Another annoying, mysterious electrical problem

Mantoga

Mantoga

Filing Flight Plan
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Mar 20, 2018
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Mantoga
Hi all,
I actually posted this on an old thread and have gotten a few good responses, but hey, the more the merrier so I’ve copied below the issue with my 182..

Sometimes when I turn on the master switch everything starts to spool up but even turning on a landing light shuts everything down. Sometimes when I turn on the master switch all I hear is a click at the battery which must be the master solenoid because it is hooked to the side of the battery box. When I jump the two main poles of the master solenoid which is affixed to the side of the battery box everything spools up, works, of course regardless of the master switch position. The master solenoid was replaced less than a year ago and of course did not fix the problem, it obviously must work sometime but takes almost nothing to kick it off/open.
 
I would suspect the master switch or its wiring and connections. When you jump the solenoid it bypasses that circuit.

It appears the switch provides just enough voltage and current to the solenoid to operate it, but when loads are applied, the weak magnetic field in the solenoid coil cannot hold it engaged.

This diagram is not exactly correct for your plane but the master switch circuit is pretty straightforward. Look at the top left corner. The two pole master switch provides a ground to the solenoid coil and also switches on the generator circuit.

cessna-172-wiring-diagram-of-cessna-172-alternator-wiring-diagram-2.jpg
 
kgruber said:
Cessna master switches are of poor quality.
Click to expand...

In 22 years of fixing airplanes full-time, I have replaced maybe one or two master switches. I have replaced many solenoids.

That's the rocker-type master switch. Old Cessnas (early 60s and before) had a pull-type switch that truly is troublesome.
 
My SWAG, power so marginal for some reason, that sometimes not enough, sometimes enough but, added load from turning something on drags voltage down? Maybe a relay sticks in the wrong position after shutdown. Do you watch your amp meter through preflight and start up? Make sense?

You could try opening all the breakers before turning on the master, then close them one at a time to see if it made a difference. Just to eliminate possibilities.
 
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I had a 182 come in with a similar problem. It would shut down intermittently when we engaged the starter. The master relay, master switch, starter solenoid, starter, and starter switch were all replaced in that order. I eventually took another look at the master relay. Turns out we got a bad one. Replaced it and haven't had a problem since. When it fails, do you hear the relay click off? If not, check the power output. On our problem child, the relay clicked on and stayed that way. It just couldn't carry any current.
 
RPaulson said:
When it fails, do you hear the relay click off?
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I had the exact same question when I read the first symptom.
We need more information to help troubleshoot. But ultimately, you should use a smart mechanic (or a smart electrician) before throwing money at it. It seems like you can reproduce it and if so, it should be easy to diagnose properly.
As a point in case: I preside our local auto club and as an electrician, you have no idea how many times I've seen people waste hundreds of dollars on relays, batteries, starters etc, just to come to me for help and finding a simple thing such as a bad contact.

Let us know what you find! And if you don't find it, post more symptoms and troubleshooting, we'll try to help (as much as we can over them Ynterwebs)
 
BigBadLou said:
As a point in case: I preside our local auto club and as an electrician, you have no idea how many times I've seen people waste hundreds of dollars on relays, batteries, starters etc, just to come to me for help and finding a simple thing such as a bad contact.
Click to expand...

I always find it amusing when I see cars at a show that have hugely expensive engines in them and the electrical wiring is draped around the perimeter of the engine compartment like multiple Christmas garlands.

They even have lots of colored insulated crimp connectors acting as ornaments.

:D
 
weirdjim said:
The common term is "shotgunning the problem".

jw
Click to expand...

Jim, that saying reminded me of TTL days, working on UPS systems in data centers. Shotgunning a board was the only hope some of the people in the local trade had of repairing it.

They didn't have the ability to find a dried out cap or identify a bad logic gate on an IC.
 
3393RP said:
Jim, that saying reminded me of TTL days, working on UPS systems in data centers. Shotgunning a board was the only hope some of the people in the local trade had of repairing it.

They didn't have the ability to find a dried out cap or identify a bad logic gate on an IC.
Click to expand...

Now you just throw out the whole
board. A new one will be here from China tomorrow for $10, FedEx even. If you can wait two weeks, $1. :)
 
An Exide 3000 series UPS had 11 control boards, and the cost for a replacement from Exide was around $1,800.

When a machine I had under contract had a board failure, I would plug in my board extender and diagnose the problem. I'd take the bad board back to the shop, spend $12 on an IC chip to fix it and resell it the next time around.
 
Dirty battery terminals, bad connections, bad ground somewhere. Start simple, with a volt meter, look for drops when you load up.
 
ATTENTION ON DECK!
It was the master solenoid...

Thanks for all the help guys
 
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