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Dead ECU causing no spark condition 850

posted by  markj227 on  Fri Oct 10 08:13 CST 2008
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last visit: Thu Nov 13 23:50 CST 2008
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Has anybody out there ever seen a wiring harness on a late model 850 just give up the ghost for no apparent reason...I have ruined four ECU modules...my initial trouble was no spark (see my other "no spark" posting)..all the usual suspects did not effect the repair.....replacing the ecu did...2 days later, that replacement is also junk...can reproduce the symptoms by placing the bad ecu in another 850......it just seems unlikely that a short in the harness would affect only the ecu....there are no other symptoms other than lack of spark while cranking, no codes set, etc....any thoughts on possible culprits would be appreciated.....Thanks








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    Dead ECU causing no spark condition 850

    posted by  someone claiming to be ksg on  Fri Oct 10 09:42 CST 2008 [RELATED]

    Looking over this and your previous post, and having the understanding that actual ECU failures are rare, I would ask the following:

    1) Were there any symptoms or codes before the car stopped running? Did it just quit suddenly while driving, or not start one day when it ran ok before?

    2) If you return the factory original ECU to each of your cars does the other car (from which you borrowed an ECU) no longer run when it ran ok before you removed that ECU? I am thinking that a car might refuse to run unless the ECU is absolutely a perfect match (or has only a few specific benign differences).

    3) Did either car experience an overvoltage condition - from an open voltage regulator, or use of a high voltage jump starting machine some time in the past? I ask because overvoltage is one way to damage the ECU for sure. I think many of the inputs and outputs are short-circuit and open-circuit protected because a designer would anticipate possible failures on harness or sensors, as you seem to have had or suspect.

    4) I don't have schematics at hand, but do these cars have a main relay or fuel system relay (not the fuel pump relay) which passes power to ECU and/or primary ignition? Did you verify it is ok?

    5) Do the injectors tick? If they do that suggests that some functions in the ECU are intact.

    good luck!








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      Dead ECU causing no spark condition 850

      posted by  someone claiming to be markj227 on  Sat Oct 11 10:26 CST 2008 [RELATED]

      Thanks for your interest....
      The vehicle in question never exhibited any symptom other than refusing to start after being used every day for a long, long time. no codes were set...when I borrowed the ecu from another 850, that module was ruined by the suspect vehicle..i.e....when the ecu is returned to it's original vehicle, that vehicle is now disabled....
      The only diff between the "donor" and the original is the last digit of the Bosch serial # which I understand to be the designate between an automatic vs. a 5 speed...which is the case....replaced the donor ecu in the second 850...it's running fine......will not try that again..
      Thought about the voltage regulation issue....there are no indications, including digital voltage checks, that would indicate the regulator is bad...never blown a bulb, never witnessed dimming and or bright lights at idle or at speed...
      Bear in mind that this issue of no spark was rectified for 2 solid days simply by replacing the ecu with a used module from a local bone yard....suddenly, one evening, the car died in an intersection....a quick check for spark revealed the exact same condition....very itermittent,I would say......
      Anyhow, as I type, I have replaced the front harness from the ecu through the starter and on to the various sensors....my replacement wiring came from an 850 with an automatic....mine is a standard.....replaced the ecu again......IT RUNS....again....I'm looking for the color code for the neutral safety switch on a vehicle with the automatic, as I had to jump the starter due to the extra piece of harness for the different tranny.....
      Thanks again for your input.....MJ







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        Dead ECU causing no spark condition 850

        posted by  KlausC on  Sun Oct 12 06:47 CST 2008
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        last visit: Fri Oct 24 10:15 CST 2014
        [RELATED]

        The only wiring problems I have seen are from the thicker red wire from the battery getting stiff and hot.

        For wiring diagrams, check this download:

        http://www.matthewsvolvosite.com/chat/phpBB2/viewtopic.php?t=8018

        Better than the haynes manual in some respects. Your manual shift has a starter cutout switch at the clutch, IIRC.

        Klaus
        --
        Just driving a 1998 V70R :)







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