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Vehicle Diagnostic Unit

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  • #16
    It begins to show signs of life....

    Tonight was wiring together the "A-Bus" which is mainly signal inputs, plus meter connections. All wiring is point-to-point soldering.

    B-Bus and C-bus are mainly the lamp outputs, arranged so that positive voltages are only on even pins while the negative (or ground) are odd pins and thus you basically solder together one row of pins and bam, grounds are done.


    • #17
      Progression. Now the single-state LEDs (altitude jumper and the two motor phases) and the red LED's are wired in.

      Keep in mind that the inputs from this side of the diagnostic connector are not loaded. The optoisolators do not enjoy floating inputs and can on occasion sporadically fire when touched or the input power is unstable. Pullup resistors are provided inside the computer for this purpose and adding more over here may interfere with the computer, plus the diagnostic unit can only work when plugged into the harness anyways so normally it won't matter.
      the other issue I've discovered is that my desk is contaminated with some sort of fine and conductive metallic dust that is getting into the flux and causing high resistance bridges. The board will have to be extensively cleaned or else recurring glitchyness will happen.


      • #18
        Hmm, a problem has developed.

        I forgot that a standard RPM meter will display speed as one pulse per revolution. On the Eagle and similar vehicles, if you are pulling the RPM from the ignition TACH signal as opposed to a crank or cam position sensor you are receiving multiple pulses per rotation. One for each firing instance to the coil. Running the meter on this results in the RPM, multiplied by six, so if you are idling at 600RPM the display will show 3600RPM, which is the redline for the 258.
        The RPM meter however that I chose includes some form of a ratio divider that it gives no specifics about. You can also toggle between acting as an RPM meter or a frequency counter, so that should fix the multiple issue, right?

        Not quite. For reasons I do not yet understand this otherwise accurate RPM meter has a variance of 300RPM in either direction. For a device that should be off by no more than 50RPM that puts it far into "unusable". Likewise while the division takes place, display refreshes will continue at the input pulse speed and not at the divided speed, or six times faster than we really need it.
        The result is a display that can vary wildly as it flashes numbers.

        An intermediate circuit that conditions and divides the TACH before it reaches the meter may be necessary.
        Last edited by MIPS; 06-11-2019, 06:10 PM.