Tightening the air conditioning nut on a4 VW TDI  (1998-2005 Golf, New beetle, Jetta)
difficulty: 1/5
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Introduction

An issue that some a4 TDI have is a loose air conditioning (AC) compressor nut.  This is the main nut that holds on the AC compressor clutch pulley on.  If it falls off, it can cause damage to the whole area.  In most cases it will be fine but since it only takes a second to tighten it, it's worth taking a look if you're already down there.  Summary of procedure: with the engine off, take a 14mm wrench and tighten the nut - if you need more details, see below.

Parts

14mm wrench
blue locktite threadlocker (optional)

Procedure

Obviously the engine should be off before you start doing any work to the car.  As a general tip, it's good practice to remove belts or watches, necklaces, or other clothing which could gouge the paint or get caught in moving parts.  The AC compressor is a silver cylinder at the bottom of the engine in front of the oil pan with two lines coming out of it.  The two lines lead to the AC "radiator" by the engine coolant radiator.  If you still don't know where it is, stand at the passenger side of the engine and look down.  Follow the serpentine belt down and see the pictures below.

The center nut should be tightened to 30 ft lbs, you may want to use a tiny dab of blue locktite since it is a one use self locking nut.  Counterhold the clutch plate when loosening the nut.

Note - the torque spec is for the Sanden compressor, if you have a bolt in the center of the AC clutch you have the Zexel compressor.  Torque for the Zexel compressor bolt is 11 ft lbs and according to bentley, should be replaced with new.  I would use some light locktite so it doesn't come off again and you will probably never have to touch it again either.