[Vwdiesel] Tach signal box

Roger Brown r.c.brown at ieee.org
Sun Jun 3 20:19:36 EDT 2007

Dave Heart wrote:
> Since this is a real odd problem I might just build a new black box. I 
> really like this approach to getting a regular tach signal which allows 
> cruise control as well.  I did draw my own circuit board using etchant 
> to make the board and used surplus parts. Seems odd possibility of a 
> malfunction due to sensor reflection becoming poor off the metal tape 
> after 10 minutes of use but fine before that. I did paint area between 
> sections flat black. Cannot figure why everything ALWAYS works fine for 
> at least 10-15 minutes from engine cold start up. As you pointed out 
> since cruise also malfunctions at same time probably the used jetta tach 
> is likely not the problem... I moved the black box to inside cabin and 
> only mounted sensor in engine compartment thinking maybe heat bothering 
> some component Same trouble...Oh well....

I have some additional troubleshooting tips on my web page if you are able to test the 
circuit board once it starts acting up:

Or for the heat side of things, get a can of freeze spray at an electronics store and 
chill each component in turn to see if you can restore proper operation.  If so you have 
found the heat sensitive part.



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