Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.
This is a 4L60E and it is totally electronically controlled, so keep this in mind. The electronics are not the only thing that might be a factor here, but you have to keep them in mind. If the trans does not upshift out of 1st, and the speedometer reads zero at all times, the problem is probably VSS failure. If so then the VSS needs to be replaced. There is a 1-2 electrically operated shift solenoid that can fail, if it bails it wont shift out of first no matter what else you do, if the VSS is good, I would look at this next.
There is also a 1 - 2 shift accumulator. It is basically a cylinder with a sealed piston, that has a spring behind it. When the fluid pressure is channeled to make the 2nd gear shift, it hits the 1 - 2 shift accumulator, it cushions the pressure so it wont bang into 2nd. If the the piston in this accumulator cracks real bad, you wont get a second gear shift. If the sprag is blown it wont move in 1st gear with the shifter in drive. But it will move in manual 1st, 2nd, and 3rd. It will actually shift 1, 2 , 3 manually with a bad sprag. I know this is a lot to dump into your lap at once, it can get a lot deeper too. These are some of the common things I have seen. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.
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