RE: Fire went out!

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

> >On the V8Q you don't even need it for starting.  Having left
> >mine disconnected when I changed the timing belt I've BTDT.
> >Must be a default in the ECU or something that allows the
> >engine to start/run sans Hall Sender?
> 
> 
> OK, I'll bite.  How can the car run/start without the Hall Effect Sensor?
> 
... well, it may be able to run without _that_ one, but there are other
timing sensors on the engine that must be working for the engine to run ...

> As far as I am aware (read: Bronco, Bronco II) the hall effect sensor
> triggers the timing much as the points would on a less modern vehicle.  On
> the Broncos, you set the timing by twisting the distributor, as you would
> on
> a points system.  In either case, failure of the hall effect sensor means
> doom for the car running or starting.
> 
Ahhh ... the pinnacle of automotive technology ... the Bronco/Bronco II ...
:-)  A fellow ur-q owner happens to own an old International Scout which has
exactly that same sort of ignition system ... :-)

> Where do these fine v8q and 5000's get any timing information in the
> absence
> of the hall effect sensor?  Why would there be a difference between
> running
> and starting?
> 
... back in the early '80s Audi produced a car which had a computer which
determined the ignition timing based on parametric inputs (e.g. engine temp,
air temp, boost pressure) ... and it used two sensors near the flywheel to
determine the rotational position of the engine (that system also included
cursory control of the fuel mixture).  One sensor is positioned to mark
every tooth on the flywheel as it passes by, and the other sensor marked a
pin in the flywheel which established a certain point in the rotation (i.e.
TDC #1).  The funky thing about a four-stroke engine is that it isn't
sufficient to know what the crankshaft position is ... OK, does this TDC of
cyl #1 signify the beginning of the power or the intake stroke?  The engine
uses the Hall Sender with a single window (as opposed to the Bronco's one
window per cylinder I would imagine ... never had the pleasure of working on
one) and all that window does is to tell the computer that last bit of info.
The thing that is screwy about many of the Audi engines is that since there
is an odd number of cylinders the computer only fires the coil around TDC #1
when that cylinder is at the top of compression and starting power ... at
the other TDC #1 there is no other cylinder that would use a spark  (it
might even cause problems).  This is why I believe that the V8 might be able
to start without the distributor Hall Sender ... the 8 cylinder engine is
firing about the same four locations on each rotation of the engine, so it
really doesn't matter which cylinder that is [yes, I'm ignoring the fact
that the V8 actually has 2 coils and 2 distributors ... in any event there
are no more than two combinations to try and the ECU can certainly notice
the lack of the distibutor signal and try each possiblity to see if it can
get the engine to fire].  I did a couple hand calculations, and there are a
couple of possible problems that might arise in the odd # of cylinders case,
which may be one of the reasons a 5000 won't start w/o the distributor
reference.  The first thing is that if the computer fires in the wrong phase
the distributor would be in a position between two cylinders.  I guess over
the short term this isn't likely to be a problem, but one of those cylinders
is somewhere around 72 crank degrees BTDC on compression, which might lead
to premature ignition of the mixture and attempting to run the engine
backwards.  

I wouldn't attempt to claim that VW/Audi was the first car to do this, but I
would dare to say that it was one of the first production cars.  Please
don't take my comments as a flame, I was just trying to inject a bit of
levity ... and to pass along my thoughts on the original question.  There
are other Audi engines that use a Hall sender with one window per cylinder
(and vacuum and centrifugal advance distributors) ... on these cars there is
no way the engine can run if the single Hall sender signal malfunctions ...

Vorsprung durch technik!
Steve Buchholz
San Jose, CA (USA)