Volkswagen Air-Cooled 1970-1981 Repair Guide

Understanding the Automatic Stick Shift Transaxle

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See Figures 1 and 2

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Fig. Fig. 1: Basic automatic stick shift transaxle components-note that the transaxle holds the engine in the vehicle



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Fig. Fig. 2: Schematic of the automatic stick shift transaxle vacuum circuits

An automatic clutch control three speed transaxle has been available on the Type 1. It is known as the Automatic Stick Shift.

It consists of a three speed gear box connected to the engine through a hydrodynamic torque converter. Between the converter and gearbox is a vacuum-operated clutch, which automatically separates the power flow from the torque converter while in the process of changing gear ratios.

While the torque converter components are illustrated here, the picture is for familiarization purposes only. The unit cannot be serviced. It is a welded unit, and must be replaced as a complete assembly.

The power flow passes from the engine via converter, clutch and gearbox to the final drive, which, as with the conventional gearbox, is located in the center of the transaxle housing.

The converter functions as a conventional clutch for starting and stopping. The shift clutch serves only for engaging and changing the speed ranges. Frictionwise, it is very lightly loaded.

There is an independent oil supply for the converter provided by an engine driven pump and a reservoir. The converter oil pump, driven off the engine oil pump, draws fluid from the reservoir and drives it around a circuit leading through the converter and back to the reservoir.

This circuit also furnishes cooling for the converter fluid.

OPERATION



The control valve is activated by a very light touch to the top of the shift selector knob which, in turn, is connected to an electromagnet. It has two functions.

At the beginning of the selection process, it has to conduct the vacuum promptly from the intake manifold to the clutch servo, so that the shift clutch disengages at once, and thus interrupts the power flow between converter and transmission. At the end of the selection process, it must, according to driving conditions, automatically ensure that the shift clutch engages at the proper speed. It may neither slip nor engage too harshly. The control valve can be adjusted for this purpose.

As soon as the selector lever is moved to the engaged position, the two contacts in the lever close the circuit. The electromagnet is then under voltage and operates the main valve. By this means the clutch servo is connected to the engine intake manifold, and at the same time the connection to the atmosphere is closed. In the vacuum space of the servo system, a vacuum is built up, the diaphragm of the clutch servo is moved by the difference with atmospheric pressure and the shift clutch is disengaged via its linkage. The power flow to the gearbox is interrupted and the required speed range can be engaged. The process of declutching, from movement of the selector lever up to full separation of the clutch, lasts about 1/10 seconds. The automatic can, therefore, declutch faster than would be possible by means of a foot-operated clutch pedal.

When the selector lever is released after changing the speed range, the switch interrupts the current flow to the electromagnet, which then returns to its rest position and closes the main valve. The vacuum is reduced by the reducing valve and the shift clutch re-engages.

Clutch engagement takes place, quickly or slowly, according to engine loading. The clutch will engage suddenly, for example, at full throttle, and can transform the full drive moment into acceleration of the car. Or, this can be effected slowly and gently if the braking force of the engine is to be used on overrun. In the part-load range, too, the duration of clutch re-engagement depends on the throttle opening, and thus the vacuum in the carburetor venturi.

Vanes on the outside of the converter housing aid in cooling. In the case of abnormal prolonged loading, however (lugging a trailer over mountain roads in second or third speed), converter heat may exceed maximum permissible temperature. This condition will cause a red warning light to function in the speedometer.

There is also a starter locking switch. This, combined with a bridging switch, is operated by the inner transmission shift lever. It performs two functions:

  1. With a speed range engaged, the electrical connection to the starter is interrupted. The engine, therefore, can only be started in neutral.
  2.  
  3. The contacts in the selector lever are not closed in the neutral position. Instead, the bridging switch transmits a voltage to the electromagnets of the control valve. This ensures that the spearator clutch is also disengaged in the neutral shifter position.
  4.  

 
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