So I did not know my systems well enough. An aircraft I've never flown before, which is being restored. Substantially rewired, and checked as much as could be at the time, prior to engine and wing installation. So I installed the engine, and hooked everything up. After a good final check, including electrics, I fired it up. After two and a half years, it ran sweet!
But, no oil pressure indication, darn
. The prop cycled just fine, so I believed I had oil pressure, but no indication = shut it down to investigate. I hooked up a non electric oil pressure indicator, started it again, and voila! 80 PSI!
. So, indication problem.
I disconnected the wires one by one, and everything checked out fine. Finally, while I was jammed very uncomfortably under the panel, I thought to measure voltage in various places. Interesting, voltage on the buss, but no voltage on the other contact of the circuit breaker for those instruments. The breaker was the non resettable type, so fiddling it was out - replace. By now I have about four hours of my time, and a very crinked neck, into this.
New breaker tested and installed. I purposefully left it tripped at installation, so I could close it, and watch what happened, when I had control - this would be a one time opportunity.
Master on, all kinds of good instrument noises and illumination, time to close the breaker for the Oil P&T, and Fuel Qty. When I did that, obviously watching the indicator, the pointer rose off the white line at the left, to the red line at the no oil pressure side of the scale. Engine not running, so no oil pressure. But, I no that the indicator works now! I have great faith it will indicate oil pressure when I next run the engine.
If I had understood exactly what indications the indicators give, I would have known that the oil pressure indicator will come up to zero with power, so correct function could be confirmed. I would have started the fault checking from the other end of the process, and saved a bunch of time....