Date: Sat, 7 Apr 2001 06:15:34 -0400
Reply-To: 72510.1173@COMPUSERVE.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Helen Fahy <72510.1173@COMPUSERVE.COM>
Subject: Re: Wet weather power loss
Content-Type: text/plain; charset="iso-8859-1"
Thanks for the great info David.
The O2, throttle position and Hall sensor leads measured acceptable. The
AFM, left in circuit & undisturbed, measured open on both 2-3 and 2-4 pins!
I pulled the AFM and measured it still open on both sets of pins. I then
took it to my shop, removed the plastic shroud for the air cleaner, and
finally got some resistance that would correspond to the temperature from
the chart ion Bentley. The vane related resistance was all over the place.
I then popped the cover, that is a bit of a pain. There are two obvious
tracks on the Bourne card, but they are not down to bare ceramic. Visual
inspection would reveal no problem. I will clean the card and remeasure.
It would seem that the AFM, "goes away" when the humidity is high. Perhaps
there is a problem with the ceramic resister card?
I have forgotten to reattach the AFM connector while say replacing the air
filter and the van will start, but it obviously will not respond to the
throttle. In retrospect, the power loss I experienced could be thought to
be the same. The tailpipe has developed a significant carbon deposit from
the 1.5 hrs that we struggled with the van, until we got back home.
Thanks again,
Joe Fahy
----- Original Message -----
From: "David Beierl" <dbeierl@attglobal.net>
To: <72510.1173@compuserve.com>
Cc: <vanagon@GERRY.VANAGON.COM>
Sent: Saturday, April 07, 2001 2:10 AM
Subject: Re: Wet weather power loss
>
> At 06:32 PM 4/6/2001, Helen Fahy wrote:
> >wet weather, though it does not have to be actually raining, it can occur
in
> >heavy fog.
>
> Ok, almost surely electrical
>
>
> >tachometer is being driven, of course the engine is slowing down, but
there
> >is nothing erratic about the motion of the tach needle.
>
> Ok, still getting some kind of ignition pulse at the coil
>
> >turning ignition switch off an on brings back normal engine performance
for
> >a short time <<<Why does this work and is this the key to the analysis
of
> >the problem???>>>
>
> This resets the ECU from whatever strange mode it may have gotten
> into. That means it's not a fuel supply problem. It *may* also put the
> system into open-loop briefly. ECU inputs are Coolant temp, Air temp, AFM
> opening, rpm and TDC from Hall sender, lambda sensor, throttle switch (WOT
> and closed throttle [should] give same signal). Outputs are fuel pump
> drive (depends on Hall pulses), injector open pulses, ignition primary
pulses.
>
> Things that can go wrong -- bad inputs: Temp II open gives very rich mix
> (lambda voltage will stay above half volt), shorted is too lean to run
> cold. Lambda sensor grounded or low output (could include bad ground
> connections at engine) gives very rich mix. Lambda sensor leakage from
> heater wire should give very lean mix. AFM noisy or intermittent output
> could give lean mix (but not rich, I think) and could put ECU in a panic
> state where van loses power -- this is cured by cycling power to
> ECU. Shorted/leaky cap at AFM would give lean mix. Note that AFM is
> ignored above some rpm -- not sure what but 4500 should be safe. Temp I
> (air temp) -- dunno how much effect it has, i.e. whether it can overpower
> the lambda sensor which is normally the final arbiter on mixture. In any
> case, open would make the mix rich I think, short would make it
> lean. Shorted throttle switch should give rich mixture above some
> rpm/throttle combination, severe surging below that but above 1500
> rpm. Open throttle switch will make lean at full throttle, possible
> backfire on decel with closed throttle (normally it shuts off the fuel on
> decel until engine rpm goes below 1500 rpm)? Intermittent Hall sender
> should show on tach if interruption is long enough.
>
> Bad outputs -- wrong ignition timing, fuel pulses too long or short,
> *alternate fuel pulses skipped* -- this is how the rpm limiter works.
>
> Lambda sensor -- bad sensor (unplug it should make situation stable as ECU
> goes closed-loop -- voltage at ECU should remain steady near half
> volt). Bad wire (short/leak to shield) -- mix very rich, unplug sensor no
> difference, voltage at ECU (probably) steady below half volt. Could be a
> very high-resistance leak, this is a high-impedance circuit. Bad engine
> grounds -- sensor is grounded through exhaust system to engine, then to
ECU
> pin 19 from engine head. ECU also grounded to body at pin 13; this is a
> big wire and appears to be the power ground, where the other is the signal
> ground for the lambda sensor and ?other signal inputs? -- AFM, Temp I,
Hall
> generator all grounded to pin 6 of ECU. The major concern is the lambda
> sensor -- it runs from 0-1 volt prox. at very high impedance and the ECU
> uses it as a threshold detector. If it gets stuck either above or below a
> half volt, ECU will exhaust its adjustment range trying to shove the
> mixture the other way. AFM and temp sensors are analog inputs with range
> 0-5 volts and much lower impedance. To be confident of what the ECU is
> seeing, measure lambda voltage at the ECU btw pins 2 and 19 (not 13) using
> 10 megohm-impedance voltmeter (or scope with x10 probe...). However, if
> lambda voltage measured anywhere is jumping up and down within a 1-volt
> range then ECU is in closed-loop mode and is controlling mixture correctly
> or nearly so.
>
> Quickie description of digifant operation -- ECU takes rpm and AFM voltage
> to develop a basic pulse length for injectors (all injectors fire at once,
> once per revolution, and amount injected is controlled strictly by open
> duration since fuel pressure is controlled to be constant relative to
> manifold pressure). If rpm is too high for AFM it uses an internal table
> based on rpm. It then corrects for air temp and water temp, and opens
> injectors for corrected time. When running in closed-loop mode it then
> adjusts successive pulses until the lambda sensor flops the other way,
then
> adjusts again until it flops back, forever. If throttle switch is closed
> it ignores lambda sensor and richens mix by precalculated factor.
>
> At the same time it uses internal maps of rpm vs AFM vs idunnowhat to
> advance and retard ignition timing. When idling, the idle stabilizer unit
> constantly adjusts input airflow to maintain constant rpm. It uses an
> electronic control that drives an air valve that bypasses the throttle
body
> but not the AFM.
>
> david
>
>
> David Beierl - dbeierl@attglobal.net
>
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