Nav lights

cessna1502978

cessna1502978

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Feb 11, 2019
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Butler2978
Hi all I have a 1967 150G I keep blowing the fuse to the Nav lights. I have had the inspection covers off inspected and even jiggled the wires and I can’t make them blow the fuse,but sure enough I was out tonight and the fuse blew. Any help where I should look would be appreciated. Like I said I can not find a short but something is going on. Should I throw in a 50 amp and look for smoke just kidding
 
cessna1502978 said:
I keep blowing the fuse to the Nav lights.
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Aircraft S/N? I assume the fuse blows in flight? Or running on the ground? Both? And for more troubleshooting use a piece of 8 awg solid copper wire vs a 50amp fuse--the fire starts a lot quicker. ;)
 
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Fuse or breaker?

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It is a 10 amp fuse. No corrosion thinking maybe switch? I have played with it can’t make it blow. Happens in flight
 
What is the voltage indicating? At 14v the nav lights draw 7.5A. If your voltage drops, it may be enough to blow the fuse.
I would also use an ohmmeter to check if there is a small current leak on any branch of the circuit.

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cessna1502978 said:
Serial# 15066878
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Does the fuse blow immediately when you turn the switch on? Or after a period of time?

This should be the diagram. You'll need some help from your mechanic, but look to isolate portions of the nav circuit per the diagram. Whatever point is easy to get to at first like one of the inline connectors (6/7; 12/11), until you find which portion of the circuit is hitting ground, i.e., shorting. OHM first and compare readings. Doubtful it's the switch. These are the fun problems :).
upload_2019-12-28_18-35-55.png
 
Thanks Bell206 for the diagram. It takes time to blow that is why I think there is a short somewhere. I no it is gonna take time I have been threw the wiring from one end to the other. It is very time consuming but looks like we are in for some weather tomorrow so I will get on it.
 
My guess would be corrosion in the wing break connector in the wing root. Also the light assembly itself. Open both up and clean the contact surfaces. Even just disconnecting and reconnecting may make the problem go away.
 
I am gonna look and will give updates thank you all for the advice.
 
Look at every place the wire is routed through a structural member. My Navion had an intermittant trip of the nav breaker. We found that every place the original cloth covered wire passed through the formers going to the tail the rubber grommet had deteriorated and it was binding on the metal. Replaced all that, but didn't help. Finally decided to just run new tefzel wirign to the wings as well. When we went to pull the wires from the wing we found that there were TWO going to the left wing. Gently tugging on one resulted in about three feet of disconnected wire being extracted from the wing. Looks like there may have been a piece of tape on the end, but otherwise it was just flapping loose.

The mechanic was perplexed as to what the short wire was but I knew. When you modify the Navion to have the ability to stop the flaps other than full up or full down (as it came originally), you're supposed to have a light in wing to illuminate the flap position (it's kind of useless the way it is mounted) at night. I knew I didn't have that light. Apparently, at some point in the past, I did.
 
mwagg737 said:
What is the voltage indicating? At 14v the nav lights draw 7.5A. If your voltage drops, it may be enough to blow the fuse.
I would also use an ohmmeter to check if there is a small current leak on any branch of the circuit.
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Malarkey. If the voltage drops, the current drops proportionally. Fellow by the name of Ohm told us this back in the 19th century.

Jim
 
Pull all the bulbs. Just for safety sake, pull the ground lead off of the battery. Put one of those multimeters with a closed circuit "beeper" on the hot lead coming out of the fuse. NOW start jiggling wires around and when the meter starts beeping you can do a physical examination to see where the wire is chafing against a metal structural member.

Forget corrosion. Forget tarnished switches. All this will do is prevent the lamp from lighting. Somewhere you've got a hot side wire making contact with the airframe.

Whoever said "bad bulb", why don't you come on here and explain how an incandescent bulb can draw more than double the rated current of the bulb in order to blow the fuse.

Jim
 
weirdjim said:
Malarkey. If the voltage drops, the current drops proportionally. Fellow by the name of Ohm told us this back in the 19th century.

Jim
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Yep... My bad.

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weirdjim said:
Malarkey. If the voltage drops, the current drops proportionally. Fellow by the name of Ohm told us this back in the 19th century.

Jim
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Come on, all that stuff that ohm guy said was just a theory! Never been proven.......
 
I found my problem in the tail light. It would pop the breaker every so often. Was a "wild hair" inside the bulb socket. Problem solved when I installed Tailbeacon. :D
 
Wild-Hair-For-Kids.jpg

Wild Hair!
 
weirdjim said:
Malarkey. If the voltage drops, the current drops proportionally. Fellow by the name of Ohm told us this back in the 19th century.

