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with the automatic belt tensioner..
The belt has an automatic spring tensioner and can not be "tightened".
Automatic tensioner-no provision for adjustment.
There is no adjustment you can do. It has an automatic self adjusting belt tensioner. If the belt seems to loose you may need to replace the tensioner and belt.
picture of fan belt tensioner on a vectra
Use a 1/2 or 3/8 breaker bar to relieve tension on belt tensioner to remove and replace belt - the automatic belt tensioner is the only adjustment
If it is a serpentine belt the automatic tensioner will adjust the belt for you. If it is an older "V" type belt--there should be 1/2" "play" at the middle of the belt, between the pulleys.
The fan is electric, not belt driven. The serpentine belt is has a spring loaded tensioner. If the belt is loose, the tensioner has failed.
You can tighten a fan belt by tightening the tensioner pulley. Pry to the left on the tensioner pulley and tighten it.
Loosen the tensioner pulley. Put the fan belt around the pulleys. Pry the tensioner pulley to the right until the belt is tight. Tighten the tensioner pulley nut.
If it is a serpentine belt you replace the tensioner pulley
The serpentine belt on a 406 has an automatic tensioner you need a long ring spanner to relieve the tension to be able to replace the belt. from memory ( don't quote me ) I think it's 15mm.