Cessna 172L Electrical problems

T

Tate Preece

Filing Flight Plan
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Tate Preece
Hey guys, I recently went on a long cross country flight and when I landed I noticed my hour meter (Hobbs) had only rolled over 1.1 hours on about a 3 hour flight. After returning home I assumed the hour meter was bad and pulled it out. Yes I know I can’t install as new one since I’m not a mechanic but I always have someone look over it and sign the books. Nevertheless, I hooked it up to a battery and it worked just fine. Now I assume the oil pressure switch has gone bad, I tested the voltage and the switch only to find .5V. I couldn’t believe the reading since this is a hot circuit always tied to the battery. I checked the fuse mounted on the firewall and it was fine and was getting 13V so that wasnt the problem. The only other thing on the circuit was the old Cessna clock which looking back I did mess with since I got bored on the long flight. After test the voltage on it, it also only read .5V. Does anybody have any suggestions on what to test or what might be wrong?
 
So you have 13v at the fuse and only .5 entering the switch?
 
Tate Preece said:
Hey guys, I recently went on a long cross country flight and when I landed I noticed my hour meter (Hobbs) had only rolled over 1.1 hours on about a 3 hour flight. After returning home I assumed the hour meter was bad and pulled it out. Yes I know I can’t install as new one since I’m not a mechanic but I always have someone look over it and sign the books. Nevertheless, I hooked it up to a battery and it worked just fine. Now I assume the oil pressure switch has gone bad, I tested the voltage and the switch only to find .5V. I couldn’t believe the reading since this is a hot circuit always tied to the battery. I checked the fuse mounted on the firewall and it was fine and was getting 13V so that wasnt the problem. The only other thing on the circuit was the old Cessna clock which looking back I did mess with since I got bored on the long flight. After test the voltage on it, it also only read .5V. Does anybody have any suggestions on what to test or what might be wrong?
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Bad fuse? Broken wire? Time to ohm it out.
 
Tate Preece said:
Al

Already tested the fuse, no problem with it. How would you suggest testing the wire
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Disconnect the wire at both ends and ohm it out. Or check one side to ground. But at that point I would figure a fuse would blow.

Maybe the clock or the pressure switch is dragging it down? Disconnect the clock and try voltage at the switch again and vice versa.

This sounds like problems we get on the simulators but that usually ties back to bad a DO card in a computer. And a real plane obviously don't have that. Fun stuff
 
Also the switch may be on the low (aka ground) side of the hobbs. I've seen that a few times in 172 if it wasn't a factory install.
 
A 172L is an old airplane and is going to have a lot of old wiring, which means corrosion at numerous points. The fuseholder's contacts develop a resistive oxide on them. The fuse itself can be intermittent, since the fuse strip inside the glass is merely trapped between the caps and the glass' outer wall. Every crimp terminal develops resistive oxides between the crimp barrel and the wire. There are plenty of places to check, and you have to be careful to check those voltages under some loading, which means with the Hobbs connected (if it has a motor in it and not a spring-winding solenoid) so that there is constant current flow. You can get a 12V indication at an unloaded wire end even with a high resistance somewhere along the line, since the voltmeter draws such a tiny amount of current to make its measurement. Ohm's Law applies there, you see.
 
Dan Thomas said:
and you have to be careful to check those voltages under some loading, which means with the Hobbs connected (if it has a motor in it and not a spring-winding solenoid) so that there is constant current flow. You can get a 12V indication at an unloaded wire end even with a high resistance somewhere along the line, since the voltmeter draws such a tiny amount of current to make its measurement. Ohm's Law applies there, you see.
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Very true. A simple and cheap way to test wires under load is to use a 12V test light like this:

https://www.searsoutlet.com//d/prod...MIpNjZw6C23gIVEr7ACh089gmOEAQYBSABEgLZJvD_BwE
 
You are going to have to be a little bit clearer, Tate, if you want any help. You say you "tested the voltage" on the oil pressure switch. That means nothing. You also said you "test the voltage on it" (meaning the clock). Other than the lapse in English between "test" and "tested", that also makes no sense. Take your time, spend a few more words, and tell us what you are testing and how you are doing it.

Thanks,

Jim
 
Clip4 said:
You have a bad lead or ground.
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Yeah, that's it. A bad ground. Move it to another place on the airport where there is better ground. Or maybe a loose wire or a bad connection or another problem.

If this is the only contribution you can make to the problem, I'm sorry for you.

Jim
 
weirdjim said:
Yeah, that's it. A bad ground. Move it to another place on the airport where there is better ground. Or maybe a loose wire or a bad connection or another problem.

If this is the only contribution you can make to the problem, I'm sorry for you.

Jim
Click to expand...

If you think moving the plane solve the problem.....
 
weirdjim said:
Yeah, that's it. A bad ground. Move it to another place on the airport where there is better ground. Or maybe a loose wire or a bad connection or another problem.

If this is the only contribution you can make to the problem, I'm sorry for you.

Jim
Click to expand...

That is why we need to get a Mars mission going. Pretty soon all of the ground on Earth will be bad and everything will quit working.
 
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