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