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Introduction to alternators, part eight, alternator components continued

Electric motor brushes serve as electrical conductors to and from a stationary electrical circuit or component and a rotating electro-mechanical unit. Electric motor brushes are used in electric motors, generators, and alternators. In alternators, brushes are used to provide a contact point between alternator rotor slip rings and the voltage regulator or the rest of the exciter electrical circuit in the vehicle.

Alternator brushes
Alternator brushes
A set of alternator carbon brushes

Electric motor brushes are made of various composites, usually a mixture of graphite and carbon, or copper. A type of resin or pitch is added to the mixture so that the brushes will keep their shape when they are compressed in a mold. During the compression process the brush wire, or shunt, is added to the brush. After the compression process, the brushes are heated treated up to nearly 1200°C to make the brushes durable. The brushes are then impregnated with resins or oils to allow for less friction while in contact with commutators or slip rings. Finally, because of heat treatment, the brushes are reshaped by grinding to meet manufacturer specifications.

Alternator brushes are usually the “weakest link in the chain” because they are subjected to the most wear and tear as result of constant contact to a moving component, i.e. the alternator rotor slip rings. The brushes are held against the slip rings by compression springs in the brush housing. As the brushes wear the springs continue to apply pressure on the brushes. Eventually, the brushes become so worn that the brush springs cannot keep sufficient pressure on the brushes. The brushes begin to bounce or chatter causing arcing. The arcing causes burn pits in the slip rings to the point where sanding the slip rings with fine emery cloth or sand paper will not remove the pits, then the slip rings would have to be replaced. Depending on how severe the vehicle is driven, brushes could last as long as 150,000 to 200,000 miles. The upside is that the alternator brushes are usually the least expensive part and easiest to replace. It may prudent to replace alternator brushes on a maintenance basis between 100,000 and 150,000 miles.

Alternator brushes are usually attached to the exterior of the back end of the rear alternator housing, usually held in place by, most often, two to four machine screws. Alternator brushes are sometimes part of the regulator assembly which usually is also attached to the exterior of the back of the alternator. There are some applications where the brushes and/or regulator and brush assembly are secured to the inside rear alternator housing, which would require disassembly of the alternator to replace the brushes. Alternator brushes can be purchased at most automotive parts retailers such as NAPA, Federated Auto Parts, CARQUEST, O’Reilly’s, and AutoZone.

If any of these procedures appear to be beyond the capabilities of the vehicle owner or driver, then servicing an alternator should be performed by a professional or ASE Master Certified mechanic. The vehicle would have to be taken to a repair shop that employs these types of mechanics such as A & M Alternator Services located at 2419 E. Jackson St. in Phoenix, Auto Electric Specialists located at 5216 W. Lamar Rd. in Glendale, Village Auto Electric Service located at 19 N. Miller St. in Mesa, All Start Electric located at 13501 E. Chandler Blvd. in Chandler, Tom’s Auto Care located at 63 E. McKellips Rd. in Mesa, Jordan’s Automotive Specialists located at 8718 E. McDowell Rd. #3 in Scottsdale, Rob’s Quality Automotive located at 11801 N. Cave Creek Rd. in Phoenix, Scottsdale Pro Tech located at 8245 E. Butheruand Dr. #111 in Scottsdale, and Art’s Family Auto Repair located at 915 W. Hatcher Rd. in Phoenix.

As always, before working on any automotive repairs, be sure to wear safety glasses and protective clothing such as gloves and long sleeve shirts to protect oneself from possible injury. Remember, keep the work area safe and follow safety precautions.