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Thread: microswitch intake manifold - What does it do?

  1. #1

    microswitch intake manifold - What does it do?


    Just putting my 69 engine back in the car and the microswitch that is activated via the throttle bar linkage made me think...what does it do? and how is it set?

    Part number 901 613 106 00 item number 24 on the image

    thanks in advance

  2. #2

  3. #3
    Get yourself a beer and prepare for another one of my long technical explanations.

    Bosch MFI operates in "Alpha-N" mode by which the only two inputs are 1) throttle position and 2) RPM. Alpha for the angle of the throttle plates and N for number of revs.

    However, when the throttle is closed rapidly, the reduction in fuel flow is not instantaneous. As such, the MFI pump will continue to pump fuel into the intakes, and because the throttle is closed, the mixture goes very rich. If you are using a sport muffler you can expect flames from the exhaust as the rich mixture is not burned-- it passes through the cylinders and ignites when there's enough oxygen present from the air.

    So, Bosch needed a fuel cutoff system, which they accomplished with the "speed switch." This is a metal box mounted on the electrical console with a circuit board inside.

    I'll spare you the technical description of how the speed switch works-- its FUNCTION is to close the circuit to the pump-mounted fuel shutoff solenoid when the RPM is above a certain level and the microswitch is closed.

    On the pump there is a solenoid which when energized, moves a rod that in turn contacts the main rack in the pump, driving the rack to full-lean position.

    The speed switch will energize the solenoid when: a) RPM is above 1500 AND b) the microswitch is closed. This happens when the throttle is snapped shut, such as at the end of a straightaway. The RPM hasn't yet fallen, but the throttle is in idle position.

    An adjusting screw on the throttle crossbar bellcrank contacts the microswitch when the throttle is closed, completing the circuit through the microswitch. The solenoid is energized, the rack goes full lean, the RPM falls further.

    Then, once the RPM has fallen below 1300 RPM, the speed switch cuts power to the circuit.

    A test of the speed switch is to rev the engine above 1500 by moving the crossbar, and close the microswitch with your fingers. The revs should immediately fall, and when they go below 1300 RPM, the engine should rev up again, until they cross 1500, then they fall again.

    My 71E race car never had a working speed switch. . . I didn't mind the flames
    1966 911 #304065 Irischgruen

  4. #4
    Thanks so much 304065,

    You explained exactly what I asked in simple clear detail. People like you make this forum what it is “excellent”


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