I changed the transmission filter but reused the old transmission fluid. Now it goes into gear but slips while going into second gear... has reverse good gear.
If you are reusing your old fluid you are defeating the purpose of changing the filter. I would only say this is ok if the ATF (automatic transmission fluid) was already spotlessly clean. If not, I recommend having a ATF exchange performed. This service requires a machine that will completely replace all the fluid in the transmission. A pan drop with a filter change will only replace about a third to half of the fluid in the transmission. The service I am suggesting is more of a transfusion and is performed with the vehicle running. The process is not finished until the ATF is seen to be clean in the transfer lines of the transfusion machine.
Changing the filter is good, but the condition of the transmission fluid is more important. I have repaired many transmissions simply by doing a fluid exchange. Dirty ATF will have particulates in it that can clog the small passages of the transmission. It is possible this has happened during the process of replacing the filter. Another thing to check is the wires inside the transmission. The become brittle and can be damaged during the filter changing process. But I think this is fairly unlikely if the transmission still shifts, but has a delayed response.
Something that needs to be checked for are transmission codes. It is possible to set codes while replacing the transmission. This requires a shop level scanner to do so as most scanners you can buy or rent from your local auto parts store are not capable of communicating with the transmission module.
At this point I recommend having the ATF completely flushed and then drive the car for a thorough test drive. If this doesn’t fix this, then I recommend booking and appointment with one of our experienced technicians. Gears slipping when accelerating