Transmission Question
Thread Starter
Drifting
Joined: Aug 2015
Posts: 1,500
Likes: 556
Transmission Question
I have a question about the 700R4 Transmission that was on an 84' Vette I had many years ago. It wasn't the most reliable car, but in theory the 700R4 was supposd to be a reliable transmission. One day though, one of my gears failed. I took it in have the transmission rebuilt. After the rebuild the car had a strange issue that despite three more trips back to the transmission shop for tweaking, never got solved.
Under normal driving the car behaved just fine, but under more spirited driving a shifting issue was pop-up. At around 117 MPH the car would be in third gear and screaming nearing the redline. But, it would never shift into fourth gear. The top speed of the car was supposed to be around 150 MPH, so it wasn't a HP issue.
It seemed like some sort of vaccum issue essentially where under normal pressure everything behaves fine, but at the limit under high pressure unless everything is just right it wouldn't shift.
I sold the car before the issue ever got solved, but have always wondered what caused it. Any thoughts are welcome.
Under normal driving the car behaved just fine, but under more spirited driving a shifting issue was pop-up. At around 117 MPH the car would be in third gear and screaming nearing the redline. But, it would never shift into fourth gear. The top speed of the car was supposed to be around 150 MPH, so it wasn't a HP issue.
It seemed like some sort of vaccum issue essentially where under normal pressure everything behaves fine, but at the limit under high pressure unless everything is just right it wouldn't shift.
I sold the car before the issue ever got solved, but have always wondered what caused it. Any thoughts are welcome.
Safety Car
Joined: Sep 2004
Posts: 3,953
Likes: 550
From: Mission Viejo CA
You needed a new trans shop. On a closed circuit track in my '84, with intentionally short straights to limit top speeds, I was able to run 125, with the car pulling for more. Others on this forum with '84's have reported higher speeds. There are a few trans experts here, and hopefully, someone will chime in.
Thread Starter
Drifting
Joined: Aug 2015
Posts: 1,500
Likes: 556
You needed a new trans shop. On a closed circuit track in my '84, with intentionally short straights to limit top speeds, I was able to run 125, with the car pulling for more. Others on this forum with '84's have reported higher speeds. There are a few trans experts here, and hopefully, someone will chime in.
Team Owner
Joined: Jul 2004
Posts: 28,472
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From: Hartford WI
Drifting
Joined: Jan 2005
Posts: 1,354
Likes: 325
From: San Antonio Tx
The 700R4 was a new transmission in 1982. Most of the vehicles it was put in didn't have top-speed capability that required a WOT 3-4 upshift, so the WOT 3-4 was ignored. The C4 Corvette was of course an exception. In some of the early books and articles on the development of the C4 they mention that Hydramatic Division had to develop some hardware to accomplish the WOT 3-4. It is mostly a specific valve in the valve body. I've seen this valve in the Sonnax listings.
I'm thinking the shop substituted a different valve body other than your original when they rebuilt the transmission. Back then, few 700 valve bodies had the 'good' 3-4 upshift valve in them.
I'm thinking the shop substituted a different valve body other than your original when they rebuilt the transmission. Back then, few 700 valve bodies had the 'good' 3-4 upshift valve in them.
Thread Starter
Drifting
Joined: Aug 2015
Posts: 1,500
Likes: 556
I recall that there was a valve or bearing of some kind that looked a bit like a silver marble, though I believe it was slightly smaller than a marble. After I brought the car back in the first time to correct the issue they said something about the manual they had for the 700R4 was based upon a different car's build of the 700R4. After doing some research they said that the part was incorrect to Corvette. Essentially the marble they used was the wrong material and they replaced it with a different version. They claimed the new part would fix the issue...it didn't. Not sure if this is the same valve being described in the above posts but it does seem like the valve was the source of my issues.
