Master switch issue when engine running

Discussion in 'Maintenance Bay' started by OkieAviator, Jan 31, 2017.

  1. OkieAviator

    OkieAviator Cleared for Takeoff

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    Here's an odd issue I encountered yesterday. The 172 just finished with both the annual and the Altimeter and Static check, completed by different shops.

    I have a fairly extensive post MX flight process that I go through to check everything out. I typically do an alternator check on runup but since the plane had sat around for a month at two shops and was a little sluggish on start I decided to wait and do that in flight.

    Once in the air I ran through all my little items I do to ensure everything is working well. One of the last things I do is fly a full RNAV approach to validate the GPS and CDIs are working well. Decided that would be a good time to mess with the alternator again so I turned it off... however it didn't look like it was turning off as the voltage remained fairly constant never dropping below 14. After a few minutes I decided to turn the master off... well nothing turned off, as if the alternator was still powering everything, about all that changed is I could hear some type of repetitive noise on the headphones. Voltage didn't change even with the master (and alternator) off. I cycled through this a few times with no changes, left it on and did the approach.

    In short master works like normal with the engine off, but once it's turning you can't turn the master off.. I validated this on the ground after I got gas. The A&P is going to look at it but was curious if anyone has every ran into something like this? Could it be something as simple as a messed up alternator switch where it's not actually turning off? I thought it was mechanical so not sure how that would work. Or a misplaced Blead on the contactor? Thoughts?
     
  2. hotprops

    hotprops Pre-takeoff checklist

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    bad ground on solenoid switch to firewall edit to add word switch
     
    Last edited: Jan 31, 2017
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  3. Dan Thomas

    Dan Thomas En-Route

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    The master solenoid grounds its coil through the master switch. The other side of the coil is connected to the battery terminal. It's done this way to avoid having an unloaded, unfused hot line into the cabin.

    I'd be interested to know if this is a restart Cessna (post '96), with the big junction box on the firewall, or some other airplane. It sounds like the alternator output has found a path to the master solenoid's coil, as if someone had done some rewiring and got it all wrong.
     
  4. hotprops

    hotprops Pre-takeoff checklist

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    dan yes you are correct i wanted to say wherever the switch runs its lead to ground. i thought it was wiring to the through firewall bolt to engine mount ,hence i said firewall ,you are stating facts in a more clear manner than i did .
     
  5. OkieAviator

    OkieAviator Cleared for Takeoff

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    It's a 1977 172N. The A&P is looking at it today, I really hope it's something easy...
     
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  6. hotprops

    hotprops Pre-takeoff checklist

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    well are we waiting .
     
  7. OkieAviator

    OkieAviator Cleared for Takeoff

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    Me too... it wasn't the ground, so he was thinking maybe the overvoltage regulator. That didn't correct it, so he called in some backup and now they think it's the alternator side of the master/alt switch. So that's on order and we'll see next week.

    My thought is I've only seen mechanical switches break in the 'open' position, but I'm not versed in 1977 switch technology. So we will have to wait and see.
     
  8. Norman

    Norman En-Route

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    Is he replacing parts thinking he's solved the problem? If he is, tell him to stop until he finds the problem.
     
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  9. OkieAviator

    OkieAviator Cleared for Takeoff

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    Already had that conversation with him.
     
  10. weirdjim

    weirdjim En-Route

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    Well then why the hell didn't he start the engine and pull the wire off of the master switch to see if the solenoid disconnected. Besides, it matters not to the master switch if the engine is running or not. Ask your mech if he can spell m-u-l-t-i-m-e-t-e-r.

    Jim
     
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  11. weirdjim

    weirdjim En-Route

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    That's called "shotgunning" in polite company and something altogether different between us fellers.
     
  12. OkieAviator

    OkieAviator Cleared for Takeoff

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    No joy... got a call today and it wasn't the switch. Asked if he would button it up and I'll get someone else engaged. plan to get someone out there probe it all over with a multimeter until the part/area is isolated.
     
  13. sferguson524

    sferguson524 Cleared for Takeoff

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    You mean throwing **** at the wall and seeing what sticks?
     
  14. flyingron

    flyingron Touchdown! Greaser!

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    It's like the joke about the customer engineer with the flat tire. He has to change all four before he figures out what was wrong.
     
  15. ElPaso Pilot

    ElPaso Pilot Line Up and Wait

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    My dentist was just telling me the story of a fellow dentist who went to him for a tooth extraction. Insisted he knew exactly which tooth was hurting and needed removal, and didn't need any further diagnosis.

    He sheepishly showed up several days later, as he was still in pain after realizing he asked for the wrong tooth to be pulled.
     
  16. BigBadLou

    BigBadLou En-Route

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    Sorry to hear.
    If she's flyable and you have time and avgas to waste, come on down to Nuevo Kalyfornya in the middle of TX and I'll pull out my trusty old multimeter to diagnose it for ya.
    (If your mechanic can't figure out a simple solenoid wire, how does he install things like magneto P-leads etc?)
     
  17. dmspilot

    dmspilot Pattern Altitude

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    Just had the identical problem happen on a Cessna 150. It hasn't been looked at yet.
     
  18. weirdjim

    weirdjim En-Route

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    That would be it.
     
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  19. mondtster

    mondtster Cleared for Takeoff

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    By chance does this airplane have a Plane Power voltage regulator installed? I haven't reviewed all the installation instructions but this exact problem will surface if you wire the new regulator in accordance with at least some of the application specific instructions. If you follow the generic wiring diagram you're fine.
     
  20. Norman

    Norman En-Route

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    What? Someone actually reads those things?
     
