Temp gauge inop

More than likely the cooling fans or fan for the engine is not working, ether because of a fuse or the fan its self or the temp sensor that controls the relay for the fan. So first check fuse, then check the relay, then if those two are ok, then hook power and ground to cooling fan, if fan operates then ether the wire from fan to control temp sensor is open, or the coolant temp sensor if open, faulty, sense fan is not working this causing the a/c to be inop and the over heating of engine , but only at stop and go traffic do to the lost of air flow across a/c condenser and radiator.

Use a digital multimeter--ohms setting, sensor unplugged. check the resistance across the two wire terminals on sensor. As the sensor/engine heats up the resistance should be less. From cold to hot if no resistance change then either the sensor is faulty or there is air in the system. If the resistance does change, then check the wiring connector--wiring circuits-- or the instrument cluster may have a problem? Just my opinion.
Any applicable trouble codes?

Also, besides coolant level, the heater core may be restricted and the temp blend door may be inoperative .

providing your coolant is full cause low coolant will cause no heat
and your temp gauge is normal in the middle then most likely u have a bad blend door actuator usually on the passenger side

Watch the temp gauge when the electric radiator fans come on. The temp sensor that controls when the fan comes on may be faulty.

faulty sensor or a damaged wire to the sensor

The thermostat is on top of the engine, just aft of the upper radiator hose. The temperature sensor (first one, closest to the front of the engine) is the one responsible for the coolant temperature gauge. It's an NTC with two resistors in it - one for the sensor gauge, one for the ignition computer. There is a second one for the Luftmassenmesser Hidtstraht system, bottom right side under the intake manifold.

If you have a drop of anti-boil on the water pump - it is coming from the weep hole and this is indicative of failure of the internal seals on the pump and it must be replaced.

There is a possibility that the thermostat is stuck closed, which can be easily verified in cool weather by cool air coming out of the vents at normal operating temperature. Check the heater core hoses - one should be warm.

Be careful when working on the cooling system - it can scald when hot! Always work on it with the engine cool to the touch and never put cold anti boil in a hot engine.

Checking the temp sensor is easy - remove the connector and place a 68 ohm resistor across both terminals (or one to ground if it is a single spade connector). The temp gauge should read approximately 3/4 across the scale. Don't short it to ground as this could destroy the sensitive coil inside the sensor. If no reading occurs, then the gauge or wiring is broken.

Could be the coolant temp sensor:



4.0L engine shown





5.2L engine shown



To test it:

Perform this test on a cold or cool engine.

1. Disconnect the negative (-) battery cable.
   
2. Unplug the electrical wiring from the sending unit.
   
3. Using an ohmmeter, measure the resistance between the terminal and the sending unit's metal body
   
   1. Infinite resistance or zero resistance: the sending unit is bad, replace the sender with a new one.
      
   2. Other than infinite or zero resistance: continue test.
      
4. Remove the temperature sender from the engine.
   
5. Position the sending unit so the metal shaft (opposite end from the electrical connectors) is in a pot of water. Make sure that the electrical connector is not submerged and only the tip of the sending unit's body is in the water.
   
6. Heat the pot of water at a medium rate. While the water is warming, continue to measure the resistance of the terminal and the metal body of the sending unit:
   
   1. As the water warms up, the resistance goes down in a steady manner: the sending unit is good.
      
   2. As the water warms up, the resistance does not change or changes in erratic jumps: the sender is bad, replace it with a new one.
      
7. Install the good or new sending unit into the engine, then connect the negative battery cable.
   

1. With the engine cold, remove the ECT sensor.
   
2. Immerse the tip of the sensor in container of water.
   
3. Connect a digital ohmmeter to the two terminals of the sensor.
   
4. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the sensor resistance illustration.
   
5. Repeat the test at two other temperature points, heating or cooling the water as necessary.
   
6. If the sensor does not meet specification, it must be replaced.
   

Submerge the end of the temperature sensor in cold or hot water and check resistance



Temperature-to-resistance relationship of the ECT and MAT sensors

there are 2 item located in the same area one is a little below the other.it is on the housing of the one you replaced, the top one is a low coolant sensor and the one on the bottom is the temp sensor.

It may be the thermostat