Question re Troubleshooting Cessna Alternator

eetrojan

eetrojan

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I'm reading about electrical malfunctions on pages 3-22 to 3-23 in the POH for a Cessna 172R.

For an "insufficient rate of charge" (ammeters shows discharge, VOLTS lit), it says to recycle the alternator by turning the avionics master off and cycling both sides of the master switch.

For an "excessive rate of charge" showing on the ammeter after 30 minutes of cruise flight, it says to turn off the alternator and non-essential equipment, and then land as soon as possible. It doesn't suggest recycling.

Wouldn't it also be good to recycle the alternator given an excessive rate of charge and see if that fixes it? If not, why?

Thanks!

Cessna_172R_POH_Electrical.jpg
 
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The only time I've had an excessive charge rate was in a Piper Dakota on a short, local flight. In the end, it was due to a bad battery with a bad cell so the alternator would try to charge, and charge, and charge.

We reduced the electrical load as much as possible, but the ammeter was still pegged so we shut everything off and landed because of the potential fire hazard. Recycling the alternator did nothing in that case.

I can't think of another case of an overly excessive charge rate. The recycle on the reduced charge rate is just to see if it gets going again. With an excessive charge rate, the alternator is working fine.

It might not be the type of response you were looking for, but it's just my 2 cents.

If I am wrong on anything, take it easy on me.
 
register@teamandras.com said:
...Wouldn't it also be good to recycle the alternator given an excessive rate of charge and see if that fixes it? If not, why?...
Click to expand...

The reset is for the over-voltage trip and has nothing to do with excessive charge rate.
 
Excessive charge will usually be due to a bad regulator. The old electromechanical units would sometimes weld their regulator contact points shut (they're making a small arc constantly) and the field would get too much current. Modern electronic regulators are less susceptible to that but they can still malfunction. Transistors can short closed and no longer regulate. Cycling either type won't fix anything.

Dan
 
Because the reset has nothing to do with high amp charging. If the alternator is still putting out high current after the battery should be replenished, and you aren't using a bunch of high draw gear, it means you have excess heat building somewhere which can lead to a wire fire or an exploded battery.
 
Henning said:
Because the reset has nothing to do with high amp charging. If the alternator is still putting out high current after the battery should be replenished, and you aren't using a bunch of high draw gear, it means you have excess heat building somewhere which can lead to a wire fire or an exploded battery.
Click to expand...

Thanks all. My takeaway from your comments is that an overcharge situation means that the alternator is working just fine but, due to a defective battery, component, or owing to the regulator aka "alternator control unit", it's pumping out more current than normal and that current is going somewhere, potentially being dissipated as heat which may lead to fire. So, best course of action is shut the alternator down. No need to recycle it.
 
register@teamandras.com said:
Thanks all. My takeaway from your comments is that an overcharge situation means that the alternator is working just fine but, due to a defective battery, component, or owing to the regulator aka "alternator control unit", it's pumping out more current than normal and that current is going somewhere, potentially being dissipated as heat which may lead to fire. So, best course of action is shut the alternator down. No need to recycle it.
Click to expand...

You have it correct.
 
Henning said:
Works with airplane radios too, only you beat them on the face.:rofl:
Click to expand...

And it sort of works on panel lighting.

Had half the panel lighting (the important half, of course) blow out 10 miles out the other night.

But that's why we have flashlights.
 
Electrical problems can be insidious. What seems like a minor problem can turn into a major emergency. Just look at the NASCAR 310 crash which helped lead to recommendations from the NTSB and the FAA on dealing with things such as popped circuit breakers.
I use to fly a well maintained 310 for a flight school that still had generators. Minor electrical problems kept coming up. Dead batteries. Blown generator. Popped circuit breakers. None of them seemed related. Everything came to a head one day when I had a student and, upon gear retraction, everything went dead and I detected a burning smell. After doing a manual gear extension I came back and landed. The problem was traced to a faulty generator control unit (the original 45 year old part), which is located under the front right seat. The mechanic showed it to me- it was melted. I'm not sure how it did not actually catch fire. The owner finally bit the bullet and purchased alternators.
 
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