tachometer signal revisted, LONG

[Richard Hoffman]

Hmm my V8 redlines at 7000, same as my 20v did.  As the TEC only allows me
to define pulses per revolution, I am a little screwed.

Audi says the tachs are the same.  Javad (and others) say that they need a
signal based on number of ignition events per engine cycle.  I have no doubt
that Javad's info is correct, after all, he's doing this precise thing with
his 034EFI and I'm certain he has the attention to detail to make sure his
tach reading matches the 034EFI's tach reading (from the crank trigger).  I
just don't understand how the 4 and 5- cyl tachs can be the same physical
part and interpret different signals to produce the same output.  It just
boggles my mind!  I'm taking apart my cluster for any helpful hints it might
provide (jumpers maybe?)
(pause for 20 minutes while I take it apart)

Ok I am looking at the insides of the cluster. No visible jumpers on the
tach, BUT... On the side of the tach in the back, it says
110. 902/129/12  (VDO P/N)
12v 258/323Hz  (obviously 12 volt.  258 and 323 are a 4:5 ratio....4 vs 5
cyl methinks)
4/5zyl/ 4takt       (obviously 4/5 cylinder (zylinder is german for
cylinder))  4 takt?  I don't know.

This is speculation, which I am pretty good at.  the frequency rating 323hz
corresponds to 7752 rpm on a 5-cyl  (323/5 x2 x60) and 258hz corresponds to
6192 rpm on a 4-cyl.  The tach goes to 8000 rpm, the 4 cyls redline at
6300rpm   What does this mean?  Why am I getting so in depth on such a
simple issue? Am I crazy?  Should I just hook it up, tell TEC it was a 4-cyl
tach and see if it works?

On the housing that covers the tach (on the rear) there are 2 sets of
jumpers.  One is 3 2-position jumpers listed as
10  5   -
           %
0    0   +

Which I interpret as a correction for some gauge.  select plus or minus and
the most appropriate value.

The other is a 6-position switch, which reads:

Diesel         ,   Benzin (gas)
RW,US,EG,RW,US,EG

with the switch under "US" on the Benzin side. I think this has more to do
with the MFA system, but I could be wrong.

Bottom line is...I need to get my bentley back! (never loan it out)  The
tachometer has 4 terminals on it.  I believe that the 4-cyl simply uses
different terminals.  There are 2 capacitors on the tach, which leads me to
believe there are 2 different inputs for the tach.  I NEED MY BENTLEY
BACK!!!!  I bet this is it....

Anyway, thanks for letting me rant aloud,  I think this helped me figure it
out
Richard

----- Original Message ----- 
From: "Dan Cordon" <cord4530 at uidaho.edu>
To: "Quattro List" <quattro at audifans.com>
Sent: Monday, December 01, 2003 11:25 PM
Subject: Re: tachometer signal revisted


> > I guess you'll have to experiment with it, I wonder what the guy in
Santa
> > Barbara did with his Electromotive run V8 conversion?
> >
>
> I don't know anything about the V8 in SB, but there's a chance that the
> V8 engine would redline about 5/8's what the I-5 would...maybe a little
> higher, but I'm guessing not a ton higher. If he sent 8 pulses per cam
> rotation (4 pulses for crank rotation) then even though the #'s were
> off, they would be getting full range on the tachometer :o)
>
> --
> Dan Cordon
> Mechanical Engineer - Engine Research Facility
> University of Idaho
>
>
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