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Thread: wiring hobbs hour meter

  1. #1

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    Default wiring hobbs hour meter

    Finishing up wiring on my series 6, and was thinking instead of wiring the hour meter to start when the master is on, can a guy wire it so it comes on when the engine sees oil pressure. If this is possible you would just splice the wire from the hour meter and the oil presser sender together right?????

    thank you
    Partick series 6 builder

  2. #2
    Senior Member Av8r_Sed's Avatar
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    Default Re: wiring hobbs hour meter

    Wiring to the pressure sender won't work with the Hobbs and it would mess with your oil pressure reading. It would work if you had a separate oil pressure switch.
    -- Paul S
    Model III SN910
    582 IVO Med

  3. #3
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    Default Re: wiring hobbs hour meter

    Try wiring it to the tach, that way you get an hours reading when the engine is at max continous operating power.

    Dick B

  4. #4
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    Default Re: wiring hobbs hour meter

    Pick a low oil pressure switch for pretty much any older model car, and
    get the sending unit from the auto parts. They are almost all 1/8 npt. Then just
    use a tee fitting with your oil pressure sender. Have it toggle your hobbes
    on when the engine starts. I run mine to a relay that flip flops, and turns
    on an idiot light if master is on and oil pressure is zero, or hobbes on when
    master is on and oil pressure is > 5 psi

    summitracing.com has them

    http://www.summitracing.com/parts/sum-g1438/overview/

    Regards,
    Jeff Hays

  5. #5
    Senior Member Dave S's Avatar
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    Default Re: wiring hobbs hour meter

    Patrick,

    Many ways to accomplish this........to some extent, it depends on what other equipment a person has on the panel.

    Please check the attached photo. In this case, the UMA electronic tachometer just below the Quad gauge (conveniently labeled for a 912) has a hobbs tap on the back.(hobbs just below the tach) ......when the engine is not running, no signal is produced and the hobbs stops.

    Maybe I ran across a few too many rental planes which had battery powered hobbs meters which would just keep on running till the battery died if a careless renter happened to have left the master on...then I would have to get both the hours straightened out as well as the dead battery before I could use the plane in the morning. (admittedly not so much a problem with owner operated planes)

    However, the Hobbs tagged onto the UMA tachometer (if a person happens to have this tachometer) is a pretty simple system and effective.

    Sincerely

    Dave S
    KF7 Trigear
    912ULS Warp Drive

    St Paul, MN
    Attached Images Attached Images

  6. #6
    Senior Member colospace's Avatar
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    Default Re: wiring hobbs hour meter

    Unrelated to original question, but Dave, I was wondering about what appears to be a small window in your left boot cowl adjacent to your left foot.
    - Gary
    S7 SuperSport Tri-gear
    w/Rotax 912, Oratex, Dynon

  7. #7
    Senior Member Dave S's Avatar
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    Default Re: wiring hobbs hour meter

    Hi Gary,

    The window you are looking at in the photo is a cutout for the pilot's side air vent...that was just after my first flight after deciding that flying a Kitfox without air vents and without a heater shutoff valve in 85 degree F. weather would be more comfortable with some fresh air..... At that point the vent was not installed yet. I fabricated one for each side....sort of RV style installed in the high pressure area on each side of the cowl and so the pilot and copilot can easily reach their respective air vents for adjustment.

    If you do a thread search on Teamkitfox for cowl vents, you can find photos of this completed installation as well as vent installations of other builders.

    Dorsal started a thread titled "Thoughts on cabin ventilation" back a while....several photos and discussion there.

    Sincerely,

    Dave S
    Last edited by Dave S; 11-19-2013 at 08:41 AM.

  8. #8
    tommg13780's Avatar
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    Default Re: wiring hobbs hour meter

    A very simple and inexpensive solution is to take A/C from the engine charge/lighting output. Run this through a rectifier which is available for about $5. Mine came from an ebay store. Then connect the D/C output to the Hobbs meter. This circuit is totally independent of the D/C power feeding the master and remaining electrical system. If anyone is interested I'd have to do a little research to be more specific. This method provides 100% failsafe protection from introducing erroneous hours into your hobbs circuit. I'd like to take credit for this myself but actually it came from one of my flying chums that works in a quality testing lab for Amphenol Corp.

  9. #9
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    Default Re: wiring hobbs hour meter

    Hmm, I still think the oil pressure switch is the simplest method since it's simply
    a switch ... Pressure goes up, switch closes, voila hobbes meter runs ... It's
    also very useful for turning on the big red "you're about to fall out of the sky
    oil pressure is zero" light.

  10. #10
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    Default Re: wiring hobbs hour meter

    If the switch closes to operate the hobbs, how does the switch opening turn on the light?

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