transmission is stuck in 3rd gear. can I get it unstuck if I remove the access plate on the tunnel and go into the remote ?
Yes, if its stuck due to rust in the linkage. If it is stuck due to a broken fork, then that wont work.
The tranny was making a thumping noise in 1st and reverse before it froze up. I have taken the access plate off the tunnel. Should I separate the remote and check?
Just lift the cover and see what’s there. It doesn’t sound good though.
Back when your car was new a friend of mine had a new MGB-GT. He had some tranny work done at the dealer one day (I don’t remember what). Next day, I’m riding along with him, and the shifter jams. Holy Moley! We coast to a stop, and he hikes to a pay phone to call the dealer. Car gets towed in. Of course, he’s on pins and needles all day and night and next morning. Then the dealer calls: “Mr. [Doe], we’ve found the problem with your transmission. We’re checking to see which of our mechanics is missing a half-inch socket.”
But that’s probably not the cause of your problem.
(That dealer might seem more honest than we’re used to nowadays, but just a couple of years later my brother bought an MGB from them, trading in a Dart(?). He had ordered a roll bar; it didn’t come; and they were jerking him around something awful. One day he’s there trying to resolve his issue He sees his old Dart on the dealer’s lot, and takes a look. Hmmm. Odometer reads many kilomiles less than when he traded it in. He squealed on them, and they got shut down for a few days.)
Before you start pulling the tranny apart, does this car have overdrive ?
If it does, make sure that overdrive is OFF - this could be a failed inhibitor switch or jammed solenoid. The thumping noise you describe could be the OD stuck in the ON position, overdrives make a terrible racket when reversed.
Also ~ when you say stuck in 3rd gear, do you mean you can’t shift from 3rd or that you can move the gearstick but the car’s always in third ? The MGB box has a side access panel, you could drain the box and remove this for inspection but you’ll need to drop the box or remove it to get to the remote shift assembly.