1990 jeep cherokee 6cyl inline- electric radiator fan inop.

Several possibilities, including blockage in coolant tubes of radiator, damaged or slipping impeller on water pump, blockage of air flow through radiator, inoperative radiator fan(s), damaged head or head gasket...

Does the radiator fan come on? If not, problem could be bad temp sensor or fan motor or wiring.

most 93 to 95 have viscus clutch not electric fan (A/C evap fan sure) but some do have the Aux fan too. and the PCM reports DTC errors, if the relay fails. did it? report that?
ok the 95 XJ with 4.0 does have fan.... have FSM in hand.
If the ECT has air pockets it well never report hot temps.
it will not energize the Fan relay if the ECT fibs. for any reasons.
page 35 Chapter 7
the viscous fan locksup at (165° to 180° F).
the electric fan is as follows.
i guess you never looked in the PDC before or its lables
"The fan is controlled by the cooling
fan relay, which is located in the power distribution
center (PDC). For the location of relay within
the PDC (Fig. 41), refer to the label on PDC cover"
and now AUX Fan , quote2.
"When coolant temperature is above 88°C (190°F),
the powertrain control module (PCM) provides a
ground path for the fan relay. This ground is provided
through pin/connector #31 of the PCM 60-way
connector. Battery voltage is then applied to the fan
through the relay. When coolant temperature is below
88°C (190°F), the PCM opens the ground path to
the relay. This will prevent the cooling fan from being
energized.
Whenever the air conditioning is operated, the
PCM engages the auxiliary cooling fan

given these facts? why not turn on A/C and end the guessing.

I use my scan tool , and innova 3140 for jeep OBD1.
it shows me what he pcm sees for ECT readings
if the engine is at 100C and the ECT shows way lower
its bad. or has air pockets around it,
no ect runs in air and works, none,.
if say the ECT is at 100C and the fan relay is not closed
the relay is bad or ECU driver to it blown. (bad ecu)
if the relay coil is hot, and the fan is not the relay is bad.

if the relay is closed and 12vdc out of it, then fan is dead.
if the fan is good (hot wired), and the relay out is hot , the wires are bad from relay to fan.
a $10 voltmeter and the guessing ends.

Clean the blower resistor connector and retest.

Sample pictures here

sounds like sludge build up in the radiator, have it flushed, however that might cause leaks that the sludge is currently plugging up, so be prepared to replace the radiator

try the tps.worked for my 89

Check your radiator for external and internal obstructions (leaves, bugs etc on the a/c condenser or leaves between the condenser and radiator, rust or calcium deposits inside the radiator core all cause loss of heat transfer)
Also make sure that the system is full and that the electric fan and the clutch fan are operating properly. (electric fan should come on just above normal operating temp and clutch fan should not be "freewheeling" above temp either)
Note: If you need to have the radiator flushed, do it separately from the engine. especially the inline 6 and 4 engines rust up inside and if the system is flushed with the radiator connected, the debris will pack up inside it and actually can worsen the condition.

Where is this fan located and what does it cool?

I need to know what all has to be taken off, the easiest way, to replace a radiator fan motor for a 2001 grand cherokee?

Locate the relay in the under hood relay center remove it and swap it with the A/C relay or a relay that looks the same. If the fan still doesn't come on, the fan is not working anymore. If it does come on, go to the parts store and replace the relay. You can use a jump wire to the hot side of the fan from the pos side of the battery to see if the fan comes on. Also run a jump ground. Disconnect the harness to the fan so you don't back feed the ECM. Just use a couple of flat female blade connectors and wires to jump to the battery to test fan. $600.00 is way too much to replace the fan. You can find a good replacement fan assy. at the local wrecking yard for cheap. They work out just fine.

Radiator may be plugged up inside or a bad headgasket. Try the radiator first since it is far easier.