Mechanical Gauge for oil

Someone on Van’s Air Force asked about the backup oil pressure gauge I installed.  After combing through this blog and not finding any pics, I decided to make a post about it, since it was a super-easy fix and a qualified A&P could do this in their sleep if they had to add one later.   The reason for this was a gnarly scare I got during test flight back in 2014, when I had what appeared to be a complete loss of oil pressure over the Ojai foothills.   

It turns out I was getting bad data from the RDAC on my EFIS/EMS.  For some reason, and I was never really sure why, the oil temperature didn’t seem to map to reality, and when it got above 200 degrees, other readings became erratic. When the oil temp cooled back down to ~180 degrees, everything leveled out. What I suspect is that heat-soaking the RDAC may have damaged some of the components on the board, or the heat expanded something just enough to cause a short or disconnect a path to ground. 

This problem went away completely after I replaced the RDAC with the new version, which is mounted on a thick aluminum plate and has much more robust hardware.

I put in a mechanical oil pressure gauge from Autozone, because oil pressure is the one instrument I can’t afford to have lying to me.  The rationale was that if the digital and analog readings mostly agree with each other,  I can make good operational decisions. The analog gauge didn’t have to be dot-accurate either; as long as it was consistently showing a number in line with the expectations in the Lycoming 360 manual, I could reasonably conclude that it was safe to fly.

There’s a manifold mounted on the firewall for engine fluids, typically fuel and oil.  The manifold has 3 places to plug in some type of fluid under pressure.  To install the AutoZone gauge, I simply removed the plug from the 3rd hole on the oil input and installed a fitting appropriate for the connector on the nylon oil line.  I don’t own the plane anymore, so I can’t check, but generally, the idea was to undo the firewall pass-through seal, run the line from the gauge out from the cockpit through the pass-through, connect it to the fitting on the manifold, then reseal the pass-through with high-temp RTV and high-temp zip ties.

The pic below shows the situation before I actually did the work, but the annotations describe what happened.   NOTE:  The squiggly yellow line from the manifold plug running through the pass-through is where the mechanical gauge’s oil line runs.

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