APPLICATION: This should apply to all air-cooled Volkswagens fitted with the upright fan housing Type 1 engine. This would be all Beetles and Super Beetles, all Karmann Ghias, all Things (181s), and Buses through 1971.
TOOLS NEEDED: Hefty screwdriver; wrench to fit alternator/generator pulley nut. This nut was 19mm on my Karmann Ghia.
Adjustment for the fan belt tension on a Type 1 engine is kind of goofy compared to more modern cars. Since the generator/alternator is in a fixed place on this engine, VW had to provide for adjustment of the fan belt another way. They did this with shims. The alternator/generator pulley is really two separate halves. To adjust the belt tension, the pulley nut is removed and shims are added to or subtracted from the space between the pulley halves. Adding shims decreases belt tension, and subtracting shims increases belt tension.
The belt should be at least tight enough so that turning the generator/alternator pulley with a wrench also turns the engine. If it is too loose, turning the pulley will just turn the generator/alternator shaft while the belt slips on the pulley. In this particular engine, proper fan belt tension is vital, since any belt slipping reduces the amount of cooling air that is supplied by the fan to the engine. Of course, having the belt too tight can cause premature wear on the alternator/generator bearings.
When depressed by thumb pressure midway between the two pulleys, the belt should give approximately 1/2″.
If the tension is more or less than this, it must be adjusted to bring the tension into specification.
To adjust the tension, first you must remove the pulley nut. Observe that there are some notches in the front half of the pulley (front is front). Take your big screwdriver and stick that in one of the notches so you can hold the pulley stationary while using your wrench to loosen the pulley bolt. Once you have the bolt off, you see a metal bell-looking thing, and under that are some shims. Remove all that stuff and then remove the rear pulley half. Then add or subtract shims as required to bring your belt tension into specification.
The problem with this adjustment method is that you have to guess how many shims you need to add or subtract, then put it all back together and check it. If it’s still not correct, then you have to go back and do it again. This is not the best design, but I have to admit it works.
Once you’ve got the belt adjusted correctly, make sure you tighten the alternator/generator pulley nut sufficiently, and you’re done.
— Sean Bartnik, February 10, 1998