since it's a TDI,
I would ask what signal is going to the tach ..
is it alternator-sourced or is it directly from the ECU ?
if the former ...one might look there some.

"tdi conversion "
  is a very good list for this sort of thing.

On 9/13/2015 3:27 PM, Mike Miller wrote:
> Yeah, I forgot. with a van always start with the grounds. Even for a flat tire.
>
>
> --------------------------------------------
> On Sat, 9/12/15, Dennis Haynes <d23haynes57@hotmail.com> wrote:
>
>   Subject: RE: bouncing tach
>   To: "'Mike Miller'" <mwmiller6@ATT.NET>, vanagon@GERRY.VANAGON.COM
>   Date: Saturday, September 12, 2015, 6:05 AM
>
>   You either have a bad connection
>   somewhere causing false pulses, interference from something,
>   or the tach itself. I would start by looking for wiring
>   issues starting at the chassis, (ground) connections.
>
>   Dennis
>
>
>   -----Original Message-----
>   From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com]
>   On Behalf Of Mike Miller
>   Sent: Friday, September 11, 2015 8:09 PM
>   To: vanagon@GERRY.VANAGON.COM
>   Subject: bouncing tach
>
>   Vanners,
>
>   For the second time the tach in my 87 syncro westfakia
>   converted to TDI went crazy for a while. Bounced up and down
>   and all over then after a few minutes went back to regular.
>   It a diesel dash pod with a diesel tach.
>
>   It was very hot yesterday but the first time it happened it
>   wasn't hot so….?
>
>   I have a tech tool hooked up  and it shows a steady
>   tach reading. but it's kind of disconcerting.watching the
>   dash board tach go nuts.
>
>   Any ideas on what goes wrong and how to fix it.
>
>   Mike
>