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Charging system nightmare - DFM alternator
Posted to Technical Discussion Forum on 1/16/2014 7 Replies

It is a very weird problem. At least to me.

This 2008 TFSI Passat has the charging lamp ON. Every time engine is turned on alternator will not charge. But, after accelerating engine to about 1800 / 2000 rpm it starts to charge normally and continuously. Even at idle speed. But charging lamp (red battery symbol) will never not go OFF.

There is no codes on ECM memory.

I used a known good alternator on car and problem continues exactly the same. Assembled car alternator on other Passat and alternator/regulator are working normally.

My doubt about DFM alternators equipped cars:

-The signal DFM to ECM has something to charging control or is just used by PCM to measure alternator load in order to control idle, engine load???

-What problem would prevent alternator to charge after engine start, while in idle. And why it starts charging after accelerating engine to 1800 rpm?

- In the case of the Passat what kind of information is used to turn the charging lamp OFF? In other works: what control module will ask instrument panel to turn charging lamp OFF? ECM, J519 power supply unit???

I made a Erwin subscription but found no explanation about the DFM + L alternator charging system logic.


Mauro Brisola
Multi Marcas
Curitiba, Brazil

7 Replies Received (View Replies)


car Vehicle Data

2008 Volkswagen Passat 2.0T 2.0L

Engine2.0 L
Trans6-speed Automatic (Electronic)