Engine accessory systems, such as the alternator and coolant pump, are powered by the engine through the crank shaft pulley. This small and lightweight component is one of the most important outside of the engine block. It is connected to the various systems by one or more long belts, and often has an internal dampener to reduce the transfer of vibration to these systems. While the belts can be replaced without removing the crank shaft pulley, failure of the dampener requires a new pulley to be installed. Replacing the pulley can be a difficult task since either the engine or transmission usually needs to be removed to access it.
The crank shaft pulley is located at the base of the engine, on the end of the crank case. It is separate from the crankshaft sprocket which drives the camshaft timing gears. The diameter of the pulley varies between different engines but is usually between six and eight inches. It is made from forged aluminum that has been finished by a computerized milling machine, and is connected by rubber belts to the various accessory pulleys above it. Grooves are machined along the outside of the pulley to provide extra surface area for the belts to grip. An idler pulley and a spring-loaded tensioner pulley are also used to keep the belts from slipping off the other pulleys.
There are many essential components that are powered by the engine through the crank shaft pulley. The alternator provides electrical power for the distributor and spark plugs, as well charging the battery. The engine coolant pump is also powered by the pulley, as well as the power steering pump which provides the pressure required to move the steering rods. The air conditioning system pump is also powered by the pulley. One problem with this complex setup is that all these systems stop working if the belts breaks.
The crankshaft pulley is often combined with a dampener, also called a harmonic balancer, to reduce the torsional vibration from the crank shaft. This often takes the form of a thick rubber disc sandwiched between two metal drive rings, joined together with adhesive. One problem with this arrangement is that the adhesive can lose strength over time, resulting in slippage between the discs. This can cause vibration and a loss of power in the accessories, leading to other problems with the engine.
Changing the crank shaft pulley can be a difficult and time-consuming task if the engine needs to be removed. Only a few vehicles have enough space in their engine bay to allow the pulley to be changed with the engine in place. The tensioner pulley first need to be moved aside so that there is enough slack to remove the belts from the crank shaft pulley. The large nut holding the pulley in place is difficult to remove and requires a large spanner or an impact wrench.
Before installing the new pulley, check the shaft key and key slot for any damage. Once the pulley and shaft key are in place, screw up the large nut with the appropriate amount of torque, as specified in the service manual. Slide the belts back over the pulley and adjust the tensioner pulley to remove most of the slack. The belts will break if they are too tight so leave a small amount of slack in them, as specified in the service manual.
Using the engine to power the accessories results in slightly reduced engine performance. An underdrive pulley can be used in place of a regular sized crank shaft pulley to improve performance. It has a smaller diameter and turns the accessories at a slower rate, freeing up more energy to drive the transmission and wheels. However, there may also be a noticeable reduction in the function of the accessories, especially the air conditioning system. Using a smaller pulley can also result in engine overheating if the coolant pump does not provide sufficient pressure.