All the parts which control the opening and the closing of the intake and exhaust valves make up the timing system, a complex organ of vital importance in terms of engine operation.
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The term timing gear defines the set of mechanical components (camshaft, tappets, rods and rockers) that allow intake and exhaust valve opening and closure so that air, fuel and exhaust gases can pass through, based on the timing diagram. The movement of the camshaft, usually fitted in the cylinder head or in a cylinder head extension secured to the cylinder head, is achieved by means of connection to the crankshaft via a rubber toothed belt or via a chain.
A toothed belt timing system consists of a rubber belt which joins a pulley secured to the crankshaft and one or two pulleys secured to the camshafts.
In accordance with the engine design layout, with a transmission ratio
of 2 to 1, two crankshaft revolutions are equivalent to one camshaft
revolution. Its main role is to synchronise the movement of the crankshaft with that of the camshaft. The toothed belt is placed under tension by a belt tensioner that may be fixed, sprung, or hydraulic.
As well as the camshaft, the belt may send drive to other components, such as the coolant pump, commonly known as the water pump. In diesel engines, the belt also sends drive to the high pressure pump.
The timing belt is made of rubber with internal teeth.
Each tooth is profiled so that it can couple with the outer teeth of the
camshaft and crankshaft pulley gear. The belt is made of a glass fibre layer
that supports the traction load or cords made out of similar material:
these cords, or strands, are resistant, flexible and do not give rise to
permanent stretching. The neoprene teeth are connected
to the inner part of the frame that supports the load, to ensure that a
continuous and positive pressure is maintained between the belt and the
pulley tooth. The synthetic rubber mount is soft on the outside and is
connected to the strand: this keeps the layer of strands, continuously
subject to bending forces, in position. The support layer also exercises
contact with the belt tensioner. A sturdy, wear-resistant nylon cloth coating is also fastened to the belt inner tooth in order to lengthen its effective lifetime.
Nowadays, the belt's working conditions are much more intense than in the past, due to the higher operating temperatures created by the new 16V or direct injection diesel engines. This has led to the ongoing development of increasingly more resistant materials, to increase service intervals, reduce engine faults and limit noise.Belts offer the following advantages:
On the other hand, however, because the belt is wound on to the pulleys, the belt is continuously subject to fatigue cycles and its durability is significantly lower than that of the chains. The belt-pulley coupling is particularly sensitive to dirt and therefore, like the chain, it requires watertight protection. The timing belt is never folded, twisted or forced inside the pulley: its components include glass fibre, which could be damaged by these movements.A timing belt may yield due to a worn pulley surface or tensioners which can no longer maintain sufficient tension, or even due to worn bearings.
A chain timing system has identical operating
specifications to a toothed belt timing system, except for the means by
which the camshaft pulleys are driven. Chain based timing systems offer
the benefit of being sturdier, more precise and longer-lasting than systems with toothed belts. On the other hand, they must be lubricated,
and consequently require watertight casing and special oil ducts. Over
time, chain systems can become noisy, due to the elongation of the chain
itself. To compensate for the natural lengthening of the chain due to the thermal expansion of the cylinder head and engine block, and for toothed wheel wear and tear, automatic hydraulic tensioners are used which combine the pressure of oil with the thrust of the spring to maintain the chain under optimum tension. These
apply their action to the non-pulling branch. Both the tensioner pads
and the guides are made out of plastic with highly wear-resistant
Roller chains consist of numerous inner and outer
connection strip pairs. A wide hole in the ends of the inner strips
supports a steel cylindrical bushing. Each pair of outer strips is
connected with a pin that moves with clearance in relation to the
bushing but is interference-fitted in the outer strip holes. To reduce
wear due to friction, a cylindrical roller is freely fitted to each
bushing, which meshes with the toothed wheel to share the transmission
The coolant pump works actively in harmony with the timing components
because it is driven by the toothed belt. The moving parts of the water
pump, just like other components, suffer the same wear and tear over
time. Therefore, it is not convenient to service the timing system and
later on the water pump. When servicing the timing system, it is recommended that you replace all connected components at the same time as the timing belt, such as the tensioner bearings, the transmission roll and the water pump.
The service belt is connected to the timing belt, and controls the alternator and the climatecontrol compressor or the water pump, in the case that it is not controlled directly by the timing belt. The service belt should also be regularly checked and replaced according to the instructions in the owner handbook. If the service belt breaks, all of the
aforementioned components will stop functioning, and there is a risk
that the breakage may lead to fragments inside the pulley, causing
serious problems for the timing system.
Not all vehicles have a timing belt. Some models are fitted with a timing chain,
which requires completely different servicing and maintenance
procedures: the chain does not need to be replaced as often as systems
The timing belt runs on a series of fixed and mobile bearings, and also passes through the toothed pulley of the water pump: it is used continuously and undergoes severe mechanical stresses at very high engine compartment temperatures. If it breaks, this may cause serious, and sometimes irreparable, damage.It is crucial to pay close attention to the mileages and time periods set out in the owner handbook for your car, in order to avoid accidental breakage of the belt with the engine running, which could cause serious and significant engine damage: bent valves, tilted rods, holes in pistons, etc.Timing system maintenance is very delicate and must always be carried out:
When replacing the timing system during a service, it is recommended that you replace all connected components at the same time as the timing belt, such as the tensioner bearings, transmission roll and, where present, the water pump, which is an active component that works alongside the timing system components.For the scheduled servicing of your vehicle, ask for the Mopar® timing kit: a complete solution containing the timing belt plus all the related components, including the water pump.The timing kit guarantees the most accurate and convenient servicing possible. The Fiat Professional's Service Network's highly specialised personnel will be able to recommend the best repair solution for your needs.
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