I have a td7g 1990 no first or third gear, in reverse but all gears work in forward ???
I wish I had the service manual for your particular machine. I do have a Td8e. The transmission unfortunately is the long version in my machine. I believe you are going to need a service manual and you will have to place pressure taps into the range valve on the side of your transmission. I have attached a photo of your range valve. In my older model there was no mention of a common failure mode for losing both 1st and 3rd only in reverse.
thank you for the info