Transmission won't go into reverse

What is your question?

My brother brought me his s40 and said it has no reverse and wanted me to check it out. I opened up the trans and it smelled like gear oil so I asked him about it and he said he put gear oil in the front differential. WTF! I explained to him how this transaxle works and he said woops. I rebuilt the valve body and replaced two solenoids that were bad, flushed it out and replaced the fluid. The car was now trying to go into reverse, so I put it through the gears and kept checking the fluid until it was full and tried again. I put it in drive and it worked fine, I put it in reverse and the rpms dropped but would not move, except for a feeble attempt. I put it in park checked the fluid, put in in reverse and it went forward, not backward. It now has no drive, only goes forward in reverse. How do I trouble shoot the the computer and the gear selector to find the problem. The problem isn't the cable.