[alfa] Re: how did you test the mixture

["Simon Body" <body@xxxxxxxxxxxxxxxxxxxx>]

Hi John

I'm most likely going to be wrong on this, but I think the
series 4 Spider has essentially no way of manually
adjusting the mixture. The Motronic ML4.1 relies on the O2
sensor to set the correct stoichiometric mixture (14.7:1)
by reading the O2% in the exhaust before the catalytic
converter, thus sensing the rich/lean condition of the
mixture. The "brain" then uses the voltage signal of the O2
sensor, along with input from the coolant temp sensor, the
throttle position switch and the mass flow meter (and
others) to decide on a duration for the fuel injectors to
cycle on. My reading of the manual says that the manual
adjustment of the voltage output on the mass flow meter
that you could adjust using a small covered screw on
earlier Spiders has been disabled inside the "brain".

When I measured the O2 sensor voltage on the car it cycled
from about 0.2V to 0.9V. From my reading this seems
correct. Bench testing showed a low value of 0.17V in air
and a high value of 0.91V in a propane torch flame. I also
bought a new O2 sensor and got exactly the same values.
From my reading I still don't understand why the books say
the voltage signal from the O2 sensor will cycle across the
0.45V value when in the exhaust, while on the bench it
gives a constant voltage signal. The only explanation I can
think of, is that the brain deliberately varies the
fuel:air mixture at a rate of 0.5-1Hz to hunt for a correct
value of the mixture, thus crossing the 0.45V set point. I
bet I'm wrong about this bit.

I've misplaced my copy of Probst (Bosch fuel injection
book) so I can't be sure of all this. But I bet someone
will put me straight on the details.

Thanks for your help. My "brain" is now mush.
Regards

Simon



On Tue, 07 Oct 2003 17:36:37 -0700 (PDT)
 "John D. Fox" <jdfox@domain.elided> wrote:
> Simon - when you ran the car open-loop what was the O2
> sensor voltage?
> Your car was excessively, grossly rich, and the base
> mixture must be
> adjusted before anything, or spending cash on a catalyst,
> is going to do
> anything for you. Have you set the mixture CO level, per
> the manual? How
> did you "test" the mixture and the O2 sensor? John
> 
> 



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