Electrical Problem C172N

Electrical Problem C172N

I have a 1979 C172N that has been "acting up" electrically lately.

1. DEC 04 - Lost all electrical power during IFR flight (luckily I had my handheld).  Mechanic could not repoduce failure. Replaced master relay as most likely cause.

2. Feb 05 - Turned on master and no power. Cycled master numerous times - no go.  Mechanic again could not reproduce failure the next day (seemed to work fine). He replaced the master switch.

3. Apr 05 - Turned on master and no power. Cycled master numerous times - no go. Went back to hanger 8 hours later and turned on master - everything seems fine.



Help!


tom.

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Re: Electrical Problem C172N


And...

The plane had been flown 1-2 times between the electrical inops.

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Re: Electrical Problem C172N

I've found many approaches out there as to how to do troubleshooting.  The initial approach is usually, very little diagnostics and just replace the usual suspects.  Sounds like your problem does not lend itself to the usual approach. That's why your asking.  Leave it to me to restate the obvious!  Here's some questions that need aswering. Others may have more, this is just to get you started.

When you lost electrical, did the alternator output or field breaker breaker trip?

What did you have operating (switched on) at the time of the failures?

What were the weather conditions?

Have you reviewed the sequence that you use before turning the master on verus the sequence the your mechanic uses?  If you go by a check list before turning the master on and he doesn't, that might be a clue.

Do you have an external power plug for your battery?  there is a circuitry associated with them that bypasses most of the electrical in the plane.  Is there an intermmitent failure mode in it that could cause this?

You obviously have been sitting in the airplane when you get this failure, is the mechanic sitting in it or was he standing next to it?  I was thinking of some kind of stress related failure with some connection?

Well this is just food for thought.. let us know what you find out.



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Re: Electrical Problem C172N

Lets see now...replaced master switch and relay. Seems to me what remains to be checked is the continuity of the wire in between. I would check for loose or poorly crimped lugs at each end and any damage to the wires overall.

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Re: Electrical Problem C172N



Ans to some of Barry's questions:

When you lost electrical, did the alternator output or field breaker breaker trip?

No.


Q  What did you have operating (switched on) at the time of the failures?

A  First time, in flight everything except lights.  Afterward failure occurs at the start of pre-flight while standing outside the plane with nothing on.


Q  What were the weather conditions?

A  1st time -5C -SN in flight, after that it hass occured in the morning when it is about 5C in the hanger.

Q  Have you reviewed the sequence that you use before turning the master on verus the sequence the your mechanic uses? If you go by a check list before turning the master on and he doesn't, that might be a clue.



Q  Do you have an external power plug for your battery? there is a circuitry associated with them that bypasses most of the electrical in the plane. Is there an intermmitent failure mode in it that could cause this?

A  No.

Q  You obviously have been sitting in the airplane when you get this failure, is the mechanic sitting in it or was he standing next to it? I was thinking of some kind of stress related failure with some connection?

A  Both. See above.

Well this is just food for thought.. let us know what you find out.

A thanks, I appreciate the help.

After flying last night and having it work OK I went back this AM to check and found the master does  not work again.  Wonder if it is broken wire or poor connection that contracts in cold and loses continuity...  (maybe like message from Rich...)


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Re: Electrical Problem C172N



When master does not work, you do not hear familiar "clunk" from solenoid (?) in relay.  Just nothing when flip switch.

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Re: Electrical Problem C172N

Tom, you might try this. Turn the master switch on and gently flex the wires terminated at the switch and also the relay.

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Re: Electrical Problem C172N

"When you lost electrical, did the alternator output or field breaker breaker trip?

No."

Conclusion: So we can rule out the alternator and battery because I am assuming you lost electrical all of a sudden and not slowly.

"Q What did you have operating (switched on) at the time of the failures?

A First time, in flight everything except lights. Afterward failure occurs at the start of pre-flight while standing outside the plane with nothing on. "

Conclusion: it is unlikely that your operating with a lot of current through a particular connection that's expanding due to high resistance causing it to heatup; just the electric Gyro(s) and beacon (typically left on).


"Q What were the weather conditions?

A 1st time -5C -SN in flight, after that it hass occured in the morning when it is about 5C in the hanger. "

Conclusion: Nothing unusual about moisture conditions.

"Q You obviously have been sitting in the airplane when you get this failure, is the mechanic sitting in it or was he standing next to it? I was thinking of some kind of stress related failure with some connection?

A Both. See above."

Conclusion: Don't think there's anything furhter here.

"Q Have you reviewed the sequence that you use before turning the master on verus the sequence the your mechanic uses? If you go by a check list before turning the master on and he doesn't, that might be a clue. "


Conclusion: Have your mechanic start your aiplane when you want to fly and make sure he is in the seat next to you so you have no more failures! (Just kidding)


Was the key in or out of the ignition switch when you were on the ground?  Just wondering if anything is bypassed by the ignition switch when you start the plane to protect against surges.  And could there be something wrong with that?

Are there any thermal overloads in the circuit? 

Other than that it sounds like a continuity problem. 

Rich has a good idea.  Also you may just need to do continuity checks with an ohm meter and isolate the portion of the circuit involved.  Do you have a wiring diagram so you can formulate a plan?

How old is the alternator output cirbuit breaker?  I'm thinking about continuity problems at the breaker?

Good luck

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Re: Electrical Problem C172N

Had yopu checked the connections and terminals at the battery to be clean and tight along with them not broken at the battery itself. Also check to see that the connections at the solenoid are also clean and tight. Also make sure that the battery is in a good charged condition. You may want to try another battery as a jumper when it fails to work.

Good Luck
Bill


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Re: Electrical Problem C172N

Tom, I agree with Rich, check your wires thouroughly.  Sounds like intermittent open circuit affecting master relay.  I have seen where a diode rendered a starter relay totaly inoperative, removed diode and starter relay worked fine.  Does not make sense at times, but with new diode on starter relay no problem for over a year.

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Re: Electrical Problem C172N



Having mechanic replace the diode.  this is part of 1993 AD that has now (it seems) caused further problems!!   Took diode out of circuit until part arrives  -no problems thus far.

Also tried the advice in other notes (connections, battery, etc) Thanks for the help!!!

I'll follow-up once we know for certain.  It seems like the failure would occur more in the AM when it was cool.  Will test old diode after cooling.


tom.

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Re: Electrical Problem C172N


Now it looks like it was not the diode.

Looks like it was the master relay.  This was replaced after the first failure in Dec 04, but the Cessna replacement relay was also bad.  It would not work, especially after a cold night when tried in the morning.  As the day warmed up, it seemed to work. This was also observed with the master left in the on position while inop in the morning, and when the AC warmed up the master relay kicked in and the power came on.

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