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RE:Duty Cycle Test Connection

on 1/22 Mark Pollan asked:

Excuse my ignorance. I am and probably will continue to be an
moron.  I have a question(s) about connecting a dwell meter
to check duty cycle. The Bentley refers to a few Audi tools and is
confusing - to me at least.  My ride is an '86 5KCSTQ.

1)  Is the test connector the two wire unit right behind the waffle-like
    air duct?
2)  Do I just connect the dwell meter directly to the test connector
    and read away?
3)  Will the polarity of the connection affect the reading?  That is,
    do I have to have + to + and - to -?here is the way  hook up

The connector is the small wihte plastic two conductor located between
the waffle duct and fuel filter/ISV. It has two connections one wire is
brown and the other is blu-blk.  Brown is ground (vehicle chassis
ground) and the blu-blk is the 12V supply from the ECU. I don't know how
some dwell meters would respond to a negative voltage so to be safe I
always connect + lead to blu-blk and - lead to brown.

This connection will give you a +12V when the OXS F/V is closed (lean)
and 0V when OXS F/V is open (rich). I'm a little unclear here but I
think this would read "opposite duty cycle". In other words when you
first start the car cold it is supposed to read ~80% DC until the 02
sensor warms up and then the lambda circuit takes over. I think with
this connection you will get 20% DC - no big deal as long as you know

A couple of terms defined:The lambda ciruit is always trying to keep the
O2 sensor at 0.5V for 14.7 air fuel. When the fuel mixture goes rich the
lambda compensates by going lean (towards 0%) and vise versa. Therefore
a rich mixture will result in a lean lambda (lower DC). 

If you are setting the DC for 50% it doesn't matter either way. I think
the current wisdom is is to set the DC slightly on the rich mixture/lean
lambda side so the lambda circuit will have more "head room" to enrichen
the mixture.

One other way to verify this is to pull the dip stick and see which way
you DC goes. When dip stick is pulled you introduce air which leans out
the mixture and the lambda circuit tries to compensate by going richer.
Which ever way the dwell goes is the opposite way you want to go (from
50%) to adjust the DC for the optimum 40% DC (rich mixture/lean lambda).

I've adjusted my DC with an oscilliscope and have tried from 40% to 60%
and I do think there is a little better throttle response at 40% (the
opposite direction from the dip stick pull test).

Any comments are welome.

Littleton, CO