09 Merc Sable bang/thud
 

Just purchased an 09 Mercury Sable to replace my A-B car that was on its last legs. Love the car but has a mysterious transmission issue that only pops up every once in a while. When coasting at low speeds, and then stepping on the gas, it will slam into gear, and make you feel like you just drove over the biggest pothole of your life. Not normal. Previous owner said the car has done a "hard shift" ever since it was new if you put your foot on the gas at the exact right time, and that they learned to let it downshift into 1st before stepping on it and it never happened to them again. As far as they knew, it was normal and "how it was designed" but transmissions are not designed to do this. This was not a hard shift, it was an explosive shift. I have owned the car for 3 weeks and this has only happened twice even though I have tried to get it to happen and it won't, but it's enough to scare the piss out of you and make you wonder if it is damaging the car. Additionally, once it happens, the next 2 times that the car has to shift from first to second, it will shift roughly. No matter how you accelerate, it will do that for the next 2 accelerations from a stop. Then, the car figures it out and it doesn't do it anymore. These random rough 1-2 shifts don't always just happen after the explosive bang/thud. These have occurred on their own an additional 2 or 3 times. The car seems to downshift abnormally roughly sometimes when slowing down as well. Honestly if I could just get the bang/thud to go away, i'd be fat dumb and happy. Everyone else that has documented this issue says that mechanics can't duplicate it (because it doesn't happen enough) and there doesnt seem to be much of a solution for it. One guy back in 2011 said that his transmission was doing the same thing and it turned out that the transmission was "cavitating" and getting air bubbles in the fluid, and Ford gave him a new transmission for it. The fluid is still a light red color. Thoughts? Could it be a VSS? Solenoid? Computer problem? Problem with the transmission cooler/cooling lines? If it's something I'm just gonna have to live with, is it safe for the car? I just need it to last me about 3 years before I'm in a position to buy the car of my choosing. Turned down an 06 Ford Fusion because of heavily documented transmission problems, and this is exactly what I was trying to avoid when buying a car, so I'm totally bummed about this! Smooth ride but the bang/thud sucks! Thanks so much in advance!

Very few dealers will get involved in internal transmission work for various reasons. What most will do is replace the trans and send the problem trans out to a trans shop or authorized remanufacturer.
We know if you take it to a trans shop they will talk you into a complete rebuild or replace it.
Options are few and sooner or later the trans will need to be opened up to see exactly what is going on.
Abrupt shifts do take a toll on drivetrain parts and the longer it continues , could involve more expense.
Check with a trans shop or two and get some idea of the cost involved. Never hurts to get other opinions from knowledgeable sources..

okay thank you sir! Appreciate your insight