VW Passat & Audi A4 1990-2000

Understanding the Manual Transaxle

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Because of the way an internal combustion engine breathes, it produces torque, or twisting force, only within a narrow speed range. Most modern, overhead valve engines must turn at about 2500 rpm to produce their maximum torque. In general, by 4500 rpm the amount of torque the engine produces begins to drop off such that continued increase in engine speed often does not produce any further power increases.

The manual transaxle and clutch are employed to vary the relationship between engine speed and the speed of the wheels so that adequate engine power can be produced under all circumstances. The clutch allows engine torque to be applied to the transaxle input shaft gradually, due to its mechanical slippage. Consequently, the vehicle may be started smoothly from a full stop. The transaxle changes the ratio between the rotating speeds of the engine and the wheels by the use of gears. The gear ratios allow full engine power to be applied to the wheels during acceleration at low speeds and at highway/passing speeds.

In a front wheel drive transaxle, power is transmitted from the input shaft/mainshaft to the output shaft located slightly beneath and to the side of the input shaft. The gears of the mainshaft mesh with gears on the output shaft, allowing power to be carried from one to the other.

All forward gears are in constant mesh and will rotate at a different speed as the shaft unless the synchronizer/sliding sleeve assembly is engaged. Shifting from one gear to the next causes the selected gear to rotate at the same speed as the shaft while the disengaged gear is allowed to spin freely at a different speed from the shaft.

Gears are engaged and disengaged by a series of engagement dogs, which are engaged into a grooved sliding sleeve, which is attached to the shaft via splines, thus allowing it to slide to engage the selected gear and rotate at the same speed as the shaft. A cone-shaped synchronizer is used by the sliding sleeve to act as a small brake, which smoothly brings the gear and shaft to the same speed, allowing the sliding sleeve and the gear's engagement dogs to mesh. The gear is fully engaged once the sliding sleeve slides completely over the gear's engagement dogs.

As the synchronizers wear over an extended length of time, the engagement from one gear to another may exhibit a slight "notchy" feel, or in extreme cases, a grinding sensation may be felt. If the clutch does not disengage smoothly or completely, this creates additional strain and wear on the synchronizers, sliding sleeve and gear engagement dogs.

The following is a list of manual transaxle symptoms and possible causes.

Gears feel "notchy" or grind when shifting:



Improper clutch adjustment
 
Incorrect transaxle lubricant
 
Pilot bearing worn or seizing
 
Flywheel surface uneven or warped
 
Improper flywheel machined surface
 
Clutch disk thickness above specification
 
Clutch disk binding on input shaft splines
 
Flywheel surface damaged from hot spots
 
Crankshaft thrust bearings excessively worn
 
Worn synchronizer, engagement dogs and sliding sleeve
 

Transmission jumps out of gear:



Worn shift fork
 
Weak shift detent springs
 
Worn or damaged shaft bearings
 
Worn engagement shaft and detents
 
Worn engagement dogs and sliding sleeve
 

Transmission noisy, whines or grumbling noise



Worn gears
 
Worn bearings
 
Insufficient or improper lubricant
 

Transmission will not go into gear



Seized pilot bearing
 
Broken clutch damper springs
 
Clutch lining above specification
 
Bent or damaged clutch release fork
 
Failed hydraulic slave or master cylinder
 
Excessively worn crankshaft thrust bearings
 



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Fig. The sliding sleeve is installed onto the hub. The detents keep the sleeve engaged or in its neutral position



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Fig. A close up view of the sleeve and the engagement area for the for the gear dogs



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Fig. A close up view of the synchronizer and the gear's engagement dogs. The engagement dogs are a part of a ring that is pressed onto the gear



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Fig. When the sliding sleeve slides toward the gear to engage it, the sleeve slides over the dogs on the synchronizer and the engagement dogs of the gear

 
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