automatic transmission hard shifting problems

[mamaro]

old transmission had exact same issues. Hard shift into first. Then the whole tranmission went out. I bought a rebuilt one. Then my engine went out so I replaced that. I am now getting chance to test out my engine. and this transmission will hard shift into first also. I tried reading up and heard metion of drop resitor and thermal sensor. I hope that is some type of sensor that I clean/replace and not a bad transmission. What advice can you give me? If is some sensor that needs to be replaced can I get a pic or where it would be, please. BTW Thank you for everyone that helped me with my electrical issue, it is now working!