Alternator Question

Bryan Wood · #1 02-10-2007, 10:48 AM

A friend with a 1978 172 has had 2 alternators fail. He has had the plane about 2 years and it has happened again. While talking about it he mentioned that part of his checklist during run up is to turn off the alt switch to see if the annunciator lights on the panel. Then switching it on the light goes off. Something about this seems like it would blow the diodes in the alternator by always bringing it back on line with the panel all lit up and everything running. What are you thoughts? He says that the POH has this in the checklist. It's been so long since I've flown a spam can I can't remember for sure but don't remember doing anything like this on my Cherokee. Anyway, I told him that I'd ask this here on the RV group since there is such a broad knowledge base to tap into.

Thanks,

osxuser · #2 02-10-2007, 06:55 PM

Thats normal procedures at the flight school I work at, and it doesn't seem to have an adverse affect, I'd be checking out the regulator or wireing, could be something it CAUSING the failures. It also could be that he is using Kelly Aerospace rebuilds and the just suck.

Bryan Wood · #3 02-10-2007, 07:07 PM

Thanks Stephan, we found the problem today. The shielded wire between the alternator and the regulator was rubbing against a clamp and wore thru. It looks like he replaced an alternator a couple of months ago for nothing.

Best,

Geico266 · #4 02-10-2007, 07:59 PM

I just happen to have a 1977 Cessna 172 POH in my flight bag for stayovers in hotels when I can't sleep well. Anyway, I looked up the Preflight check list on Page 4-7, Paragraph #2, Item #8C = "Engine Instrucment and Ammeter --- CHECK"

Doesn't mention anything about cycling the switch, just says to look at the ameter and see if the alternator is charging. Could be the 1978 had an idiot light?

Okay, I'm going back to sleep now, this has been my contribution to the aviation world for the day.