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Let me just start by thanking everyone for assisting me in trouble shooting this issue. The recommendations have been spot on and it's been a great learning experience. As a former Air Force avionics tech who worked on F105's and F4 Phantoms many years ago, to becoming an Army Warrant Officer flying and working on all kinds of aircraft both fixed wing and Rotorcraft, to now Captaining large Boeing aircraft for a major Airline, I can honestly say; I am humbled by the simple charging system of my 68' T337. To own an aircraft for as many years and not truly understand the heart and soul of the charging system is somewhat embarrassing. But when it works great and kinda lasts a long time, that's a testament to Cessna and the engineers back then with their slide rules.
Ok, tonight we made some testing progress but are still not getting any charging from either Alt on either VR. Based on the arrangement of my Turbo to include extra hoses for an aftermarket oil screen system, the ability to balance a wrench on top of either alternator was not fesible. We started back at the main bus and the Alt Field CB. The main bus where this CB is mounted was damaged and the screw whole was cracked through on one side. We had noticed this before and cleaned up the bus and used a washer to better support the CB. Today we removed the original CB and bench checked it. It seemed solid until light vibrations or jarring occurred, then some resistance was noted. I purchased a new 5amp CB from a Cessna dealer and it tested rock solid. We also added an extra length of aluminum where the cracked bus was to beafen up the area. Removing and replacing the bus will occur in a couple of months during the annual inspection. We were hanging our hopes on the bad Alt Field CB being the heart of my dual charging issue. After running both engines, we still got no charge. So we started checking line voltages and came up with the following:
--All the switches were cleaned and then checked. No apparent voltage loss through them.
--Charging circuit diodes both checked good. 0.4 ohms in one direction and open the other way. At least these seems like reasonable diode results and they were identical.
--The voltage at either VR input was exactly the same as Batt voltage. No apparent loss of voltage through the switches and harness to the VR. The output at either VR was approx 1.5 volts lower than Batt voltage. Not sure if that is a normal reading.
--We checked the coil or field resistance from each Alt and noted both around 16.5 ohms. But when the prop was moved, the readings would jump anywhere from 0-200 ohms. Either engine. Always settling back to around 16 ohms when movement stopped. This allowed us to believe neither shaft was sheared but curious why the readings fluctuated so under slight rotation. I can only surmise that worn brushes making weak contact on a rotating surface would cause such fluctuations.
I will have a new VR on hand in a few days and after test running with it, we will be stuck pulling Altenators if that doesn't change the results. Unless we are still missing something else. If not, I would take suggestions on sourcing of Altenators.
Thanks,
Mike
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