I actually have two comments for you and a question for you:
The question: Did he try to sell you magic beans to go along with your “new transmission?”
And, now the comments:
1. I think it’s time for a new mechanic.
2. He did not “replace the transmission.”
My comments may seem a bit harsh but unless your mechanic went to a local salvage yard and found an exact copy of your TC’s automatic in another non-working TC there is no way he can say that he replaced your transmission.
You said so yourself in your question. The TC went in with a forward-reverse transmission thump and came out of the shop with the same thump. If it is doing the same thing now as it was doing before, there’s no way he could have swapped out the transmission.
More telling is the price he charged you. As I noted, unless he obtained a copy of the same transmission from another TC at a salvage yard, there’s no way he could have replaced the transmission for $1,000. Even with the usual 10 to 15 percent break in your repair bill that would still leave you with a $2,550 to $2,700 bill to pay, based on a $3,000 rebuilt transmission.
So, I would save the $500 and find another mechanic right away.
As to the areas that might be a problem. I’d start with the transmission fluid levels, first.
Yes, I know you said the tranny had been swapped but given that it is still doing the same thing, I have my doubts, though I am sure the mechanic to whom you took the TC will show you that work had been done.
So, assuming that very little was done, in the first place — maybe the case was cleaned and bolts were reset — I’d check the transmission fluid. You can find the procedure on our site by using this link: https://www.transmissionrepaircostguide.com/change-transmission-fluid/.
I’d also suggest looking at the torque converter, if the fluid is up to the mark, because forward-reverse slip can indicate a torque converter problem. If the torque converter is okay, then it is likely either that the clutch/band pairs for either Drive or Reverse are starting to go. If they are, then you are looking at major transmission work. I’d suggest going to an independent transmission shop with certified technicians to have the work done.
So, save your extra $500 and plan to invest it in a new round of work.