Jetta L4-1781cc 1.8L SOHC (ACC) Can (1995)
Timing Belt: Fundamentals and Basics
TIMING BELT, GENERAL INFORMATION
The timing belt synchronizes (times) the opening and closing of the intake and exhaust valves with the movement/stroke of the pistons. The timing
belt also matches the rotation of the distributor with the movement of the pistons.
The crankshaft represents the movement of the pistons while the camshaft controls the opening and closing of the valves. The timing belt connects
the crankshaft to the camshaft and uses the rotation of the crankshaft to drive the camshaft in a 2 to 1 relationship. For every two revolutions of the
crankshaft the camshaft is rotated only once.
In 4 stoke engine each piston must complete 4 separate strokes (intake, compression, power, and exhaust) to complete a cycle. For a single
piston to complete all four strokes the crankshaft must rotate two full revolutions.
The camshaft controls the opening and closing of the valves. During the four piston strokes of an engine cycle the intake and exhaust valves
(for each cylinder) will open and close only once. This requires only one revolution of the camshaft.
NOTE: The intake valves will open and close only during the intake stroke and the exhaust valves will open and close only during the
exhaust stroke. During the compression and power strokes the valves remain closed.
This 2 to 1 reduction is accomplished by making the circumference of the camshaft pulley twice as large as the crankshaft pulley. This allows
the smaller crankshaft to complete two revolutions while the larger camshaft completes only one revolution.
NOTE: The distributor also requires only one rotation for every two rotations of the crankshaft.
FIRST INDICATIONS OF A BROKEN BELT
1. The engine will immediately die or fail to start.
2. When attempting to restart the engine, the cranking speed (rpm) will be unusually fast.
3. The sound of the engine while attempting to restart will be different than normal. Instead of the usual rhythmic "Rir. . .Rir. . .Rir..." cranking
sound. it will sound more like "RirRirRirRir" with no detectable rhythm.
When the timing belt breaks the camshaft and distributor quit rotating.
Without distributor rotation the ignition system stops producing spark and the engine immediately dies or fails to start.
Without the rotation of the camshaft the intake and exhaust valves stop opening and closing. This prevents the pistons from developing
compression which also causes the engine to die. This lack of compression also produces the unusually fast and non- rhythmic cranking speed
when attempting to restart the engine.
NOTE: The normal rhythmic cranking sound is a result of the starter having to work harder as each piston begins its compression stroke.
VERIFYING TIMING BELT IS BROKEN
Check for camshaft rotation while cranking the engine (with the starter or by hand, using the crankshaft bolt).
WARNING: When rotating the crankshaft by hand always disconnect the battery.
Camshaft rotation can be verified by:
Remove oil filler cap and check if camshaft is visible.
Remove valve cover and directly observe camshaft.
Remove distributor cap and check for rotor rotation (the rotor is driven by the camshaft)
Remove upper timing belt cover and check camshaft pulley rotation and belt integrity.
Other Indications the Timing Belt may be broken.
No ignition spark.
Very low compression (<15 psi) on all cylinders. Due to the low compression the following additional symptoms may be present.
While attempting to start the car, the battery strength remains high for an unusually long length of time.
The sound of the engine while attempting to restart will be different than normal. Instead of the usual rhythmic "Rir.. .Rir. . .Rir..."
cranking sound it will sound more like "RirRirRirRir" with no detectable rhythm.
NOTE: For an experienced technician this unique sound will often be the initial symptom that leads to a more in-depth check of the