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The POH for a PA28 (engine driven vacuum pump) says a decrease in pressure on the gyro suction gauge may indicate a possible sticky vacuum regulator. A zero pressure indication means a sheared pump drive. What exactly does that mean?

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There is a small vacuum pump attached to the engine by a small rotating shaft which generates vacuum whenever the engine is running. That vacuum is used to spin up the gyro instruments in the plane.

Because the pump produces different amounts of vacuum at different engine speeds, its vacuum output must be sent through a device called a regulator to furnish a constant vacuum to the instruments at all times.

If the vacuum pump drive shaft is broken ("sheared"), the pump produces no vacuum and the gyros go dead- and the suction gauge reads no suction. If the regulator mechanism is stuck or jammed, it stops working as a regulator and allows full suction pressure to be applied to the instruments- and the suction gauge reads too much suction.

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    $\begingroup$ Do you know if the gauge is before or after the regulator? $\endgroup$
    – Dan
    Oct 8 '21 at 7:30
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    $\begingroup$ Good answer, I'd just add that the regulator can get stuck the other way, giving too little suction. $\endgroup$
    – GdD
    Oct 8 '21 at 7:40
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    $\begingroup$ The gauge is after the regulator @Dan, you are seeing the regulated pressure as that's what the instruments see. $\endgroup$
    – GdD
    Oct 8 '21 at 12:40
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    $\begingroup$ Those vane suction pumps are not very reliable because the vanes are made of carbon and are brittle. The vanes break apart and jam the pump, and shear the drive. Also, there is a filter on the inlet side of the system, prior to the vacuum instruments. If it's clogged more than the regulator can accommodate, it can result in a excessively high vacuum indication due to the restriction. $\endgroup$
    – John K
    Oct 8 '21 at 16:36
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    $\begingroup$ Another thing that result in low suction is a vacuum system leak. Vacuum pumps are life limited, and in many cases, especially if reliability is required such as for IFR flying, many operators and owners will replace the vacuum pump every 500 hours as a preventive maintenance measure. $\endgroup$ Oct 8 '21 at 17:48

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