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I had the same problem with my Cessna and was cured by cleaning every
contact and replacing the Cessna with a more robust military switch. David Lesher wrote: "DoodyButch" writes: There is a usual problem that causes this on Cessnas. Cessna solid state voltage regulators on pre-1980 alternator equipped Cessnas had three wires. One for the aircraft bus, one for the alternator field and one for ground. THIS IS THE PROBLEM: The "Aircraft bus" lead on the regulator serves both as the voltage sense lead and as the power lead to ultimately provide power to the alternator field. When energized, the field draws about 2 amps. ..... This is obviously a big design defect. In about 1980, Cessna apparently realized this and changed to a voltage regulator with four leads, the extra one being a separate "sense" lead connected directly to the aircraft bus so that the alternator field did not draw current through it. This is one of those "damned if you do, damned if you don't" issues. Many auto regulators also draw current down the sense lead to run things and here is why. You're zipping along, and the sense lead falls off. "Ooops, Zero volts!" says Mr. Regulator... "Full Power, Scotty, the Romulans are headed this way.." and it raises the field current to try to get back to 14.4 vdc on the sense lead. No good, it's still zero. Of course, meanwhile your avionics stack and battery are in rigor mortis as the main bus voltage is not zero, but 30 or 40 or higher volts. Did I mention the landing light? Or is it a landing {in the} dark? If you run the field supply off the sense lead, and it falls off, then the alternator produces no output. Not great if in heavy IFR, but a better deal then in the same situation with NO working radios/navs. There is another aspect. Ma Bell figured out eons ago that copper-copper joints, say at binding posts/splices etc. develop intermittent connections. [Little whiskers grow between 'em..] They perfected "sealing current" -- run a small DC current and it cures the issue. So you want to draw some current down the sense lead anyhow.. -- A host is a host from coast to & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 -- Regards, Ross C-172F 180HP |
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