i replaced the temp sending unit thats the one on the side of the head right but anyway i replaced it before the head job with one from oreilly's and it didnt make the gauge work so i got a ac delco temp sending unit and the gauge started working then we did the head job and the gauge was reading differently so replaced the sending unit with one from oreilly's again and the gauge still didnt work i dont understand how when i replaced the temp sending unit one time the gauge started workin but now it wont maybe i need to get another ac delco temp sending unit thanks for the help and tell me what you think about this

 

They probably sold you the wrong sensor. There are two temp sensora: one for the temp gage, and one for the PCM (ECT sensor). The ECT is on the water pump. You can verify the accuracy of your gage by using a scanner to read the coolant temp, and comparing it to what the gage shows.

 

try grounding the sensor wire. it should peg your gauge one way or another. if not u have a bad wire, or connection somewhere or the gage or sensor is bad.