Re: Can tach be disconnectd?

[Dave Saad <dsaad@micron.net>]

hootspa@zzapp.org wrote:
> 
> I've got an 89 Audi 100 wagon w/automatic trans. The tach started acting
> up and the needle is flying all over the place.
> 
> 1. Don't really need a tach on an automatic trans car.  Could I just
> disconnect it without it screwing up the input to the fuel injection
> computer (does it have RPM's as in input??)?
> 
> 2. How do I trouble shoot it? Is it more likely to be the tach or the
> sending unit?
> 
> - hootspa


I would'nt!!
I had an experience where my '91 100 would not idle but would run fine.
After a week of fiddling with the fuel injection computer, I learned
that the
tach sends a needed RPM signel to the ECU. I also learned that the
Bently manual
was not too helpful in this area because it said if the idle stablizer
valve was good
and the wires were OK then the ECU had to be the fault. Wrongo.
I found a trim pot adjustment on the fuel distributor that was a tad out
of spec.
The adjustment was in the manual - but in a different area. Adjusting
this cured
my problem. (the dealer here in Boise had no idea it was even there)
In the process I also learned - and this is probably what you want -
that some
soldered connections on the instrument panel for the speedometer and
tach can come loose.
If you can solder, and take out the instrument panel, you can fix it for
nothing.
My speedo had the same symptom as your tach.

Dave