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Thread: Vacuum System Question

  1. #1

    Vacuum System Question

    Greetings All,

    A couple weeks ago I had to replace a bad VAC pump on my Sport. After
    describing the symptoms (no movement of vac gauge needle, and no spin
    up of DG or AI), mechanic removed the pump and found the shaft was
    sheered and the pump seized. $460 for a new pump and installation, the
    vac gauge stands right up at '5' and instruments spin up nicely even at
    fast idle. (it was barely making the green at idle before, run up was

    Anyway, a few days later I wanted to learn were all the VAC hoses and
    components were located and followed the system through. I looked
    through my maintenance manual and could not find a diagram of how the
    system should be hooked up. Where would one find this? The one thing
    I did notice was the VAC gauge had only ONE hose connected to it. I
    did not notice any hoses laying around that might have fallen off. I
    would think that this open port would be a place were unfiltered air
    would enter the system. I also read (I think on BAC) that many of our
    planes have "pressure" vice "Vacuum" systems. I asked the mechanic
    about that and he said the system is definitely hooked up as a VACUUM
    system. (I have not been able to ask him about the gauge hose yet) I
    am hesitant to keep the system running if it is not put together right.

    Any thoughts appreciated and many thanks in advance

    '78 Sport 150
    Ser MB 901

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  2. #2
    If you have a Parts Manual, check out Figure 194. It shows very clear diagrams of all the hoses and routings. It is very typical to have only one hose (usually 1/4") attached to the vacuum gage. The vacuum line should be connected to the port marked "P", not "V" as you would logically think.

    Check out the following link at Parker's website for additional information for evaluating vacuum system performance (Yawn!!!)

  3. #3

    vacuum system question

    You probably had worn vanes on the old pump to make such a low pressure.

    You don't really need a diagram, but just to spend some time behind your panel. Each gyro should have 2 ports with hoses on them (there are exceptions, I'm keeping it simple), One should say filtered air, one suction. Sometimes they are inlet and outlet. Each suction/outlet port should connect by hose to the vacuum pressure regulator. Each filtered air/inlet to a gyro filter. Most planes these days have a central gyro filter. Both of these filters need to be clean, snow white clean. Check the hoses that they are not cracking or weatherchecked. The hoses should still be flexible. If for some reason you do not have a gyro filter, put one in, it will save hundreds of dollars of gyro repair.

    About your gauge, usually it is connected by hose to a smaller port on one of the gyros, sometimes that is marked, others not. The vacuum gauge can also be teed into the suction line. To answer the gauge hose ports, P is for vacuum pressure, V is for vent. Vent is used for pressurized aircraft to vent the gauge to ambient air and not the pressurized air. This is left open on nonpressurized planes.

    Now, one bit of advise for the beech planes is to be sure that neither the windshield, nor the airvent plenums leak onto either the gyro filter or vacuum regulator. The introduction of water into the gyros is a very expensive proposition.
    I chewed through my restraints for this?

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