Description

This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.

44 How do I check the Alternator?

A: [From Jens]:
There is one correct way to check the alternator. All you need is an
oscilloscope which has to be connected to D+. Let the engine run at
2000-3000 rpm, switch on everything that draws a greater current
(lights, ventilator, rear window heating) and look at the scope:
A correct alternator shows 12-13VDC plus a 2V "sine" (A). A defective
alternator breaks down to 1-2V every time the "sine" comes down (B).

           __    __                         __    __    __
    A:    /  \__/  \__                B:   /  \__/  \__/  \__    - 12V
                                               |     |     |
                                               |     |     |
                                               |     |     |     - 6V
                                               |     |     |
  
         --------------------------------------------------------- 0V

This is just a vague description, every book about car electrics
should show you the "error pictures" (e.g. the "red Bosch book").
With the scope, you can even tell WHAT is wrong with the alternator,
e.g. short exciter diodes, broken ground diodes, shortened stator,
without tearing it apart and check each single element.

 

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