Temperature gauge not working

I have a 2000 Dodge Dakota SLT, recently installed a rebuilt engine. I now have a check engine light and it says temp switch defective. I replaced the temp switch and I still have the check engine light. The temp guage is not working and the electric fan stays on. When you shut the key off and turn the key back on the temp guage reads approx 150 degrees, but after it runs for a minute it drops to 0. Someone told me to replace the PCM, which I did with a used PCM, still having same problem. I had the wire check from the temp switch (tan w/black stripe) and it has no voltage. Any help is greatly appreciated.