1990 XLT 2x4, 2.3 Oil Gauge Issues

My Oil Gauge doesn't Rise. I changed the Sensor, but it didn't help. I hear that it may be my Sending unit. And I'm confused, because I thought that 'That' was the Oil Pump.... So I could use some advise on That...
Another thing; I was reading "Doityourself" link that wasn't Vehicle Specific, and it said that I can Test the Sending unit... So, I'm asking:
1. Assuming that the Oil Pump is Different than the Sending Unit, Is my Oil Sending unit OUTSIDE of my Block/Oil Pan?
2. When I start the Motor, and run it for a few seconds, then turn it off; the Oil on the Dip-Stick is Way Above the Full Line. Does this mean that the Pump is Good?
3. Why does my Gauge show "0"? What should I be looking at?
NOTE: All Other Gauges on the Dash work, and I am getting No "Service Engine" Light.
"Thank you, for your help!!!"

Hi Steven and welcome to TRS~~!

The sending unit for the oil should be on the side of the head near the back on the driver side...at least that's were mine was...and it sorta sticks out like a filter if you have the more advanced kind...but it is actually inside the engine...with some external parts...

The oil on the dipstick usually gets thrown about by the crank splashing around down there and you may also have a bit of blow-by that forces the oil up the tube...sometimes out onto everything else that doesn't need oil...

The oil "pressure" is a function of the oil in the bearings...and all that...I'm not the expert on this but from what I've read on here and other places...RonD gave an explanation of this a few times over the last few years but my eyes sort of gloss over and I start thinking about beer when I read it...so maybe he can post again the details while I go buy some beer...but the 0 pressure can mean either you have no pressure in the bearings or your sending unit has become non-functional or a wire came off...check the location beside the back of the intake on the driver side...look for a single wire coming off the unit...

There may also be one on the block on the driver side but you won't be able to see that without crawling under the truck...it is actually in the oil pan and has two wires coming off it...that may be the real sending unit...

Hope that keeps you going until someone else replies...but it will keep your post up the top...

Thank you Mark, I will check into this.

The sending unit location depends on engine, and the 2.3 Lima engine has the sending unit on the side of the cylinder head. Near the rear, and has one wire connected.
Most have black plastic(bakeline) 'dome' with the wire connector sticking out the top. If you ground the wire, with the key on, the gauge will read in its 'normal' range if it is the 'dummy' with the resistor. It will read high if it is a true gauge.
The other sender is metal colored and is similar looking to a very small oil filter, again with one wire coming out of the top. Internally, it has a diaphragm, spring and sweeping resistor. As the pressure comes up, it moves the diaphragm against spring pressure, and moves the resistor 'sweep' changing the effective resistance. The gauge responds to the resistance change, to reflect apparent oil pressure.
If you have a gauge, ground it on some bare metal of the engine, a bolt, say, with the key ON. The gauge should go to normal or full pressure. If it does, your sending unit may be defective.
You should also look for a loose connector before changing anything, it's cheaper.

The pump is at the bottom of the engine, near the front, inside the oil pan. It is driven mechanically by the timing belt, sprocket, and auxiliary shaft. If you had a distributor, it would also be driven by the same shaft & gear. It actually pumps the oil to the bearings and valve lifters. In the 2.3, the lifters are hydraulic, needing oil pressure to work. If you lack oil pressure, they will clatter so much you'll think your engine is dying. Which it is. If they have oil pressure, the rest of the engine likely has sufficient pressure, as they are at the end of the line, furthest from the pump, of all the things the pump pressurizes and lubes. The sending unit is right near the passages to the lifters, so it senses the lowest pressure in the system.
If you want, you can buy a low price mechanical gauge, and connect it in place of the sending unit to check your 'actual' oil pressure. Permanently, or on a temporary basis.
tom