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  21. AKBill

    AKBill Line Up and Wait

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    This is the schematic I found for your C172N. I don't have an answer for you but I'm thinking about problem.. I'm thinking if you can't control the alternator with the ALT switch it's the Alternator Control Unit. Try taking the field wire off the alternator.


    upload_2017-2-17_16-42-49.png
     
    Last edited: Feb 17, 2017 at 8:18 PM
  22. Plano Pilot

    Plano Pilot Filing Flight Plan

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    My best guess is that your Master SW and relay are working correctly. For some reason when you turn off the Alternator SW the Alternator is not dropping off line and continuing to power the airplane. With the Master SW off and its relay open the battery, ie capacitor, is off line and that is why you hear the clicking noise in your head set.

    Find out how the Alternator Control Unit, ACU, is actually getting power. If it is wired according to the wiring diagram above pulling the ALT Field CB should kill the power to the ACU thus turning off the ALT.
     
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  23. AKBill

    AKBill Line Up and Wait

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    I had the same thoughts and felt a positive way to determine if it's the ACU would be to pull the field wire. Anyway you look at it if the master is off and the alternator can not be controlled by the ALT switch it's the ACU that is not correct or bad. My concerns are if you pull the breaker for the ALT and it's outputting power, could you damage the alternator? I don't think so but just a precaution in my mind to pull field wire first.
     
  24. OkieAviator

    OkieAviator Cleared for Takeoff

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    Thanks for the responses. Just got back from a work trip, will make my way out to the hangar at some point in the next week.
     
  25. Plano Pilot

    Plano Pilot Filing Flight Plan

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    FYI - I said to pull the "ALT Field CB". I agree with you pulling the resetting the ALT CB has a high chance of blowing the diodes in the ALT.

    Does this 172 even have the CB's that you can pull? It has been a long time since I was in a 172.
     
  26. hotprops

    hotprops Pre-takeoff checklist

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    well when you pay the bill i would tell the repair person you will pay for one hour labor as that is more than any trained a and p would take to find this problem.plus pay only for the part that fixed the issue. can you fly it to a real tech?
     
  27. weirdjim

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  28. Plano Pilot

    Plano Pilot Filing Flight Plan

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    It is more of a problem on boats. Here is a good description of what happens.

    https://www.jamestowndistributors.com/userportal/document.do?docId=100

    Alternator Field Disconnect

    [​IMG]
    Technical article from Blue Sea Systems

    An alternator field disconnect is used to protect the diodes in an alternator when the battery switch is inadvertently turned to the "OFF" position. Essentially it turns off the alternator output. Below is a description of the operation of an alternator and how the AFD works.

    An alternator is basically an AC generator powered by the engine. It consists of electromagnets mounted on a rotor spinning inside windings of wire, called a stator. A relatively small field current is supplied to the rotor, powering the electromagnets to create a magnetic field. When the rotor is spinning and a field current is supplied to the rotor the alternator generates AC current.

    There are typically three separate windings of wire in the stator positioned so the AC current generated for each winding is out of phase with the others. This allows a smoother, more continuous output.

    To change AC current into DC current, diodes are used as a rectifier. A diode will only allow current to flow in one direction, essentially a one way valve, allowing AC current to become DC current. The regulator is the device which supplies the field current and controls the power output of the alternator. An external regulator is mounted close to the alternator in the engine compartment. An internal regulator is contained within the alternator housing.

    There are typically four connections on the alternator, an output terminal to the battery, the ground terminal, the field connection, and a separate connection to the stator.

    If a battery switch is turned off while the alternator is producing current, the voltage will increase due to the sudden elimination of the load. This will burn the diodes out in the rectifier quickly. To prevent this, a battery switch with an alternator field disconnect (AFD) can be used. The AFD is a secondary, isolated, single pole switch within the battery switch, through which the alternator field current source wire from the voltage regulator is wired.

    The AFD is constructed in such a way that the AFD switch does not close until slightly after the main switch contacts have closed and it opens slightly before the main switch contacts open. This insures that there will always be a path for the alternator's output current when the alternator is producing power. The field disconnect will only work on alternators with an external regulator. The diagram below illustrates the connections. Blue Sea Systems battery switches 9002 and 9004 have an AFD.

    [​IMG]

    Original article from Blue Sea Systems
     
  29. OkieAviator

    OkieAviator Cleared for Takeoff

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    Went to fly today and now the alternator isn't charging. So I'll have to reengage someone to take a look at it, there's a new shop on field maybe they can check it out. Also there's no Alt Field CB to pull, it can get tripped but not pulled on or off.
     
  30. weirdjim

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    Completely different way of doing things. Please don't post off-topic stuff like this.

    Thanks,

    Jim
     
  31. AKBill

    AKBill Line Up and Wait

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    By bad I miss read your post. I don't think Cessna's have breakers you can pull.

    Personally I am in the process of changing out the breakers in my Beech with those you can pull to disconnect. PITA I must say
     
  32. Plano Pilot

    Plano Pilot Filing Flight Plan

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    WOW. Sorry!
     
  33. Palmpilot

    Palmpilot Touchdown! Greaser!

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    Having flown a lot of Cessnas, my recollection is that a typical panel will have a small number of pullable breakers among the non-pullable ones.
     
  34. SCCutler

    SCCutler Administrator Management Council Member PoA Supporter

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    By this time, most of the fleet has been modded and ad-hoc repaired so many times, you just cannot predict what is and is not "typical."
     
  35. Palmpilot

    Palmpilot Touchdown! Greaser!

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    I was talking about what I've seen among the numerous rental planes I've flown, not making a prediction. (I could be mis-remembering about the "typical" part, of course.)
     
  36. Plano Pilot

    Plano Pilot Filing Flight Plan

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    I bet it is! I have done it on some boats and it is not fun but worth the time and expense.
     
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