Any electric fuel pump power circuit (beyond an ignition switch connection and maybe primer switch) is going to involve a continuous duty relay.
Gauge == sensor == variable resistance == variable voltage.
Indicator ('idiot') light == switch == on/off == relatively stable voltage.
It's worth noting virtually
all motor vehicle gauges are 5-6 volt devices. (There's a voltage regulator in OEM instrument clusters that reduces the vehicle's nominal 12VDC to 5-6VDC for the gauges.). This, along with the variable resistance/voltage, creates issues when using an oil pressure
sensor for determining if an engine is actually running. (At a minimum, you'd need an electronic voltage comparator in addition to a continuous duty relay.)
Note: The same issue exists with using the alternator as an indicator of engine operation. The alternator's voltage regulator (the one most people are familiar with) turns the alternator on and off in response to sensed system voltage. So, the alternator is likewise a problematic indication of whether the engine is running or not.
Oil pressure indicator lights alert a driver to a sudden, catastrophic drop in oil pressure. Oil pressure gauges require the driver to notice the drop in oil pressure and are more useful for noticing changes over time.
Back in the 50's and 60's (and probably into the 70's and 80's), race car builders installed both oil pressure switches and sensors. Because the switches/sensors were an either-or situations, this required plumbing to install both.
Computerized engine management units only need to know if there is or is not sufficient oil pressure so a switch is preferable. On the other hand, consumers strongly preferred gauges even if they didn't know how to read them or what the hell the needle position meant.
So, manufacturers began producing components that combined both a switch and sensor in a single unit. These combined units is the key to what Leeann has done, in terms of both an oil pressure gauge and indicator (warning) light.
Here's a very good Allpar article on the subject.. The article is reprinted from Mopar Action magazine, originally written and subsequently updated by Richard Ehrenberg. (Richard Ehrenberg is extremely knowledgeable on motor vehicle electrical systems, particularly in terms of Dodge and other Mopar vehicles ... I tend to trust anything he's written.)
If you look about two-thirds way down in that article, you'll see "Michael D. Griffin" cited -- that's me.
