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Thread: CHT gauge working after recycling alternator

  1. #1

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    CHT gauge working after recycling alternator

    Had an odd thing happen to me today. I've got an Alcor CHT gauge with a single bayonet probe. Just after taking off from a little lake coming home tonight I look down and notice CHT gauge was cold. Interesting, it was fine on the warm up and takeoff run. Engine seems to be running fine - making full RPM, running smooth. I continue back home. If I have to put down, I'd rather do it closer to home rather than in the middle of the woods with no roads. I recycle the alternator, because why not, BOOM, CHT gauge is working again, seems to be reading about normal. The CHT gauge isn't electric at all, so why would that do anything?

    Any thoughts?

    I was going to do a mag check once I got back home, but completely forgot. It was 9:30 and I was hungry and I had snacks in my truck. Forgive me.

    PA-12 with a O-320 150 hp.

  2. #2
    wireweinie's Avatar
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    Alcor systems are usually self powered. That means that there should be no electrical connection to any other items or even to airframe ground. Check for loose connections on any Alcor wiring. Then see if the probe wiring is bundled with the alternator/regulator wiring. Separate them for testing purposes and check for loose connections. Let us know what you find.

    Web
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  3. #3
    www.SkupTech.com mike mcs repair's Avatar
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    CHT gauge working after recycling alternator

    I would guess ground issues. Even if itís not using aircraft ground, itís shield might be acting as engine to airframe ground (2 wire in shield ?) and getting mighty hot. or is it a single wire like a Cessna???


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  4. #4

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    I looked at the wiring. It's a two wire system, with no loose connections. There was a spot where the insulation had somehow gotten cut and one of the wires was exposed. I didn't see a spot where the shield would be grounding itself. The wiring is mostly within a plastic shield, only about a foot by the thermocouple had a metal shield until it got butt connected into the plastic shielded wire. I'm going to wrap up the exposed spot and separate the wire from any alternator wiring and see what that gets me. If not, running a new wiring may be in my future.

    Thanks for the help guys! I'll report back if it happens again.

  5. #5
    BC12D-4-85's Avatar
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    What do you do with the CHT when it works that replaces the manufacturer's engine instruments required for certification or STC? Just curious and no argument intended.

    Gary

  6. #6
    wireweinie's Avatar
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    Mike brings up a good point. You might want to check/clean the grounds from the alternator to the crank case and from the crank case to the airframe.

    Web
    Life's tough . . . wear a cup.

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    Gary, I mostly use it because it came with the plane . I have been using it to ensure my cylinders aren't getting too hot in the climbs. It doesn't replace any manufacturer's engine instruments, just in addition. From what I can tell CHT is not a required instrument for a PA-12 (this gauge was STCed).

    Mike/Web - Will definitely take a look at those when I go out to the lake today.

    -Anson

  8. #8
    BC12D-4-85's Avatar
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    Ok Anson thanks for the reply. Like anything gauges should be calibrated and the numbers shown real in the range of interest. I prefer multi probe units to get a look at all cylinders for the hottest or any odd one, rather than a single output unless the airframe manufacturer has done prior testing and installs it for certification.

    I did have an EGT try to be a source of ground on one plane until a savvy mechanic properly grounded the engine to the frame. It would act weird when I ran the starter was the indication.

    Gary

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    Gary,

    Eventually I'd like to install something like an EI UBG-16 or a JPI EDM-700. They seem to have the same functions, do you prefer one over the other? Or are there other options I haven't seen? For now I'm rolling with what I've got though.

    I'll add it to the list along with a Reiff system and wheel skis.

  10. #10
    BC12D-4-85's Avatar
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    Anson there's good folks here that would know what's best today for multi-point sensors and displays. They install and maintain them. Better to ask them than me.

    I'm just a user or envy-er of the equipment. In the past I've added selector switches and more probes to single point gauges like the Alcors. That works. Then I flew a couple of planes with scanning EGT/CHT's and was amazed. Now there's the EDM line and similar that boxes it all and offers recording and a focused display that alerts the pilot when + or - range is exceeded. Good stuff if you're into that.

