PeachParts Mercedes-Benz Forum - View Single Post

Diseasel300 · #12 11-28-2017, 10:01 AM

The squeak could have been either bearings or brushes. It isn't uncommon for hard brushes to squeak at times, especially if they're worn and the spring tension is lowered. The bearings in the fan motors also tend to be sintered bronze bushings that will squeak/squeal when they dry out. Adding oil to a squealing sintered bronze bushing is a band-aid, the squeak will be back or it'll seize since the burned residue that caused the squeak is still there.

The sintered bronze bushing is porous by design and holds a given quantity of oil that it "sweats" while the shaft rotates in it, creating a very thin oil layer (exactly the same mechanism that's undesirable on the brushes/comm, but very desirable for a bearing). Once the oil evaporates or polymerizes, the bearing stops working and the shaft rubs. You can often restore one of these bearings by cleaning the contact area with solvent to clean away the sludge and re-open the pores, then soaking it in thin oil. You can often get several years of good service that way.

The comm and brushes will wear just from friction over time, but the wear is very even and predictable. Most dust/dirt/debris is self-clearing since there is nothing to hold it there. The brush tends to simply sweep it away harmlessly. This is where the oil contamination comes in - As it gets hot it polymerizes and becomes sticky. In addition to creating a poor brush/commutator interface, it holds dust and dirt in place and turns the brush into a grinding stone. As the brush starts eating the commutator, the oil residue holds more and more debris and the cycle accelerates. Cleaning the comm/brushes with a clean-drying solvent like Acetone will generally do the job well. Acetone has the additional benefit that any oil residue that is leftover will turn to a powder and will disperse when the motor runs.