Master switch issue when engine running

OkieAviator

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OkieAviator
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?
 
bad ground on solenoid switch to firewall edit to add word switch
 
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bad ground on solenoid to firewall

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

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

Already had that conversation with him.
 
Already had that conversation with him.
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
 
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.
 
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.
 
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.

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

What? Someone actually reads those things?
 
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
 
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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.
 
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.

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.
 
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.
 
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.

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.
 
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?
 
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.

Could you explain how this happens?

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

An alligator cliplead to ground would work quite well.
 

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


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.

100a.jpg


Original article from Blue Sea Systems
 
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.
 
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.

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
 
...I don't think Cessna's have breakers you can pull....
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.
 
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.

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."
 
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."
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.)
 
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

I bet it is! I have done it on some boats and it is not fun but worth the time and expense.
 
And the saga is finally over. What I assumed would be a simple fix turned into a mess. Last I left it the 3rd A&P to work on the plane over the last 3 years was coming over in attempt to return the plane back to post annual, non master turning off form. He wasn't able to get it fixed and essentially fired himself said he wasn't going to charge me anything and gave me contact who happened to be out at my field.

The new 4th A&P not only has a mobile operation going he recently took over the big hangars on my field from the old A&P #2 MX facility. Well A&P #4 found errors with the way A&P #1 had the OV Regulator connected which may have been the original issue and just not noticed until recently. After that was fixed now the battery was intermitantly chanrging and then just stopped. I got the plane going full RPMs around the pattern and after about 5 mins it quit charging completely. Landed and went and talked with him again. He wanted to do some more troubleshooting and swap some connections, came back and said the new Volt regulator A&P #3 put in is not functioning and probably broke due to being connected wrong (Solid State). A&P #4 went to A&P #3 to pick up the old Volt Regulator and installed that.

Now all is working well, went flying for about an hour this morning with no issues. Yah for progress!!!
 
Wow. That was an ordeal. Glad you found someone who knew what they were doing on the fourth try! LOL.
 
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