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Electrical Gremlins

asw20c

Well Known Member
Hi All,
I have an electrical problem with my Cessna 180, and although it isn't RV-related, I was hoping the collective here might be able to shed some light, which might also be helpful to the RV community.
Upon startup I have an immediate discharge, proportional to the equipment that is powered on. The discharge can range between -1 to -4 amps, or up to -12 or more amps depending on whether the panel master is turned on.
This discharge continues through taxi and run-up, but as soon as I start my takeoff roll the electrical system begins behaving normally and I get positive charge. It spikes up as one might expect to charge the battery that I had been running off of, and then tapers down after the battery is topped off again. Again, normal behavior. I have discovered by cycling the alternator master that the alternator is completely out of the circuit between startup and the takeoff roll, but as soon as I'm wheels up it kicks in and behaves normally for the remainder of the flight. If I start up again just after shutdown, it still behaves normally. It doesn't begin the faulty/discharge behavior again unless I let the plane sit overnight.
I know the problem isn't my voltage regulator, because I replaced it thinking it was the problem, but I am still experiencing the same symptoms. There is something that changes when going to full power, and/or a fully warmed engine.
There are three major components in the electrical circuit; the battery, the alternator, and the voltage regulator. The voltage regulator can't be the problem because I just replaced it. The battery appears to be working properly based on what I can see on my JPI 930. And the alternator also works as one would expect, once it kicks in on the takeoff roll. I'm completely baffled by this behavior and was hoping someone might have some experience with a similar problem that they can share.
Thanks,
 
Alternator

It could be:

1. Belt slipping on the alternator -- as the flywheel heats up, it expands and results in less/no belt slippage.

2. Alternator field &/or wiring issues to alternator. Bad terminations/crimps? Alt Field Switch flakey?

3. Other...
 
If your airplane is an early model and still has the original generator, this indication is completely normal. Generators don't work well at low RPMs. Sorry, I just reread your original post and see that you have an alternator.
 
Thought I would post a follow-up now that we have figured out the problem. I originally thought this was a bad voltage regulator but replacing it had no effect. What we ended up doing, and which should have been one of the first troubleshooting steps, was to pull the alternator and put it on a test stand. Turns out there was a bad diode. I'm still baffled by how this problem only manifested when the engine was cold, but worked fine when fully warmed up, but apparently the diode was the culprit. So, a 50 cent diode goes bad and I get to buy a $600 new rebuilt alternator. This is an expensive sport.
 
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