[Vwdiesel] 85 Jetta turbo diesel GL tachometer going whacky

Roger Brown r.c.brown at ieee.org
Thu Jun 9 21:31:07 PDT 2011

On 6/9/2011 9:04 PM, W Davis wrote:
> Awhile ago, I had a sudden problem..driving home from work;
> all of a sudden, the alt light began to glow, dash lights went out, and the tach
> started going all over the place.
> Long story short, I pulled the voltage regulator from the alternator, and simple
> visual
> inspection showed it was bad...replaced the v-reg with a new one I had, and all
> was
> well again, except I still have intermittent "whacky tach syndrome".
> 1-  from what source does the tachometer sense what it's reading?..is it the
> alternator?
> 2-  what might be the problem..the tach itself,, or the source from which it
> gets its' info?
> Many thanks,
> William in Fla
> 1985 Jetta GL turbo diesel, my daily driver,
>   running fine otherwise.

Yes, the OEM diesel tach works off the alternator, specifically a special terminal called 
the W-terminal:
Basically an unfiltered tap off the raw alternator AC voltage.

Might be a loose connection or could be brushes inside the alternator or maybe an 
intermittent diode that is connected to the W-terminal winding.



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