    Now in my Taylorcraft I rely on basic instruments provided by the builder but then I don't fly that much as I used to either. If I ever upgrade airplanes I'll sure consider a singular display for it all.

    Check your grounds and enjoy your PA-12 they are great (owned one and miss it).

    Gary

  11. #11

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    This looks like as good as an excuse as any to oogle shiny things at Northern Lights Avionics. But I'll probably stick with the basics that I have now.

    And I'm doing my best to fly the snot out of the thing. I just got into flying a few years ago and lucked into a co-ownership and am loving it! Thanks for all your help!

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    BC12D-4-85's Avatar
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    Try to work with your maintenance people and make sure the cylinder baffling is good and doesn't leak cooling air. That plus any air leaks into the intake system between the carb and engine can bring up the heat. Adding a 4-point switch and three more CHT probes isn't as expensive as a new system and would give you all cylinders to compare.

    For example: https://alcorinc.com/index.php/produ...tch-2-section/ and https://alcorinc.com/index.php/sensors/

    Extensions may be needed to the probe wiring.

    Gary

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    Iíll most likely be called out for heresy, but there is no benefit to a multi sensor EGT/CHT beyond troubleshooting on a carbureted engine! The fuel mixture is far to great from the leanest to the richest cylinder for leaning to get better fuel economy. If you are worried about CHT, get a single CHT gage and put it on the hottest running cylinder.

    Iím all for multi channel engine gages for injected engines with tuned injectors so you can run lean of peak, used it all the time on the A36. Could go from about 17gph down to about 14.5 gph by going lean of peak with only loosing about 2kts. Beyond that, for a carbureted engine there are better ways to spend your money!

    Iíve spent about 2000 hrs towing gliders, and another 2500 hours towing signs. Only one of the dozens of Cubs, Citabrias, Pawnees, L-19s and other airplanes had a multi channel engine monitor, and nobody even bothered using it.


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    BC12D-4-85's Avatar
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    The secret is finding the hottest cylinder. Swapping probes works until it's discovered.

    Gary
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  15. #15

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    So a quick update on this. I examined all the grounds as well as the grounding of the probe and all is good on that front. However the wiring runs along the return line for the alternator. So I'm guessing in periods of high workload on the alternator it is inducing a current in the line and causing issues with the readings. Granted I have yet to test this hypothesis by changing where the probe wiring comes through the firewall, but it's really all I can think of.

    However, I did the old boiling water test and my gauge read 280F or so. I'll probably just replace the probe and rerun the wiring when I do that.

    Thanks Web, Mike, and Gary!

  16. #16
    BC12D-4-85's Avatar
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    The others mentioned are pro's...I'm just the Idea Guy Fly on the wall as usual. The boiling water test was a good choice. Rerun the wiring and test. Is there any alternator AC current noise on the radios? If the diode(s) pack gets wonky they get noisy

    Gary
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  17. #17
    skywagon8a's Avatar
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    Quote Originally Posted by amoxness View Post
    However, I did the old boiling water test and my gauge read 280F or so. I'll probably just replace the probe and rerun the wiring when I do that.
    It's more likely to be an instrument calibration than a bad probe. Is there an external screw on that instrument for adjustment? Disconnect the probe, clean the connection then re-calibrate the instrument.
    N1PA
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  18. #18
    BC12D-4-85's Avatar
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    Alcor setup: https://alcorinc.com/wp-content/uplo.../11/591851.pdf

    CHT calibration page 4 also see page 21 for hard boiled egg method with millivoltmeter: https://alcorinc.com/PDF/59207.pdf

    Gary
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  19. #19
    wireweinie's Avatar
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    Page four specifically states that you need an Alcor system tester to check calibration for CHT. Also says that there is no in flight calibration for CHT. So if you want to adjust it, do it in small increments. It's your wallet.

    Web
    Life's tough . . . wear a cup.

  20. #20

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    The hottest cylinder location can and will change depending on several factors. Once you start using a 4 cylinder CHT/EGT unit properly you will realize that you may not have seen hot cylinders but they have seen you.
    DENNY

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