The primary function of an air conditioning (AC) bypass pulley is to stand in for an AC compressor that has stopped working. In most cases, the AC bypass pulley will bolt up in place of a malfunctioning compressor unit. This can allow all of the other accessories to continue functioning just as if the AC compressor was still in place. If the diameter of an AC bypass pulley is smaller than the compressor pulley it is replacing, a smaller belt is typically needed. Some situations allow for a smaller belt to be installed without removing the AC compressor, and in other applications the AC belt can be removed without any adverse affects.
Many different factors can cause the air conditioning system in a vehicle to malfunction. If the AC system develops a leak, low refrigerant levels can cause the compressor to constantly turn on and then back off, which may have adverse effects on the engine idle speed and other factors. Other malfunctioning components can have similar effects, though a seized compressor can cause a catastrophic failure. When an air conditioning compressor pulley seizes up altogether it can cause the belt to break, which may contribute to a loss of power steering or other vital components.
Since air conditioning systems can be very expensive to repair, an AC bypass pulley is often a cost effective way to continue driving a vehicle. Before installing an AC bypass pulley, it is important to inspect factors such as belt positioning and the diameter of the pulleys. In some cases it may be possible to simply leave the AC compressor in place and use a smaller belt, though this will often result in routing issues that can lead to premature belt failure. If the AC bypass pulley is smaller in diameter than the original compressor pulley, the vehicle manufacturer may also specify a smaller belt.
There are certain other circumstances where an AC bypass pulley may not be necessary. If the AC compressor is driven by its own belt, that belt can usually be removed without any adverse effects. This is often seen in vehicles that use v-belts, though there are some cases where a v-belt will drive both the AC compressor and another accessory. If it is determined that the AC belt can be safely removed, it is often possible to cut the belt out and leave the compressor in place to save time.