Do I have to change the wheel sensor when replacing the rotor

  • 1 POST
  • 2002 MERCEDES BENZ C320
  • 3.2L
  • V6
  • RWD
  • 118,000 MILES

How do you change the rotors and can you keep the wheel sensor or
do you have to change it too?

Do you
have the same problem?
Sunday, November 13th, 2016 AT 7:37 PM

1 Reply

  • 29,775 POSTS

The ABS wheel speed sensor is built into the bearing assembly and has nothing to do with the rotor. The rotor design is identical to that on most other late model cars. Remove the wheel, then remove the brake caliper. Be sure to support the caliper so it is not tugging on the rubber flex hose. The rotor will just slide off, at this point I should mention I have never worked on this model, so if what I am describing does not look right, you will need a service manual to find the correct procedure. On most European cars, there will be one or two small screws that must be removed. If the rotor does not slide off easily, run a larger-diameter bolt into the threaded hole, and tighten it to push the rotor off the hub. Those screws do not have to be replaced. They are there to prevent the rotors from falling off when the chassis is flipped over on the assembly line. If you do reinstall them, that will prevent rust from forming in the threaded hole and will make life easier next time you need to do this.

Before you slide the new rotor on, check for any rust or scale that chipped off that could get stuck between the rotor and hub. Clean that surface as necessary. To prevent a harmless but irritating crunching sound when cornering, put a light coating of high-temperature brake grease on the hub where the rotor's center hole will make contact. Be careful to not get any grease on the rotor's or pads' friction surfaces. Professionals even avoid getting fingerprint grease on brake parts.

Always use a click-type torque wrench to tighten the lug nuts to specs. Every tire and alignment shop will have a wall chart with the correct setting for your car and they will be happy to tell you that spec.

Hint: When you are doing nothing else besides replacing the rotor, expect it to be thicker than the old one. Before you unbolt the caliper, use a large flat-blade screwdriver to pry the piston back into the caliper housing. That will provide the clearance needed to reinstall the caliper over the rotor.

Hint 2: The first time you press the brake pedal, it will be possible to push it all the way to the floor to run the piston back out of the caliper housing. Do not do that. After about a year, crud and corrosion build up in the lower halves of the bores in the master cylinder where its pistons do not normally travel. Pushing the brake pedal over half way runs the rubber lip seals over that crud and can rip them. That will result in a slowly-sinking pedal, and that often does not show up until two or three days later. People who think they need a helper to pedal-bleed the system often run into this problem.

Be aware that if some well-meaning but uninformed person previously topped off the brake fluid in the reservoir, during other routine service, like oil changes, pushing the piston into the caliper housing is going to push all the brake fluid behind it up into the reservoir where it will overflow and make a mess. Brake fluid eats paint, so have a bucket of water handy in case you need to rinse it off. If you need to add brake fluid, be extremely careful to not get the slightest hint of petroleum product mixed in with it. I even wash my hands before poking the rubber bladder seal back into the cap. One drop of engine oil, transmission fluid, power steering fluid, axle grease, or penetrating oil will contaminate the entire hydraulic system. The only fix for that is to remove every part that contains rubber that contacts the fluid, flush and dry the steel lines, then install all new parts that have rubber in them. That one drop of oil can easily make a car this age not worth repair, especially when it also has an ABS hydraulic controller.

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Sunday, November 13th, 2016 AT 8:52 PM

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