Temp gauge does not work

the most expensive part of 3 possible replaced 2 times.
wow.
the sensor or the wires (end to end) are bad. (just one gauge fails?)
the service manual covers how this work.

I guess you meant the IP cluster engine temperature meter reads wrong and the engine thermostat is not bad, 23 years old and thermostats have design life of 7 years../
a scan tool set to scan PID, ECT sensor tells what coolant is doing.
189f to 213F is spec, temp range

the gauge coolant temp has:

Water Temperature Indicator Sender Unit
When the engine temperature is low, the resistance of the water temperature indicator sender unit (10884) is high, thus restricting the flow of current through the gauge and moving the pointer only a short distance. As the temperature of the coolant increases, the resistance decreases, allowing more current to flow through the gauge and resulting in a corresponding movement of the pointer.

these year trucks the gauge sender is not the ECU ECT sensor
and may be single wire sender.. try mostly fail this old, not the gauge
the wire on the sender can rust or even fall off.

You may have disconnected the wire on the sensor.it's close the the thermostat.Check this out. 1995 ford ranger coolant temperature sensor

when a temp gauge drops to cold ,it indicates a problem in the cooling system
temp gauge sender units need water to activate the resistance that makes the gauge read so a falling gauge indicates no water around the sender unit --- air lock in the system--faulty wire connection to the gauge sender unit
the ac unit is not connected to the temp gauge so there may be another separate problem there
the temp sensor is an entirely different unit to the temp gauge sender unit and the short term working would be from an altered coolant level from changing that sensor --air lock in system
the temp sensor connects to the ECM to activate the fans and monitor engine operating temp so as to keep the air/fuel mixture exact for the engine conditions
the temp sender unit just makes the temp gauge read
a faulty thermostat will either be stuck open and the engine temp will take a long time to rise or it will be stuck closed and that will cause an overheat very quickly

Change the temp sender

There are two possible candidates here, both of which you claim to have "checked" as good. They are the ECT (Coolant Sensor) or the thermostat. As the gauge works "occasionally and when it does fuel mileage is much improved", I would take the cheap route and change the T-stat and see what happens. If the thermostat is intermittent sticking in a wide open condition, this will result in the inability of the engine to regulate its coolant temperature. Engines run more efficiently at higher block temps because it aids in the combustion process. Many DIYers often change out 180 degree thermostats for 195 degree ones because they see marked improvement in power and fuel mileage. The ECT samples the temperature of the coolant and sends a signal to the PCM. The PCM uses that to determine rate of fuel delivery and also sends the signal to the temperature gauge on the instrument panel. A cooler engine needs more fuel, thus the decrease in gas mileage. You don't mention anything about the vehicle running rough, so I'm assuming that the ECT has it right. If the ECT were faulty and the engine were warmer than the ECT says, you would probably start having running issues since the engine would be running too rich, and that could possible even set error codes and the Check Engine Light.

your coolant temp sender could be bad. locate the temp sender. this is a 1 wire sensor. the other sensor in the block has 2 wires and starts the cooling fan.

If you are not getting a signal to your gauge, your car can still run. You just won't know if your car is overheating until it is too late. A bad thermostat won't keep your gauge from working. The temperature sensor that sends the signal to the gauge will keep it from working.

The thermostat helps your car in extreme weather. It helps your engine warm up in extreme cold so your heater will work sooner. And it helps you car cool itself off in extreme hot weather. As your engine water temp rises, the thermostat will open allowing the water in the radiator (that has been cooling) to flow into the engine block. The water that was in the block will go into the radiator to begin cooling.

In fair weather, you don't need a thermostat.

The connection at the gauge,or the gauge is bad,one ,or the other.Just make sure the sending unit is good,I know it is new,but make sure anyway,and check the connector on the sending unit,and the wire.

I am having the same problem with my 2001 Aztek. I changed the temperature sensor. It worked for a while and now its doing it again. One mechanic told me it problably needs a new thermostat. I might try that. Will try and report. If you have found a solution please let me know.