My 2004 grand cherokee overland oil pressure gauge does not regis

no oil in sump
wire fell off oil sender unit

The oil pressure gauge on a 2004 WJ doesn t really show the pressure, but it will go to a preset value when pressure is ok, or it will stay at o PSI. So you need a pressure switch on this jeep and not a pressure sending unit. If the right unit is installed, then or the pressure indicator is faulty, or wiring. Do a test of the instrument panel to see if the indicator works: ignition switch in off, press the odometer reset button and now switch ignition in run. Instrument panel will start a self test, all lights will come on and all meters will go full scale and return to normal.

Well the warning light is supposed to come on if any of the gauges display a dangerous condition, such as no oil pressure.
You may have a faulty sending unit for the gauge, or a wiring problem, or the engine may actually be loosing oil pressure when the gauge drops.
You could replace the sending unit as a test, or have someone check the oil pressure with a mechanical gauge.

Did you compare what your dash gauge read when it was acting up and compare it to the manual gauge at the same time? I know you said you used the manual gauge and it read fine so if you get a proper reading out of the manual gauge and an improper reading out of the dash gauge then your dash gauge needs addressed.

If your "Check Gauges" light is on, the light means pretty much whay it says...CHECK GAUGES! One or more of your gauges is/are reading outside of normal operating range. You could be having an overheating problem (Temperature gauge near or in the red) You could be having a charging system problem (Battery or Amp gauge reading in the "discharge" or "low voltage" area.) You could be nearly out of fuel (Fuel gauge reading near empty) Your oil pressure could be low (OIl Pressure gauge reading very low)....You have to look at your gauges to find out what is going on that is not normal.

first thing I should asked , when the last time you change your engine oil and filter .

If the engine isn´t making any abnormal mechanical noises, then I would expect the oil pressure sending unit at fault.
If however the engine does make more or abnormal mechanical noises, it could be a too low oil pressure and this should be addressed immediately.

Same part...make sure you get one for gauge, not idiot light though. If the engine has any knocking or ticking when pressure drops do not drive it and have a mechanical gauge installed to check actual pressure. Parts often have different names for same item...your driveshaft is actually called a propeller shaft!!!

might want to check the ignition control (the key) did same to me but with volt gauge

CHECK YOU OIL