Problem with 66 nova Power glide transmission

Why does the transmission shirt at 45 -50 mph. Should be 15-20 how do I fix is it the modulator bad ?

@j.hedtcke - Can you describe the issue a bit more? This is a two-speed powerglide correct?

First I would check to see if you have a vacuum leak. Second I would check that the kick down linkage is free and operating properly. Third you will want to check the fluid level. Any of these can cause your problem. If all of these are correct then the problem could be a defective modulator or a sticky valve in the valve body or just a dirty filter.

Thanks for your response.Vacuum is OK kick down like it is OK And fluid level is full thanks

Could it be the tour converter or the modulator being bad. ?

A bad converter wouldn’t cause this problem with up shifting but would cause slippage. If the modulator were bad it would cause this problem. Try pulling vacuum on it and see if it will hold it. If it leaks down it is bad, if it holds I would guess a sticky valve in the valve body.

Is there a additive to add to the fluid to try to get sticky valve unstuck??

There are and you can get them at any auto parts store. You should probably flush out the trans and change the filter after.

Heres. More advice.
Do you have 15 inches. Of vacume at the modulator?
Now lets talk about the VALVE THAT IS IN,THE TRANNY ,the valve the modulator works.
Remove the modulator, take a magnet to the valve in the tranny. And it should slide in and out. Freely. If not, . sand the valve with 1000 or finer sandpaper. . 3rd.,be sure your manual linlage is spot on. .