1997 Jeep Wrangler, 4.0L I6. Cam-driven mechanical oil pump. New engine in January 2018. 5-speed manual.
I've had the oil pressure gauge (top-right) drop to zero and the alarm go off, far too often for my comfort. Most of the time, it comes back off the peg quickly (but only barely) and the light goes out, but sometimes it doesn't. And it's not always at idle either, or a rough start from standstill. I've had it stay on the peg at 2500 RPM too.
I took the sensor off and put a mechanical gauge on it from Harbor Freight, and it always read 40-70 PSI depending on speed and oil temp. Never below 40 while the engine was running, even at idle after 20 minutes on the highway.
So it's probably a bad indicator, and the engine's fine. I put the sensor back on, and tapped into the wires to put an oscilloscope / data logger on it.
What I got from the 'scope, measuring both wires compared to the cigarette outlet that the 'scope was running from, suggests that I might have a switch and not a proportional sensor. Most of that conclusion comes from the experiment at the end, which I've zoomed in on. The timebase is in seconds, starting cold.
The difference between the two wires, in red, has its own scale on the right side. The individual wires, in yellow and blue, use the left scale.
That's the last of 4 trips, and I never got it to drop the needle and panic at me (except for when I killed it on purpose at the end), but it's enough to make me wonder. Do I actually have a switch? Is the gauge on the dash trying to read the variation in short-circuit resistance from non-snap-action metal contacts being pressed harder or softer? (I (ab)used some Lego switches that way as a kid, and it does work...sorta)
Also, the gauge on the dash seems to never go above 40, and often hovers around 20 at warm idle, while the HF gauge reads between 40 and 70. (not at the same time; I don't have a tee fitting)
Does anyone know what the dash gauge expects on that wire? And how to make it read accurately?
Edit after 2 weeks:
An OBDII scanner doesn't have oil pressure at all:
It was interesting to drive with this live data - it gives some interesting insights as to how much different a computer-controlled engine is from a purely mechanical one - but even the full set of live data doesn't help this problem.
