Hi,
The idea that reverse works indicated the low/reverse clutch is good. Why it doesn't have 1st gear is a good question.
Are there any noises when you shift to drive? Have you checked for diagnostic trouble codes related to the transmission? Was the replacement transmission from the same year, make, and model? Did you backflush the transmission cooler?
What happened to the original transmission? The most common cause of transmission failure when replacement is done is due to contamination from the cooler.
Let me know.
Joe
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Sunday, July 4th, 2021 AT 9:18 PM