replaced my thermostat a couple of months ago and truck heats up to proper operating temp nicely, but there's a problem. the gauge is inconsistent as it keeps going up and down between 190 to 150. i was talking to my teacher and he said its either your fan clutch or ECT sensor sending unit. i dont think its the sending unit since it looks like it functions properly. the fan spins freely when the truck is at operating temp, or as close as i can get it to operating temp before the temp goes down quickly. it could be a fan clutch, but i live in NY and its cold here now. why would the fan clutch be the problem if it only engages when its hot? that's the part im confused about, but it does look like the t-stat is doing all the work constantly opening and closing. could a bad coolant mix do this?
this might not pertain to this section of the forum but i also "repaired" the abs wiring harness that leads to the right front sensor. the wire was cut right in half, no clue how this could happen. i used butt connectors to connect the wires back together. i didnt have a shrink tube so i used electrical tape. this is just a temp fix unless it stays intact. but how much would this wiring harness cost? from the sensor to the connector. cant seem to find it online so i thought id ask here first... and i heard there pretty expensive. just dumped 165 bucks in supplies for a tune up and im trying to fix everything so i can get it off my chest.

 

You most likely still have air in the system that is causing the gauge fluctuations. Steam will not register at true temperature.

As far as the wiring harness is concerned, if it is cut between the connector and the sensor, you would have to replace the sensor to replace the wiring. If it is beyond the connector, you do not want to know the price.

I would get some 3M butt connectors with heat shrink tubing already on them. They are available at Walmart of all places and work great. They have a heat activated glue inside to make a perfect seal on the repaired connection.

Where in NY do you live?

 

So you think its air still trapped in the system? im not sure about that, but ill give it another look. i followed your advice on filling procedures. so should i even consider about buying a new fan clutch? thanks for the info on the harness, ill buy better butt connectors and forget about replacing it, it works fine as is now. I live in Middletown NY. Quite a way from Waterloo lol.

 

You can test the fan clutch easy enough. When the vehicle is cold, the fan blades should be relatively easy to spin. I could get at least one revolution out of my fan clutch on my old Bravada when cold. Typically, the fan clutch fails in the engaged position which will cause a very pronounced roaring noise most of the time. A failed fan clutch will not cause the temperature to bounce all over the place. If it fails to engage, it may cause an overheat condition though.

The temperature in the motor is controlled by the thermostat. The thermostat controls just the temperature inside the motor and it does not matter what the temperature of the coolant in the radiator is. The ECT and gauge sensors are sufficiently far enough away from the suction side of the water pump as to not be affected by the incoming coolant temperature. As a result, you have a problem inside the engine that is causing the fluctuating temperature from thermostat regulating temperature (you say ~190*F) down to ~150*F. Another member had this exact same thing happen to him. Low and behold, he got all of the air out of the system through many repeated steps and the condition went away.

Here is the thread I was referring to:
Temp troubles still

 

alright, i thought right about the fan clutch then. teachers think they know everything. thanks for your time and info kyle, im going to take another look at it tomorrow and tell you how it turns out.

 

I edited my post to include the link to the thread I was referring to. Must have been while you were posting. Have a look.