Jim
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Not quite! Ohm's law only applies when the resistance is constant. An incandescent bulb's resistance varies by around 10X from cold to normal operating. The current is given by (Va/Vd) ^0.55 where Va = actual voltage and Vd = design voltage:

https://en.wikipedia.org/wiki/Lamp_rerating

So, a bulb operating on 50% voltage will have 68% current, not 50%. However, the current DOES drop when the voltage drops, so it will not blow the fuse.

Note that if the "bulb" is actually an LED driven from a switching power supply, the current MAY increase when the voltage drops.

OP, you could try isolating the problem to the right, left, or tail circuits by selectively disconnecting them at the switch.
 
I had the same problem in a 1977 Skyhawk. The problem was resolved when I replaced the circuit breaker.
 
flyingbrit said:
Not quite! Ohm's law only applies when the resistance is constant. An incandescent bulb's resistance varies by around 10X from cold to normal operating. The current is given by (Va/Vd) ^0.55 where Va = actual voltage and Vd = design voltage:
Not to be pedantic, but I said PROPORTIONALLY, not linearly. And yes, Ohm's law DOES apply. At any given applied voltage the current will be the resultant of the HOT resistance of the bulb. And yes, it is a transcendental function.

Note that if the "bulb" is actually an LED driven from a switching power supply, the current MAY increase when the voltage drops.
The entire discussion, including the OP, was for incandescent bulbs. A switching power supply (properly designed) will simply stop working into a current in excess of design limits.

OP, you could try isolating the problem to the right, left, or tail circuits by selectively disconnecting them at the switch.
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Hell of a lot easier to pull the bulbs and wiggle the wires.

Jim

 
weirdjim said:
It is not a theory, it is a definition. Ohm simply said that resistance (by his definition) is applied voltage divided by the resultant current. jw
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I guess I should have added a emoji with that, or it just wasn't that funny..........
 
Ok folks so after a day of checking all Nav light wires found no dead shorts. What I did find was 2 bad ground connections on both wings. One of which needed to cut and re solder the connection. I am hoping to get her up soon will keep you all posted. Thanks again for all the input.
 
May be unrelated but, I had a plane with 3 landing
Lights, normal nose then each wingtip. The wingtip lights were add-ons of course, same circuit. My breaker would trip at times during use, routinely, I put 2 LED lights in which took the draw way down, no more tripping.

Usually as lights burn out or need replacing I look around to see what LED options are available. It may be another way to reduce the draw on that circuit. It’s been a long time for any electrical class for me, but my ‘workaround’ worked.
 
Its a caveman system. Hot wires are strung into and through the wings to the lights. The ground happens at the light fixture. Old wires crack, especially where flexed when draped over a rib. A little movement exposes a cracked wire to a ground? Pop goes the weasel.
 
Well so far so good went for a test flight still working
 
cessna1502978 said:
Ok folks so after a day of checking all Nav light wires found no dead shorts. What I did find was 2 bad ground connections on both wings. One of which needed to cut and re solder the connection. I am hoping to get her up soon will keep you all posted. Thanks again for all the input.
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I don't see any relationship between a "bad" ground and a blown fuse.


weirdjim said:
Forget corrosion. Forget tarnished switches. All this will do is prevent the lamp from lighting. Somewhere you've got a hot side wire making contact with the airframe.

Jim
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This is almost certainly the issue. Fuses blow because there is a connection between the hot wire and ground. Sure, there might be some sort of esoteric reason, but I'm going with the most probable one.
 
Not necessarily a bad ground on a lighting circuit or any circuit will produce more amp draw and heat over time in use. Have you not grabbed a battery cable that had a bad connection can get pretty dam hot. So far seemed to be my problem. Resolved week grounds and sand corroded sockets.
 
Bad grounds or bad connections in general will usually decrease the amps because resistance increases and I = V/R (Ohm's Law). The heat noticed with a bad battery cable connection is caused by the bad connection acting like a resistor and dissipating power, not increased amps.
I'm guessing your work on the grounds and sockets, while worthwhile in itself, shifted the cables enough to move the worn wire insulation away from the airframe metal. Perhaps you will be lucky, but I expect the problem will return in a few weeks or months.
 
cessna1502978 said:
Not necessarily a bad ground on a lighting circuit or any circuit will produce more amp draw and heat over time in use. Have you not grabbed a battery cable that had a bad connection can get pretty dam hot. So far seemed to be my problem. Resolved week grounds and sand corroded sockets.
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You came in here asking for answers. i know most of the folks that tried to help you and if you added up all the years of experience they have you'd probably come up with a pretty fat logbook. Try not to come to a gunfight with a knife. In the first place, what you write is just plain not true. Second of all, your writing exposes your skill level. Take the help or not, your call, but try not to tell the experts what they have spent a lifetime learning.

And for the love of the holy St. Orville, learn how to spell.

Jim

 
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Okay fellas update been almost a month and 5 hours of flying and all is good with the lights so yes poor grounding was the issue. They would of only lasted 1 flight. Thanks all for your input
 
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