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2005 Ford Transmission PRNDL Logic... need help please.

Discussion in 'Electronic Repair' started by Thomas, Apr 1, 2005.

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  1. Thomas

    Thomas Guest

    I am trying to activate a relay when the vehicle is in anything other
    than Park. The transmission module sends out 5V @ 120hz with varying
    pulse widths. In Park, I read 0.71V with the meter and looks to be
    apprx 1/7th of the pulse on the scope (square wave). In Reverse, the
    pulse is measured at 1.6V of 1/3 of the pulse with higher voltages and
    pulse widths in N,D,2. I've tried using this to enable a small dc
    relay, but only get chatter (even in Park). I've been told I could do
    the following...

    RC Integrator - Unity-Gain OpAmp Buffer - Comparator

    How would this work? Any help is greatly appreciated.
  2. Franc Zabkar

    Franc Zabkar Guest

    I think you would also need a sample-and-hold.
    I'd prefer a one-chip solution involving a PIC micro, eg a $3 PICAXE
    with on-chip BASIC. An intelligent chip could open up additional

    Otherwise I'd opt for a discrete digital solution involving a 74LS123
    retriggerable monostable multivibrator and a 74LS74 flip-flop.

    Here's my hand drawn attempt: (7KB)

    I've assumed a period of 8.3 msec (=120Hz), and pulse widths of 1.2ms
    (1/7 x 8.3) and 2.8ms (1/3 x 8.3) for Park and Reverse signalling.
    I've also assumed the pulses to be positive going.

    The 74123 is triggered by the transmission pulse (P) and produces a
    negative reference pulse, P1 (2.0ms). P1 clocks the 7474 flip-flop on
    its rising edge, ie after 2.0ms. If pulse P is shorter than P1, then
    the input to the FF will be low, otherwise it will be high. The Q or
    Q* outputs can then be used to control a relay via a transistor.

    Cext and Rext are chosen so that ...

    2.0ms = 0.45 x Cext x Rext.
    Rext >= 5Kohm.

    I've selected a pulse width of 2.0ms because it falls half way between
    Park and Reverse. Values of Rext = 10K and Cext = 0.47uF would give an
    acceptable pulse width of 2.1ms.

    - Franc Zabkar
  3. Franc Zabkar

    Franc Zabkar Guest

    Oops, the relay driver transistor in my circuit should connect to the
    Q output of the 7474, not its Q* output. I would also add clamping
    diodes to the P input of the 74123:

    /_\ 1N4148
    P ---|--> 74123
    /_\ 1N4148


    - Franc Zabkar
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