Turbo 400 TH400 Trans Shift Problem

My 71 Monte Carlo has a cable actuated B&M Shifter in it.
Last week, it started acting like the cable was out of adjustment.
I could not get it adjusted to where it would shift properly going up or down, (and adjustment doesn't normaly change once it is set correctly). This setup has been in the car for over 10 years, although I replaced the cable a couple years ago because exhaust heat damaged it.
Changed cable yesterday, problem sill exists. From Neutral, to get into Reverse, I have to shift past R, almost into Park. But going the other way, from N into D, I have to go down to where 2nd gear is. So obviously it is not cable adjustment, because I have to go 2 shifts forward to move 1 gear up, and 2 shifts back to go 1 gear down. Seems to be getting worse, now I cant get into Park, or low. Acts like there is a lot of slack in cable, but there is none. Could the selector shaft inside the valve body be broken, or? Please help, any and all advice will be greatly appreciated! -Jim.

My 71 Monte Carlo has a cable actuated B&M Shifter in it.
Last week, it started acting like the cable was out of adjustment.
I could not get it adjusted to where it would shift properly going up or down, (and adjustment doesn't normaly change once it is set correctly). This setup has been in the car for over 10 years, although I replaced the cable a couple years ago because exhaust heat damaged it.
Changed cable yesterday, problem sill exists. From Neutral, to get into Reverse, I have to shift past R, almost into Park. But going the other way, from N into D, I have to go down to where 2nd gear is. So obviously it is not cable adjustment, because I have to go 2 shifts forward to move 1 gear up, and 2 shifts back to go 1 gear down. Seems to be getting worse, now I cant get into Park, or low. Acts like there is a lot of slack in cable, but there is none. Could the selector shaft inside the valve body be broken, or? Please help, any and all advice will be greatly appreciated! -Jim.

Not likely a problem with the transmission. If you're absolutely sure that the cable is properly adjusted, I'd suspect a problem with the shifter itself. It's not unheard of.

I agree with Silicon212 as to it not being a tranny problem. Seems if the selector shaft in the valve body was broken you wouldn't get any shifts at all. But I'm not saying it isn't a possibility.

Cable sheaths that are too long and not properly supported or contained tend to flex with input and not allow the output to respond correctly.

Someting to check would be to unhook the cable and see if you can manualy change gears at the tranny. If everything feels good there then it's most likely in the linkage/cable area.

Got it fixed. Drained fluid, dropped pan, and found that the nut on the inner end of the shift selector shaft had worked loose. So the Detent Pawl/Shift Valve Actuator had quite a bit of slack in it. Tightened it up, good as new!
Didn't even have to remove the valve body! (Thank You General Motors!).
Shifts better than ever, because the new shift cable is a B&M Super Duty version, .375" outer diameter, vs the standard B&M .250 dia cable, and 1 foot shorter. Thanks for your input, guys. -Jim.

Wow. I suppose that'd do it! Good to see you got it fixed.