76 Cadillac automatic won't shift from 3rd to 4th

SOURCE: Can't shift into reverse ( manual transmission )

I have the same problem on my 2005 cts. The shifter fork has two rubber bushings that attach at the transmission that break or give way. I assume they did this for a softer feel and to avoid vibration but it leads to the selector not engaging well when you go the extreme direction of reverse on the shifter as there is too much deflection. I replaced the bushings with washers as spacers and it seems to work as a "McGyver" fix. I do intend to get the parts needed from Cadillac to restore it to factory standards. Hopefully they have the parts. To fix the problem you have to go under the car. To get good access you have to disconnect the exhaust and the driveshaft. I did it without this and just tormented my poor hands but I wasn't willing to do all the other work while in my driveway. You simply take the clip holding the fork shaft onto the transmission selector end off and slide the small shaft out. I also took off the shifter side of the fork to study the thing a bit better. That takes a 13mm and a small allen wrench, 4.5 or 3/16, can't remember. Then replace the bushings or do as I did and stack some washers so as to get rid of the slack between the inside of the fork and the gear selector on the transmission. Assemble and try. If you have as good dexterity and tenacity at your fingertips as I do, you'll curse the day you attempted doing this without taking out all the other stuff. If you don't, take out all the other stuff to make room for your hands. The space is very confined. Good luck.

SOURCE: transmission shifting problems

You need to have it checked out by a mechanic as soon as possible, I drive a 2000 cadillac catera and it began to have the same problems about 4 months ago but last month my sport mode button began to blink which means a transmission problem. So when i had it looked at they said my 3rd gear was almost completely burnt out and it cost $2,000 to fix. The cause was a broken piece of the transmission was failing and it had to be completely rebuilt. Good luck!!

SOURCE: ''1996 Cadillac Eldorado won't shift from first gear

Have them replace the transmission solenoid. That's what fixed ours. Good luck.

SOURCE: My 1995 cadillac seville sts wont shift out of 3rd

could be shift solenoids . on ebay for $50. are located in the bottom of transmission. remove pan and u should see the valve body it has about 20 screwes in it unbolt it it is a 10mm socket u will need. take it out and unbolt the back side of throttle body be carfull to remember where the little berings . now use a screw driver to pull the clipps holding the shift solinoids in place and reinstall

SOURCE: My 96 Cadillac Deville is having transmission

Transmission repair show would be my suggestion. This sounds like the manual shift valve has a mind of its own as well as the ECU thats running your transmission.