2012 Optima (automatic) with a broken transmission casing mount (1 of the 3, was in the process of repairing). Was on the freeway yesterday and I felt a drop/shift in my engine and every dash light came on and my power steering turned off, 0 rpms and cruising on leftover momentum but the engine was still on. When I’m able to pull off safely enough to test the car, the engine won’t turn off. Every time I tried to turn it off it made a terrible clanking sound, so I figured I would attempt to gas it softly and see if it’ll move. Then the car started reversing while in drive. Is my transmission blown? No smoke or burning smell occured
There is a physical linkage between the gearshift lever and a mechanism on the transmission. This commands the selection of gears. The computer (PCM) may also be involved, but the physical linkage is at least a failsafe in case of computer malfunction.
This linkage probably has some mechanism to allow for relative motion between transmission and body. But this has limits.
If your transmission mounts are broken badly enough for you to actually get to repairing them, then most likely the transmission moved around enough to actually interfere with or break that linkage. Thus causing it to select ??? gear, possibly reverse.
This may have been caused by a marginal mount failing altogether, possibly due to strain from the now-mismatched mounts.
By the way, this happened because you extremely underestimated the amount of force on engine and transmission mounts. That transmission multiplies torque by 400% so if you're pulling 100 ft.lb. with a gentle start, those mounts are seeing 400 ft.lb.
Why were the controls weird?
There are also electrical connections between transmission and body. Some interlock functions happen through that electrical connection. So for instance the interlock that doesn't let you remove the key until the transmission is in Park, might be electrical and would care about what gear the transmission is in, not what gear the shifter handle is in.
While it is correct to immediately shut the engine off in that situation, one should only shut the engine off by moving to the Intermediate OFF or OFF/ACCY position!!! And you better know how to do that.
Moving it all the way to "OFF/LOCK” (the position from which the key can be removed) is a deadly mistake, because that will lock the steering column after a limited amount of wheel movement! So you will be steering and then suddenly click you are not steering anymore. That is a good way to wind up in opposing traffic or in the ditch! Never do that. Know the difference.
What to do about it
You will need to have it towed home. This is way beyond any roadside repair, and I am very bold with those. It cannot be done roadside.
Once it is home, you will need to tear into it to find out what exactly happened and if the sites the transmission mounts tie into are still intact enough to be usable. With any luck, it's just a matter of the failing transmission mounts having failed the rest of the way, and 2 more mounts and you're golden.
Then repair or replace the transmission linkage, which is more likely than not borked. Since it's hard to tell a bent one from a straight one, just hit Pick-n-Pull and get another.
Then you can button it up and do a standing test to make sure all the gears select properly, and take it carefully out for a very cautious 10 mph road test. Then if that checks out, then a 25 mph road test, and if that's simpatico try normal speeds. You won't know about transmission damage until you try it.
I think most likely the transmission is alright, so I would consider it worth buying the other mounts and popping them in.
Oh. And if you have to change transmission or vehicle, think about a manual. It is slightly inconvenient to drive until you catch the habit, but so much cheaper to maintain. No more having to scrap a car because the tranny went out, a manual lasts the life of the car. You replace a clutch every 100k miles or so, $150 in parts.