All (especially those with electrical system knowledge), During a recent preflight, I discovered a ground wire that broke at the crimp ring terminal connecting it to the 60 amp Cessna/Ford alternator on my O-200 powered Ercoupe. There was not enough slack in the wire to strip and attach a new crimp terminal, so today I worked with my mechanic to pull and replace the whole wire. It was indeed a ground wire connected to the airframe next to the voltage regulator behind the seat, but we discovered it was incorrectly connected to the stator terminal, not the ground terminal, on the alternator! My O-200 STC drawings show that the stator should not be connected to anything. We also confirmed this with two other sources. So, we replaced the bad wire, connected it to the correct ground terminal on the alternator (leaving the stator unconnected per the drawings), attached a voltmeter across the battery, and ran the engine up while testing all of the electronics. Everything worked and read as expected.
So, my question is, how is it possible that the system has been working all this time with the stator connected to the aircraft ground? Isn't there an A/C voltage on the stator terminal? Is it possible that the voltage regulator, with its ground tied to an A/C voltage, was alternating the field voltage accordingly to produce a corresponding wave on the rectified alternator output such that the potential between the two waves was constant? What about the negative half of the wave? Or, with the stator connected to the alternator ground through the airframe, was something happening within the alternator circuitry that prevented strange behavior or disaster? --Lance N2273H
