Accessory Belts What Are They and Who Needs Them
Accessory belts is the new term for what use to be called “fan belts”. These are the reinforced rubber belts that drive all the components on the engine like the Alternator, Air Conditioning Compressor, Power Steering Pump, Water Pump, Supercharger, and so forth.
Their function is to keep the components rotating smoothly and efficiently by gripping their respective pulleys with enough force to maintain a smooth rotation. When this smooth rotation is not happening like on an alternator, the alternator starts introducing high frequency voltage spikes into the electrical system. These voltage spikes if they become severe enough can cause driveability problems including false check engine lights, vehicles in ‘limp in mode’ and more. It could also confuse the PCM through uncontrolled changing load factors.
One of the biggest problems with a reinforced rubber belt has always been that it stretches over its life. This stretching is what allows it to get out of adjustment and thus not supply adequate force to grip the pulleys properly. Years ago, it was commonplace for a mechanic to recommend a fan belt adjustment to restore the proper gripping force and extend the service life of the belts. Manufacturers made a provision for this adjustment, however where the problem arose was ‘What was the correct adjustment’?
Many manufacturers had a stated deflection measurement method to be used on the longest side of the belt between two pulleys. Some companies engineered a belt deflection gauge measurement tool of different designs, the best known was the Burroughs. This measuring deflection became an impractical adjustment procedure as the under hood area space decreased.
The manufacturers decided to place an automatic belt adjustment capability into their designs, so the automatic belt tensioner was born. It is simply a precalibrated spring that resides inside a housing, which is attached to a pulley. This design of course has its limits on adjustment and because of the design, the spring is constantly stressed and looses force, which means the belt slips.
This slipping belt of course means lots of friction or heat which the components were never engineered to take. Thus the permanently lubed bearings in many accessories including the belt tensioner wear out.
How you can avoid the added expenses of accessory component replacement caused by a slipping belt is to replace the belts when you first hear the ‘chirping noise’ similar to a bird chirp under the hood. This ‘bird chirping’ typically goes away on light engine acceleration. If the ‘bird chirping’ has become almost a constant noise you are operating on borrowed time, and are taking the risk of extra expenses. If a new belt is ‘chirping’ on install, recheck all the pulleys and tensioner to verify no problem exists there. If no problems are encountered, replace the belt with another.
There are other indications of impending belt failure and you should consult an experienced mechanic to discover these through a proper inspection. Be mindful that accessory belts have two sides and many manufacturers are now utilizing both sides to grip pulleys, thus your inspection should include both sides.
In the USA there are 3 major manufacturers of accessory belts used on the vehicles. They are Dayco, Gates and Goodyear. All three manufacturers make a good product, however most mechanics typically gravitate to the Goodyear belts as the primary replacement choice, followed by Dayco and Gates. Keep in mind that mechanics do not like having to do a job a second time because they rarely are paid to do so. Thus, their choices are focused on the least troublesome.
The parts stores have two levels of belt quality typically, since some belts are a little expensive. Choosing which quality level of belt to be used on your car requires some thought. The cheaper line of accessory belts typically will yield only about 18 to 24 months of service, whereas the higher quality can last years. So base your choice of which quality level belt to use more on life expectancy and less on initial cost.
Copyright © 2012 JD Durham
JD Durham is a World Class Technician, an Automotive Hall of Fame honoree with 45 years experience in the automotive service and repair industry and a staff writer at Straight Talk Automotive. You are encouraged to visit for more information on OBD2 Scan Tool, and OBD2 Bluetooth Scan Tool, also accept this invitation to join our FREE members only area and access great free resources.
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