i have an 88 vert with 61,550 miles. The transmission shifts great all the way through OD and convertor lock up when the car is cool. After warming up, in order to upshift to OD, I have to let off slightly on the accelerator and the upshift occurs. Shouldn't the shift occur without letting off? I am not under full throttle here, but just normal cruising. Any ideas on what to check? Thanks.

Pete K. is the 700R4 Guru, he will have the definitive answer but it may be normal depending on how much throttle you are using. I believe 75% opening is considered full throttle. See this link, and note item 7-4-V:

http://www.transgo.com/valve_bushing.php

it is a specialty valve designed to correct the original design which won't allow full throttle upshift to OD. it doesn't explain why it works when cold, hopefully Pete will have some insight.

If it will not shift to 4th gear, I am confused a bit. If it will not lock up the converter when warm, it is likely the solenoid or the 4th gear n/o pressure switch. Both are inexpensive, and are accessed by dropping the trans pan.

Quote:The Corvette version of the 700R4 is one that allows the full throttle 4th gear upshift. Most others don't.

RACE ON!!!

Thanks guys. Pete K, it will shift to 4th when warm but I have to slightly lift off the accelerator before it will. Lock up works fine after it shifts. Maybe some minor adjustment is in order? Thanks.

Quote:

Thanks for clarification, I didn't realize that was true. I've heard of guys with Vettes having this problem but expect their transmission was a rebuilt unit using a case / valve body from another vehicle based on what you are saying.

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Because of the low miles, I assumed he had a stock vette gearbox.
The easiest way to check is to look at the bottom of the trans, just behind the rear of the pan. If there are 2 holes that are drilled and tapped to accept a trans mount, then it is a non vette case. Vette cases have that area completely eliminated because they use the tailhousing for support via the support beam.
If it is a non vette case, the 3-4 upshift valve is a non vette/policecar version.
They are no longer available from GM, but aftermarket versions are available. They require sleeve replacement of the tv plunger, allowing the same end result, a full throttle, wide open throttle shift.

Pete K, here is my original post, your answer, and my response. The subject sort of sidetracked, so i am posting again.

"Transmission question

i have an 88 vert with 61,550 miles. The transmission shifts great all the way through OD and convertor lock up when the car is cool. After warming up, in order to upshift to OD, I have to let off slightly on the accelerator and the upshift occurs. Shouldn't the shift occur without letting off? I am not under full throttle here, but just normal cruising. Any ideas on what to check? Thanks." You're response was:

If it will not shift to 4th gear, I am confused a bit. If it will not lock up the converter when warm, it is likely the solenoid or the 4th gear n/o pressure switch. Both are inexpensive, and are accessed by dropping the trans pan

My reply was:
Thanks guys. Pete K, it will shift to 4th when warm but I have to slightly lift off the accelerator before it will. Lock up works fine after it shifts. Maybe some minor adjustment is in order? Thanks.

Sorry to be a pest, but looking for suggestions.

In the original post he said "Shouldn't the shift occur without letting off? I am not under full throttle here, but just normal cruising" I think everyone is missing that. No matter what trans or valvebody he has (vette or F body or other) it should still shift to 4th without lifting out of the throttle at part throttle. All answers are assuming that he is lifting from WOT to get it to shift to 4th which would be the case with a non-vette trans, but again, that isn't his problem.