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Your Idler Pulley (Guide Pulley) worn out? » Read more about common malfunctions • Repair • Replacement manual
Every auxiliary unit its own belt with the deflection pulley
The belt drive is an invisible, though very important part of the car engine. In the era of the simple V-belt every auxiliary unit was equipped with a separate drive. For reasons of efficiency, particularly at construction, nowadays a multifunctional belt is preferred. To supply every device with the right rotation speed and torque, deflection pulleys are used. Deflection pulleys are also known as guide pulleys.
Multi-V-belt deflection pulley: tasks
The belt drive has multiple tasks. It supplies the required rotation speed of
- servo pump
- refrigerant pump
- alternator or dynamo
- refrigerant compressor
These devices have the advantage that they require a rotation speed, but not a particularly high torque and no valve timing at all. Their drive is relatively uncritical. If a belt drive fails, no serious engine damage is imminent. This is the difference with the old V-belt which additionally requires a valve timing. In many modern cars all the auxiliary units are driven by a single belt. It must be long enough to reach all output drives. A longer belt stretches more easily than a short one and the tensioning of the multi-V-belt is a challenge. To adequately drive all units, the multi-V-belt needs a specific tension at installation. If the tension is too high it can rupture and damage the auxiliary unit bearings. If its tension is too low, it slips at input or output drive, wears fast and can no longer fulfil its task. This is why multi-V-belt tensioners are used. In addition to the tensioners, the long multi-V-belts need deflection pulleys. These rollers are fixed to the engine block and guide the belt drive in the desired direction. A deflection pulley usually has a long life span. It must be replaced according to manufacturer's directions.
Deflection pulley defects
Defects rarely occur for the deflection and guide pulley of the multi-V-belt. These are robust components with few moving parts. Nevertheless the permanent operation and high tension of the belt drive affect the bearing of the deflection pulley. As a rule, robust cylinder roller bearings are used. These too, however, can over time lose their stability and smooth running. This becomes apparent in two ways:
A squeaking bearing is an indication that it jams and continuously pulls itself free. This can be observed in a running engine. The deflection pulley remains idle for a few seconds before it rotates further. This is very damaging for the multi-V-belt. When the deflection pulley jams, the belt grinds over the roller, causing wear. When the guiding pulley rattles, the bearing has too much play. This is not too damaging for the belt drive. Nevertheless, a defective bearing can always jam suddenly. A replacement of the guide pulley is then strongly recommended.
Replacement of the guide pulley
The deflection pulley should always be replaced according to maintenance directions. This is ultimately at every second replacement of the multi-V-belt. The pulley replacement is very easy. The component is fixed to the engine block with a single bolt. The immediate surrounding area of the deflection pulley should be thoroughly cleaned on occasion of its replacement. Particularly the contact points with the engine should be clean to prevent dirt from being pressed into the bearing. If the guiding pulley is replaced before reaching its replacement interval, the belt should be replaced along with it. You never know what damage the defective guiding pulley has caused in the belt drive. The maintenance of a multi-V-belt can be very difficult. In average cars the disassembly of the engine is usually not required. Installation of the pulley requires tightening the central bolt with the prescribed torque. If it is tightened too strongly, the roller can jam. If it is too loose, it can loosen itself during driving. The central bolt of the deflection pulley possibly has a left thread. Be certain to inform yourself about it prior to starting the repair. Otherwise you might risk breaking off the central bolt when attempting to unscrew it.
Costs for a guiding pulley
The price of a deflection pulley starts at 5 EUR (± £4) and is one of the cheapest components of the entire belt drive.