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CISe system q's [Answers - long]
At 12:58 PM 7/24/2003, Ryan H wrote:
>Well, if your car was running limp-mode, you'd really notice it, cause
>you'd have NO power. None.
She actually was -- I saw the 10mA reading.
Whether the car will actually run like shizzynit is going to be a factor of
where the mixture is set. I'll explain in a minute.
>The connector on the back of the fuel block is what you measured (DPR)
>that read 10mA. Not exactly sure how this unit works, though. A reading
>that is stuck usually means a dead O2 sensor, but I was under the
>impression by others that the reading would be 20mA in such a case, not 10.
Actually, I've seen it read 0 mA with a dead O2 sensor, too. As O2 sensors
age, they put less and less voltage out, which causes the computer to think
the car is becoming increasingly lean. It'll compensate for it by
richening the mixture (reducing the DPR current, eventually to 0). At some
point, the sensor will be totally dead, and then the computer will go into
limp-home mode. (More below).
> And the sensor on the front really has nothing to do with the CIS-E
> loop - it's an airflow sensor that is only used when you first start the
> car - it supplies additional fuel when you hit the throttle for the first
> 30 seconds or so after starting it. If you were to unplug it then blip
> the throttle right after starting, the car would bog nicely because of
> the missing signal. HTH.
It actually works all the time -- not just the first 30 seconds. It's a
potentiometer that measures rate of change of the airflow meter. It will
respond by richening the mixture under any quick throttle
transitions. Unplug it, and you'll feel no big difference under
steady-state cruising. But any time you make a sudden downward movement of
that pedal, the engine will bog for a second until the mixture catches
up. There are a lot of reasons why you want a richer mixture under
transitional throttle, but one of the big reasons this device is used is to
compensate for the time it takes for the air flow plate to move and to
actually supply extra fuel to the injectors when the throttle is suddenly
opened.
Julie, I recommend getting the Bosch Injection book. I'm going to look to
see if I have it on CD-ROM (or at least parts of it) for you to
download. It's a great resource and explains everything in great
detail. It's very technical, but I know you'll have no problem
understanding it.
Here's the long and short of it: CIS-E meters fuel by measuring airflow
with a plate. As the plate moves further upward in response to more air
travelling past it, it opens 4 valves in the fuel distributor which
increase the amount of fuel going to the injectors. It's a brilliantly
simple system.
Certain situations call for a mixture leaner or richer than the baseline
level (somewhere around 'Stoich') [cold starting and running, overheating
engine, throttle transitions, full throttle, fuel cutoff,
etc]. Furthermore, using feedback from the O2 sensor, it can determine the
*actual* mixture, and make very fast continual adjustments to even more
precisely control mixture. The CIS-E computer sends signals to the DPR,
which allows it to make these corrections to the mixture.
There are two "halves" of the fuel distributor on CIS-E cars. The lower
chamber is fuel pressurized by the fuel pump and sitting there waiting to
be fed to the injectors. The upper chambers are fuel that is on its way to
the injectors. There are two ways that fuel can pass from the lower
chamber to the upper chambers:
(1) Through one of the 4 valves that are opened by the airflow
plate, and
(2) The differential pressure regulator (DPR).
If your engine is perfectly tuned, you can disconnect all of the
electronics from a CIS-E car and it will run fine. (Mine runs
fantastically when it's warm with no CIS-E computer hooked up to it at
all). It will meter fuel on a purely mechanical basis (i.e. through those
4 valves controlled by the airflow plate), and you'll lose all the
electronic babysitting services that the "E" added to CIS-E: It'll start
hard, since it won't have cold-start enrichment. It'll run rough when cold
because it won't have cold-running enrichment. It will lose some power
under full-throttle because it can't richen the mixture. Your injectors
won't shut off when coasting under no throttle, causing a slight backfiring
and a reduction of fuel mileage. Your emissions will be higher because the
computer can't adjust mixture based on O2-sensor feedback. Some CIS-E
systems (like Mercedes CIS-E systems, for example) will richen the mixture
at high engine temps to cool the motor down, or use an altitude-sensor to
compensate for high altitudes...
The computer does all of these neat little tricks by using the DPR. The
DPR is a connection between the lower chamber of the fuel distributor and
the upper chambers of the fuel distributor. It uses an electro-magnetic
membrane that acts as a valve to allow additional fuel to 'sneak' past the
other 4 valves and supply more fuel to the injectors.
When no current is supplied to the DPR, the valve is like half closed. The
fuel getting to the injectors is the fuel that passes by the 4 fuel
distributor valves and through the DPR. The more current that is supplied
to the DPR, the more fuel it allows to pass through it. (So the mixture
will be richer for any given position of the air flow plate). Likewise,
the less current that goes to the DPR, the less fuel that is allowed to
pass through it, and correspondingly, less fuel is delivered to the
injectors for any given position of the plate.
