tachometer signal revisted, LONG

[Bob Rossato]

> 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!

Jumping in a little late here so I'm not sure which car this is in relation
to but, I believe the tach input, wherever it originates from, is controlled
by the circuit board in the IC to allow the tach to read properly.  At least
in the later cars.  I had an issue with my '95 S6 that my tach was reading
low.  I was hitting the rev limiter at an indicated 6000 rpm instead of the
7200 rpm redline for the AAN.  Exactly a 5/6ths ratio.  After swapping
tachs, ECUs, etc. I finally determined the problem was with the instrument
cluster circuitry.  Somebody else eventually figured out that, at least on
the '95 and later cars with digital odometers, you can reprogram the IC via
VAG-COM or VAG1551/2.  I plugged in my VAG-COM and sure enough the software
coding for the IC was set to 6 cyls.  Switched it to 5 cyls and the tach
read accurately again.  So, that's how you can have the same tachometer
being used for a 4, 5, 6, or 8 cyl engine.  Don't know if this applies to
earlier cars, or if it does, whether they are reprogrammable.

Bob