Vanagon EuroVan
Previous messageNext messagePrevious in topicNext in topicPrevious by same authorNext by same authorPrevious page (April 2001, week 1)Back to main VANAGON pageJoin or leave VANAGON (or change settings)ReplyPost a new messageSearchProportional fontNon-proportional font
Date:         Sat, 7 Apr 2001 02:10:49 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Wet weather power loss
Comments: To: 72510.1173@COMPUSERVE.COM
In-Reply-To:  <092f01c0bee9$843b2560$175ec0d8@oemcomputer>
Content-Type: text/plain; charset="us-ascii"; format=flowed

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


Back to: Top of message | Previous page | Main VANAGON page

Please note - During the past 17 years of operation, several gigabytes of Vanagon mail messages have been archived. Searching the entire collection will take up to five minutes to complete. Please be patient!


Return to the archives @ gerry.vanagon.com


The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.

Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.