Losing 4th gear

[bud36light]

Havent pulled no codes yet(maybe this weekend). Each time I have taken this car on this same trip the same problem has occured within 10-15 miles of each other. Last year running balls to the wall around an exit ramp ribbon the tranny on its own down shifted from 4th to 3rd and lost lock up. It stayed like this for atleast 75 miles(interstate driving) running 70mph and 3200 to 3500rpm. The rest of the shifting is perfect. Till I pulled over and let it set for 45min. Drove another 500 miles of interstate with 4th and lockup no more probs. 6 months later the tranny strips a clutch drum and I have it rebuilt. 6 more months same trip a little further down same road only driving strait, it down shifts to 3rd no lockup 70-75 mph 3200-3500 rpm. Stays this way till we make a pit stop(20 min). Tranny shifts fine only it loses 4thn lockup about 30 min later......... On the Way back coming out of the mountains it loses 4th ... for 150 miles. pull in to eat. Damn thing shifts perfect for the next 3 hours home. Ill take any suggestions on where to start here. Kind of seems like something is getting heat soaked. Thanks for any input.

[RednBlack]

I don't have an answer, but mine did the same thing before it quit the first time. It also started making noise at startup. Finally got to where it was really loud and slipped a lot. I had it rebuilt. The input carrier had broken out the hub. Yours may be a different thing altogether. I hope it is.

[TurboGTU]

This wouldn't happen to be a shift selector problem. You might want to check the tras cooler. Its prob the TCC Apply Valve. Do you have Shudder at first or any...your falling out of OD or no lock-up, its running hot , and maybe reduced cooler flow. Sonnax trasmissons says that the cause of this is: Valve body bore wear at large diameter B, creates a loss of signal oil, causing the apply valve to stroke off or stall in mid-position; bore wear at small diameter A, allows valve hang-up and reduces converter/cooler flow.

You might want to put some additive before thinking of dropping the tras for a repair.

Heres the link... http://www.transmissionspecialty.com.../84754-16K.htm