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Protect the alternator with overrun
An overrun generator ensures a long life in a highly loaded engine. It decouples the engine torque variations and minimizes the vibrations resulting in the generator. The overrun generator is mounted on the generator instead of the previously used v-belt pulley alternator. Its life is limited to about 120,000 km.
Increasing demands lead to new strains
The demand for more and more comfort with less consumption of fuel leads to ever higher strained components in the design of vehicles: Nowadays, engines have much higher compression and yet have a longer life than a few years ago. The alternators, which used to only feed ignition and lighting, supply many more consumers with power these days. Heated seats, navigation instruments, control units, electric window, mirror and seat adjustment, and the significant increase in sensors have led to a much higher demand for electricity in modern cars. However, rather than fitting larger alternators, existing generators are operated at higher speeds in order to save on cost and weight. Higher compressions in the engine lead to stronger torque, which also exhibits itself through increased vibrations. Where high speeds, large fluctuations in the torque and high vibrations meet, wear of all components involved rises significantly. The overrun is a small yet very effective component, which can intercept most of the vibrations.
How does an overrun generator work?
The specialty of the overrun generator is that the pulley wheel, at which the engine torque is transferred to the alternator, can only be turned in one direction. When the speed of the motor increases, the speed in the generator also increases proportionally. However, if the engine's speed decreases, the generator continues to run due to its momentum. Thus, the transitions between the speeds do not occur as directly and suddenly as used to be the case with rigidly transferred torque. The mechanical implementation of this decoupling of the speeds is naturally very complex from a mechanic standpoint. The unilaterally acting, steadied running direction requires many movable small parts in the overrun. Therefore, the life of this component is limited.
Overrun generator defects
When the overrun at the generator is faulty, it is blocked. Then it acts as a simple pulley, which directly transmits the torques. The generator with overrun operation includes a separate tensioning fixture. In older v-belt drives, the belt's tension was produced right above the alternator. In an overrun generator, a separate tension pulley is installed, which always keeps the flat belts for the drive under tension.
A defective or blocked overrun is now relatively easy to detect, because the flat belt will vibrate strongly when the engine is idling. It also audibly changes the noise from the engine. To check the overrun, simply turn the engine on. The generator must continue running for a while if the overrun is intact. If it stops right away or makes a loud howling or grinding noise when running, the overrun is definitely broken. Repairing overrun A generator's overrun can not be repaired, it must be replaced. It is recommended to also replace the generator's flat belt and idler pulley. The components are synchronized in their design. Therefore, if one component fails, the other two components will fail shortly later. Replacing the overrun may involve removal of the alternator. Although this is not necessary on many vehicles, it may simplify the replacement of the overrun reel. This reel can ony be replaced using special tools, which cost around 60 euros on the accessories market. The cost of the overrun repair kit are highly dependent on the car type and manufacturer. Generally, replacing an overrun reel is a task easily handled by amateur mechanics. The life of your alternator is increased by consistent adherence to the maintenance plan. A sensible step during this repair is also checking the carbon brushes of the generator and replacing them if necessary. This will completely restore the vehicle's power system.