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Discussion Starter #1
My tach reads approxamately 1/3 of what the engine is actually performing.
I checked the thin white wire to the coil and it's fine.
Questions are:
Could a "wacked" coil make it act up funny?
I have checked the coil, based on International Auto's Tech note, and it reads perfect on the primary - but - I'm getting over 200k ohms on the secondary reading. Still runs fine though.
If that's not a related problem to the tach problem, then is my tach screwed?
Who knows of a replacement source?
Or how about an aftermarket Autometer or VDO? Can they be retrofitted?
I could go forever without a tach, but what fun is it without knowing the engine's limit....
 

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Discussion Starter #3
The tach on Bosch spiders doesn't have a sender - it reads from the coil - or does it?
The manual is quite vague.
If there is a "sender" - where is it?
Also how would I go about checking the ground, is it one of the traces on the back of the instrument pod?

Alex - I noticed your car on IAP's customer pages - sweet machine!!!
 

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As far as I know the tach receives it's signal from the (-) side of the ignition coil. If the tach is working, but sluggish it probably needs a rebuild from North Hollywood or Palo Alto or any good instrument rebuilder. Remember your car is over 30 years old
 

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I read somewhere on there that the - terminal on the coil can stop working as well as it should. This may or may not manifest as a coil that feels hot to the touch.

Any insight as to the diagnostics here? Just how warm is a warm coil? Can I test resistance across these terminal? I'll have to ask the googler - these are just Bosch coils.

It's a 40 year old car. I know coils are robust, but 40 years is a long time.

Car seems to run fine (well, crappy, but is full of old gas and the injectors need cleaning).
 

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The tach in S3 cars has a mini potentiometer soldered on the circuit board that is used for adjusting or fine tuning the display. Over time, corrosion or debris on the wiper arm or micro windings of this mini pot make the tach display inaccurately (or stop working completely). Spraying some electronic contact cleaner on it, and then rotating the wiper back and forth a few times will oftentimes correct this issue, at least for several more years.
 
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