If you watch what happens to the DPR after you start the car, you can see
that it gets a high current from the computer, richening up the mixture at
first. This value then drops to the "baseline" value, which is usually
around 10mA. (Don't quote me on some of these numbers, every system is
different and I never remember which is which -- Mercedes, for example,
uses Volts instead of Amps as a measurement for the DPR). This value is
the limp-home mode.... it's the default value that the computer picks if it
can't figure out what else to do.
If you blip the throttle, you'll see the current jump up, and then drop
down to a negative value for a split second, and then return. What's
happening here is that the potentiometer I mentioned earlier is seeing a
quick change in air flow, and signaling the computer to fatten (=richen) up
the mixture temporarily. Then, the computer sees the idle switch being
activated, which means your foot is completely off the gas. So it turns
the injectors off completely by closing the DPR's valve as much as it
can. Once it returns to like 1300rpm, the fuel is switched back on, and
the value returns to "normal".
Once the O2 sensor begins to provide feedback, the DPR current will start
to fluctuate. You'll see it rise and sink, rise and sink. This is the
computer continually (over)compensating for the mixture. It'll go a little
too lean, then a little too rich, then a little too lean, and so on. Any
of the other factors we mentioned before (like fuel cutoff or cold-engine
enrichment) will vary that current by a certain amount. So, for example,
the DPR current (calculated based on O2-sensor feedback) will be reduced by
a few mA to richen the mixture when the car is still a little cold.
At full throttle, the computer completely ignores the O2 sensor and goes
into a fixed enrichment mode... I can't remember what it is, but say 8mA.
Remember, this current reading isn't the "mixture" reading, it's a
*modifier.* The real mixture is determined by the air-flow plate and its
subsequent opening of those 4 valves. The DPR current tells us how much
the computer is MODIFYING the metering that the air-flow plate is giving us.
Thus a reading of 0Ma shows is that the airflow plate is giving the engine
too much fuel, and that the computer is telling the DPR to close its valve
to help lean out the mixture. Likewise, a DPR current of 35mA would mean
that the mechanical metering was giving the engine far too little fuel, and
the computer was trying to compensate for that by letting lots of fuel flow
through the DPR to richen the mixture.
So when we look at the DPR current reading on a healthy system, we should
see a value of something like 5mA. (Don't quote me on the numbers,
remember). This number corresponds to the point where the
mechanically-metered mixture is as close to "dead on" as it can get, and
allows the DPR enough room to compensate in both directions (i.e. it can
sufficiently LEAN or RICHEN the mixture). If you set the mixture too lean
with the screw (mechanically), you risk having the DPR be out of range: It
can't provide enough fuel to compensate, and even doing all it can, you'll
still be too lean.
The CIS-E box has a bunch of inputs that it relies on to determine what the
mixture compensation (DPR current) should be. I don't remember them all
off the top of my head, but some of them are: The idle switch, the
full-throttle switch, the coolant temperature sensor, the thermo-time
switch, the Oxygen sensor, the airflow plate potentiometer, and some kind
of RPM sensor. If the computer is getting an invalid reading (or no
reading at all) from one or more of the sensors, it will go into limp-home
mode. Obviously, the O2 sensor is one of the most important sensors --
without it, the mixture compensation can't happen at all -- only the
"special" cases - like cold running, full-throttle, etc, are dealt with.
If your DPR current never moves or changes, there's something causing your
computer to just give up. You need to check that all of the input signals
are actually getting to the computer... and that all of the things the
computer controls (DPR, aux air bypass valve, etc) are functioning
properly. If I remember correctly, the computer will throw in the towel if
the aux air bypass valve is disconnected, for example. (My Mercedes CIS-E
did...)
And that's it. :)
Good luck and HTH!
Jason
PS: If you know how the aux air bypass valve works, (or the Idle
Stabilizer Valve, ISV, as it's been called), you'll recognize that the DPR
works in a similar way. The only 2 ways for air to enter the engine
through a closed throttle are (a) through the idle bypass, which is
adjusted by that screw, and (b) through the ISV. If the screw is set
properly, the ISV will be at the mid-point of its duty cycle, which means
that it's half open. This way it has maximum flexibility to raise idle (by
opening all the way), or reduce idle (by closing the rest of the way).
> >
> > From: "Julie Macfarlane" <juliemac57@hotmail.com>
> > Date: 2003/07/24 Thu AM 07:10:59 EDT
> > To: scirocco-l@scirocco.org
> > Subject: CISe system q's
> >
> > Back at Cincy, my car was identified running on "Limp home mode". That is
> > the current was at 10ma.
> >
> > Is there a "Keep your VW alive" Level description of how CISe works, what
> > the parts do etc?
> >
> > I know that there are 2 electrical connectors on the fuel dist, but not
> sure
> > what they do or what else is used to control the system.
> >
> > TIA guys
> >
> > Julie Macfarlane
> > Menlo Park Research & Development
> > Internet Application Developer
> > www.menloparkrandd.com
> > www.montgomeryweb.org
> > Amsterdam NY
> >
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