, especially older models.

In this case, the caliper does not need to be removed from the rotor unless the rotor is to be changed.

In the case of these calipers, there are 2 pins running through the pads, these pins need to be knocked out, then the pads can be pulled out of the caliper. Pressing the pistons back can be fun, you press one in and the other comes out. Best bet here is to leave the pad in, press it back until both pistons behind it are all the way back, then install the new pad. Repeat the process for the other pad on the same caliper. then reinstall the pins.