Between work and the weather snap here in Texas I haven't had a chance to fight with this for a while. Here's where things stand:
I have replaced all of the potential bad actors in this farce: the automotive relay, the PC680 battery, and the Denso internally regulated alternator. The three replacements are in verified working condition. Replacing them did not solve the problem.
The system appears to work as expected when safely sitting on the ground powered by the battery. The avionics master switch grounds the automative relay. The automotive relay feeds power from the main bus to the avionics bus. All the avionics are happy.
Things do not work as expected when the engine and alternator are on. Snapping on the avionics master switch does nothing. The relay doesn't click. The bus does not power up. But other, odd stuff does happen...
I have a stand-alone volt meter (not the one in the EFIS) that I'm using to measure voltage at various places in the system. With the engine off and the master contactor on (so battery only) the master bus shows 12.7 volts between the master bus bar and the negative terminal on the battery. With the master off, it show 0.0v. So far, so good.
When I start the engine, I can see the voltage step up from battery voltage of 12.7 to alternator voltage of 14.2. The voltage then begins to climb rather quickly. When I ran it this afternoon, it climbed to 15.6 within one minute.
So I shut down and started measuring things with the engine off, and heres's where it gets really
I disconnected the master bus (the bus bar on the panel) from the system - it's not connected to the battery or to the alternator.
- With the master contactor open (off), I measure 0v between the master bus bar and the negative terminal on the battery.
- When I turn the master switch on, I see about 0.7 volts between the master bus bar and the negative terminal of the battery.
- When I unground the magneto, the voltage jumps to over 6v.
- When I disengage the alternator field breaker it jumps to about 8v.
This is with the bus bar completely disconnected from the feed. There should be no
path for power to get to the bus. But there's power there. So how?
The master contactor on this airplane has two outputs - the high amperage feed to the starter relay and the (now disconnected) to the bus. That's it.
One other oddity that may or may not matter here: for reasons I've never understood, the starter on this airplane won't budge if the alternator field breaker is open. That one is not new - it's been that way since I got it.
[EDIT: Update the above - found where the power to the starter switch comes from the alternator field breaker.]
A friend and neighbor with an RV will be coming over to take a look this weekend. I would appreciate any suggestions as to what we should check.