Did anybody over the years figure out why it starts flickering once hot at idle? Is the wiring bad or is there a redesign? What exactly is the fix?
Absolutely yes.
The problem is that people replace the sending unit (more accurately called a low oil pressure switch) with an aftermarket one. The aftermarket parts are incorrectly calibrated to close the contact at a higher pressure than the Chrysler minimum oil pressure spec. of 5 psi.
The solution is to buy the Mopar part (from a Chrysler dealer only) - do
not use an aftermarket part for this part - period.
When the connector is de-mated from the switch, thoroughly spray and rinse the connector with brake parts cleaner, and let it thoroughly air dry before plugging it into the new switch. That is to remove any oil that may have leaked into the connector from the old switch. (Oil in the connector and on the wire can create an electrical leak path to dimly light the instrument cluster low oil pressure light, which is a different problem than the low oil pressure light brightly coming on or flickering with a warm engine idling with the transmission in gear.)
Three facts to be aware of:
• The light (an LED) flickers or comes on at idle because the switch is out of calibration, and the actual pressure at idle, warm, and in gear will be at its lowest other than when the engine is off. The oil pressure spec. is at least 5 psi with warm engine at idle. It is very rare for the pressure to be anywhere near that low at idle. Again, the pressure may be 10 psi - IOW, perfectly good, but the light comes on or flickers
because the switch is out of calibration.
• When the original factory switch fails, it gets out of calibration, sometimes because it starts leaking oil thru its epoxy seal, which affects oil pressure on both sides of the pressure-sensing diaphragm of the switch, thus throwing its calibration (switch point) off. Depending on how big the switch's oil leak is, you may get oil dripping out of the switch and connector - along with the flickering light at idle (because of the out-of-calibrstion switch point), as well as the light being constantly dimly lit (because of the connector contact and the wire being wet with oil creating an electrical leak path to ground - a small leakage current is enough to dimly light an LED. (You don't always get a visible oil leak with the flickering light at idle because the oil leak may be at its early stages.)
• The "vent wire" TSB solution mentioned in this thread is a red herring. Chrysler screwed the pooch on that. If the vent wire "fixed" anything, it was because the factory switch had started leaking oil (which threw its calibration point out of whack), and the real solution was to clean any oil residue out of the connector and off of the wire (using brake parts cleaner) and replace the switch with a new Moper switch.
Bottom line:
• Clean the de-mated connector and wire of any oil with brake parts cleaner and let it thoroughly air dry.
• Replace the switch with a new one from a Chrysler dealer only.