Where is the temp sensor on 3.0 engine - 1990 Toyota 4Runner

What diognostics say its a sensor ?????? No diagnostic's ever tell's you it's a sensor . You as a technician would need to prove that by testing electrical circuits , viewing coolant temp sensor . Hooking scan tool and trying to command the gauge with by directional control . The temp gauge not showing any temp. ? No movement ? Engine Coolant Temperature Gage Inaccurate or InoperativeStep Action
Yes
No
Schematic Reference: Instrument Cluster Schematics
1
Did you perform the Instrument Cluster Diagnostic System Check?
Go to Step 2
Go to Diagnostic System Check - Instrument Cluster
2
Install a scan tool.
Turn the ignition ON, with the engine OFF.
With the scan tool, perform the Lamp and Gauge Check in the IPC Special Functions list.
Does the engine coolant temperature gage move up and down when commanded?
Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems
Go to Step 3
3
Replace the IPC. Refer to Instrument Cluster Replacement .
Did you complete the replacement?
Go to Step 4
--
4
Operate the system in order to verify the repair.
Did you correct the condition?
System OK
Go to Step 2
Would hate for you to replace the coolant temp sensor an find out that's not the problem ! Does the vehicle start an run good ,trans mission shift good ? If so it's not the coolant temp sensor .
Engine Coolant Temperature Gage
The ECM sends coolant temperature as determined by the coolant temperature sensor to the BCM via CAN serial data. The IPC displays the engine coolant temperature with the gage when the BCM sends a class 2 message to the IPC indicating coolant temperature percentage. The engine coolant temperature gage defaults to cold (0°) or below if:
• The ECM detects a malfunction in the engine coolant temperature sensor circuit.
• The IPC detects a loss of class 2 communications with the BCM.
• The BCM detects a loss of CAN communications with the ECM.

Depending on the make model and year, it could be because the engine computer goes into limp mode with the sensor disconnected, and does not use data from other sensors. That sensor could be giving a false reading, or one of the others.
For example, the sensor may tell the computer the engine temp is below zero when it is not. And that would cause the engine to get too much fuel.
Is the check engine light on ?

Check the timing, clean the radiator, check the fans.

If it's a 2.0 it'll be on the back of the cylinder head on the intake side, sort of between the engine and the firewall - it is a single unit with two separate and independent sensors, one for the dash guage, the other for the ECU.

P0710 TRANSMISSION FLUID TEMP. SENSOR MALFUNCTION
P0711 TRANSMISSION FLUID TEMP. CIRCUIT RANGE/PERFORMANCE PROBLEM

P0710 DTC Detection Condition:
Either (a) or (b) is detected for 0.5 sec. or more: (1 trip detection logic)
(c) Temp. sensor resistance is less than 79 W
(d) After engine has been operating for 15 min. or more, resistance at temp. sensor is more than 156 kW

Trouble Area:
1. Open or short in ATF temp. sensor
2. ATF temp. sensor
3. ECM

P0711 DTC Detection Condition:
Both (a) and (b) are detected: (2 trip detection logic)
(a) After 12 sec. of engine start, temp. of atmosphere and that of engine coolant is more than -10°C
(b) After normal driving for over 20 min. and 10 km, ATF temp. is less than 10°C

Trouble Area:
1. Open in ATF temp. sensor
2. ATF temp. sensor
3. ECM

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

sounds like a sticking thermostat instead of a temp sensor.

bad coolant temp sensor

if i disconect temp sensor fans come on cool engine but then no temp gauge

That's what I'd expect after unplugging.
This is sounding like a bad coolant temp sensor. Test it or replace it before you do anything else. Read this post on how to test an engine coolant temperature sensor

Test engine coolant temperature sensor Ricks Free Auto Repair Advice