Volvo 740 starup sequence

Discussion in 'Volvo 740' started by johnzab, Dec 14, 2006.

  1. johnzab

    johnzab Guest

    Doe anyone have the list of things that the onboard computer does
    during a normal startup and at aprox. the number of seconds / minutes
    into the process when the different events occur. It would help me with
    trouble shooting a startup / stall problem. Hopefully I would be able
    to isolate the function that is not working correctly.

    Thanks for any help in advance !!
     
    johnzab, Dec 14, 2006
    #1
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  2. Actually, the sequence is pretty primitive. When you first turn the key to
    the "run" position the in-tank fuel pump should run. When you turn the key
    to the "start" position the main fuel pump should run and continue to run
    for 5 seconds if the engine doesn't start or as long as the engine is
    running and the ignition is in the "run" position. When the ignition is on
    (in the "run" position) power is applied to the entire ignition system, so
    you will get spark - initiated by the Hall effect sensor in the
    distributor - when the engine turns. Also when the ignition is on, the Idle
    Air Control (IAC) valve is opened some amount (all the way? it is not a
    servo mechanism, just a 1/4 rotation motor) and a preset amount of fuel is
    injected based on normal idle requirements and trimmed by a voltage level
    from the MAF sensor. That condition changes to a different algorithm when
    the throttle is opened slightly, but this description is at idle. When the
    engine fires, the ECU monitors the engine speed (ignition rate) and adjusts
    the IAC valve to maintain proper idle speed. The whole thing settles into
    the groove within a couple seconds of the engine firing up.

    Have you cleaned the throttle body (and IAC valve as necessary) yet? That's
    what normally causes stalling at idle after a cold start. If it were not
    sensitive to the cold I'd recommend looking at the MAF (especially if it is
    not only at idle) and the fuel pump relay (especially if it was worse when
    hot in the car), but the cold stalling problem points strongly at the
    throttle body.

    Mike
     
    Michael Pardee, Dec 15, 2006
    #2
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  3. johnzab

    Per Mathisen Guest

    A vacuum leak in the intake system can cause similar symptoms, as the
    fuel/air-mixture is leaned out because some air is drawn into the combustion
    chamber without being measered by the MAF-sensor.
    Check all tubes for cracks.

    Good luck.

    Per Mathisen
    -88 780 w/ SBC350TPI/TH700
     
    Per Mathisen, Dec 15, 2006
    #3
  4. johnzab

    James Sweet Guest


    There isn't much of a sequence, as far as you'll perceive, the processes
    all happen pretty much instantaneously.

    Have you tried unplugging the air mass meter to see if that makes a
    difference? That and the wiring harness in pre-89 cars are the usual
    problems.
     
    James Sweet, Dec 18, 2006
    #4
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