98 oldsmobile silhouette. Does it only have one temp sensor that sends the reading to the gauge and pcm. Or does it have one for each?

Has CHT - cylinder head temp. sensor . This sensor is a input to the PCM - engine computer . The PCM sends engine temp info on serial data network to the instrument cluster . I/C has a microprocessor inside . Have it check for DTC'S - diagnostic trouble codes . Not OBD2 codes , B - body codes . The gauge could be the problem , which means replacing the instrument cluster .
Engine coolant temperature information is received by the instrument cluster from the PCM over the SCP network. Temperatures over 121°C (275°F) will display hot (H). If the engine coolant temperature data is missing or invalid for five seconds, the instrument cluster will move the engine coolant temperature gauge below the cold (C) position.
The best way to diagnose this is with a scan tool , factory or a professional type . Know how the system works and not replacing parts is the proper way to diagnose .
U1073 D073 SCP (J1850) Invalid or Missing Data for Engine Coolant PCM CARRY OUT the PCM self-test.

• A single gauge does not return to zero with the ignition switch off

• Instrument cluster.

• INSTALL a new instrument cluster; REFER to Instrument Cluster . TEST the system for normal operation.

• Incorrect temperature gauge indication

• Circuitry.
• Instrument cluster.
• PCM.
• Engine coolant temperature sensor.

• GO to Pinpoint Test F .

I should also point out that if the temp sensor were bad the vehicle may not run right .
Using the factory scan tool
Trigger instrument cluster active command ENGCOOLNT. Monitor the engine coolant temperature gauge while adjusting the ENGCOOLNT active command to read 50% and 100%.

• Does the temperature gauge start at cold, move to half at 50%, and full hot at 100%?

Yes REFER to the Powertrain Control/Emissions Diagnosis (PC/ED) manual.

No INSTALL a new instrument cluster; REFER to Instrument Cluster . REPAIR the circuit. CLEAR the DTCs. REPEAT the self-test.
Your best bet , take it to a qualified repair shop !

One advantage us dealer tech's have over a week end warrior is that we have factory service repair information and have factory scan tool ,Tech 2 , to check data coming into the instrument cluster . Most of you probably don't even think about how that coolant temp infor shows up on the gauge , except by a coolant sensor !

High Temperature Indicator
The high temperature Indicator function handles the control of the Engine Temperature Gauge Display and the red High Temperature indicator. The Powertrain Control Module (PCM) reads the Coolant Temperature Sensor, filters the messages, and sends the coolant temperature to the instrument cluster via a Class 2 serial data message. The PCM monitors the coolant sensor in order to determine a fault. The PCM transmits the Class 2 serial data coolant temperature report messages to the instrument cluster for the temperature gauge display and for the high temperature indicator control.
The instrument cluster receives the engine temperature data in the Class 2 serial data message from the PCM. The instrument cluster converts this data into a temperature gauge pointer position which drives the gauge pointer. The instrument cluster also uses this data in order to determine the high temperature indicator control.
The Body Control Module (BCM) looks for the high temperature indicator Class 2 serial data report from the instrument cluster and triggers a chime when the indicator is ON.

Your best bet , take it to a qualified repair shop , rather then just replacing parts .

with a scan tool read data of transmission look for trans,temp.
if it said the same thing as the temp gauge in cluster
temp sensor inside transmission send info to PCM then pcm send message to cluster

The hose from the valve cover to the air cleaner is just a fresh air source for the crankcase.
What you are describing is a radiator low on coolant or has air in the cooling system.

look for a sensor in front of the raid it has 2 wires

I think you correct in thinking the sending unit is bad, this is a common problem. if the temp gauge reads hot and the engine is running normal and there is no coolant overflowing onto the ground and the radidator is full then i would have to say that the car is not really overheating but rather the sending unit is bad, i would replace the unit and go from there. these trucks run up to above the three quater mark on the gauge at times before the cooling fans are turned on by the computer. hope this is helpful.

Sounds like you are low on coolant. Have this checked right away.

You need to make sure all your wiring is good especially the terminals in the connectors.Check all the wiring from the front to the back where the connectors hook up.If you know someone with a scanner then you can usually do a sweep test on the gauge and it should show the level reading also.But not all scanners have this ability.If it shows the level reading on the scanner then this would eliminate the pcm.Because it is seeing the data.Throttle position has nothing to do with fuel level.Do you think you have a problem with the throttle position?

One guess is there is air in the cooling system. The air pockets will cause temporary overheating (fluctuating guage).

It sounds like a bad head gasket is creating these air pockets.

On the 98 Silhouette, the PCM should be located in the air filter housing.