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  1. #1
    Sam Carlson
    Tutorial Bot euromobile900's Avatar
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    14 Aug 2010
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    Medford, MA
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    683
    Saab(s)
    '90 900 LPT with a flat-nose conversion
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    LH 2.4 N/A ECU failure (AKA '89 N/A ECU failure)

    The story has long been told that non-turbo cars from 1989 have bad ECUs. Something about them seems to cause them to fail. I recently looked at a 1990 car, whose owner complained of sputtering, bucking, and misfires while driving. I suspected plug wires, because his car was a 1990, not an '89. But when I listened to the engine run, I found something surprising. At idle, injectors normally make a nice loud rhythmic ticking. On this car, when one of the misfires occurred, I could hear the injectors issue a rapid series of ticking sounds, before returning to their normal cadence. The "shift-up" light also flashed whenever there was a misfire. I replaced the ECU with a known-good one, and the misfires disappeared and the injector ticking went back to normal rhythm.

    The failure of his ECU (the start of this missing, sputtering, and injector arrhythmia) occurred after the owner's repeated attempts to jump-start the car. It had run fine the previous day, but it would not start (would only crank). He used a 50-amp jump-pack to resuscitate the car, and when it finally started, it misfired and sputtered because of bad ECU. I assume the ECU failed after shutoff, and would not start the car at all until a lot of current was put through the system. An alternative hypothesis is that he fried it doing a jump, but I find this unlikely, as his battery was fine to start the car even before he tried the jump. So it seems his ECU failed hard (no start) and then mended itself to the point where it caused only misfires, before I got a chance to look at it.

    At any rate, this may prove helpful to those of us with LH 2.4, trying to diagnose a problem. If the injectors are ticking out of rhythm and shift-up light is flashing during misfires/stumbling, suspect the ECU!

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    Last edited by euromobile900; 15 January 2013 at 19:43.
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  2. #2
    Dave T.
    Super Moderator Dave T's Avatar
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    03 Aug 2010
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    near Seattle, Washington
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    1,337
    Saab(s)
    1999 9-3SE (2013-2015), 2005 9-3 (2005-2013), 1990 900 (1990-2003)
    Thumbs Up:   25
    Hmm, the ECU in my 1990 normally aspirated 900 went out one day in 2002. The battery was getting old so I bought a new battery but it didn't help.

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  3. #3
    Sam Carlson
    Tutorial Bot euromobile900's Avatar
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    14 Aug 2010
    Location
    Medford, MA
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    683
    Saab(s)
    '90 900 LPT with a flat-nose conversion
    Thumbs Up:   2
    Quote Originally Posted by Dave T View Post
    Hmm, the ECU in my 1990 normally aspirated 900 went out one day in 2002. The battery was getting old so I bought a new battery but it didn't help.
    Sounds strikingly similar to what the owner of this car encountered, except that he was able to goose the system with current and somehow force it to go. I will forever wonder what happened inside the circuitry when he attached that jump pack.

    I have a hunch that all the N/A LH 2.4 cars had ECU failure issues, and for some reason '89 got pegged as a problem year. How many times have you ever heard of a LH 2.2 or 2.4.2 ECU failing? In all of my contact with SAABists, I've only heard of one LH 2.2 ECU failure and NO LH 2.4.2 ECU failures. But I've heard of a lot of non-turbo '88, '89, and '90 ECU failures. The thing I really think is interesting about all this is that I now know how to diagnose a failed ECU, something that hasn't really been discussed much on the internet. It's just "replace with known-good", a luxury that not all of us have.

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    Last edited by euromobile900; 10 January 2013 at 05:02.
    Ask me a question about your c900! I promise I either can answer it or know someone who can

 

 

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