I’m considering a 700-R4 overdrive for my C-10 pickup project since I have a trans that I got for a really good price. I know that a lot of people say the 700-R4 has problems killing clutches because of the setup on the TV cable. I’m running a Holley carburetor on a small-block Chevy. I’m worried about setting up the TV cable properly but I’ve also heard about something called a constant pressure valve body that can help. I have no idea what this is. Is this a drag race only piece or can it be used in a street transmission?
The big potential failure point for 700-R4 automatics is installation error in setting up the throttle valve (TV) cable. In stock vehicles this is generally no problem. However, with aftermarket applications this often is where failures can occur. We’ll get to your question about the constant pressure valve body as it plays directly into this area.
What is an Automatic Transmission TV Cable?
The cable controls the throttle valve (TV) in the valve body. This valve sets the transmission line pressure which is responsible for applying and maintaining the hydraulic load on the clutches in this transmission. The TV cable replaces the old vacuum modulator valve on older transmissions. The modulator valve set line pressure by using manifold vacuum as an indicator of throttle opening. High engine vacuum signaled low throttle opening and low load. A low manifold vacuum indicated high load and a large throttle opening. The TV cable replaced the modulator valve but demands a precise linkage arrangement in order to maintain proper line pressure in the transmission at various throttle openings.
Where TV Cable Problems Arise
Problems can occur when using an aftermarket carburetor like a Holley, Edelbrock, or other carburetor that requires the proper arc of movement so that the TV cable does not set the pressure too low or too high. Over the years, companies like Holley, Lokar, TCI and others have created precise brackets for these carburetors that will position the TV cable in the right position most of the time. We say “most of the time” because there are so many different engine power, torque converter, gear ratio, and tire size variables that adjusting the TV cable so that line pressure will be ideal for all throttle positions is a daunting task.
We have had experience with this in a couple of different transmissions and applications. For years we ran an Art Carr/California Performance Transmissions-built 200-4R that also uses a TV cable and that trans worked near perfectly using a Holley carburetor and the proper linkage arrangement. We had to manually shift the trans at the drag strip to achieve the 6,300 rpm shift points we wanted as this wasn’t possible even after multiple attempts by modifying the governor and leaving the shifter in “Drive.” But other than that, the TV cable for that trans shifted great at part-throttle on the street.
The second application was a performance-built 700-R4 behind a 468ci big block Chevy that did not work out as well. The freshly-rebuilt trans failed the clutches in a matter of a couple of days despite the fact that we very carefully set the TV cable up using a line pressure gauge to measure pressure at various throttle positions.
The point here is not to bash the 700-R4 but instead to point out how difficult it is to set up the TV cable even when using all the correct linkages and procedures.
Understanding Pressure Inside an Automatic Transmission
As a quick overview of how the pressures are applied, a typical mild 700-R4 will idle in Drive with 90 to 100 psi and should increase to almost 200 psi at wide-open-throttle (WOT). A good min-range pressure at mid-throttle would be around 130 to 140 psi. What often happens is that WOT pressure is good but a light throttle line pressure is not sufficient, allowing the clutches to slip and burn up literally in a matter of a few miles.
Automatic Transmission TV Cable Solutions
One solution would be to use a 4L60E electronically-controlled transmission which is basically an electronically-controlled version of the 700-R4. The 4L60E does not use a TV cable. This trans in your C-10 would also require a standalone automatic transmission controller. There are over 10 different versions of this controller on the market that can meet that need. However, this does present the cost of buying a different transmission and then spending an additional $500 to as much as $1,000 for a controller.
The better solution that applies to your situation since you already have a 700-R4 would be TCI’s constant pressure valve body. TCI came out with this product many years ago in response to these TV cable problems. This is a complete valve body kit that also includes options for heavy-duty, street, and what TCI calls street-plus approaches. Each step increases the firmness of the shift even at part throttle. For your application it would probably be best to have a professional trans shop install the valve body using either the heavy-duty or street application.
The advantage of the constant pressure valve body, as you may have already surmised, is that once the throttle comes up off idle, the line pressure immediately increases to around 200 psi. The TV cable and its adjustment are still important because it indicates to the trans exact throttle position. But what the TV cable no longer controls is line pressure.
The TV cable adjustment is still important because a proper setting is necessary to set the light-throttle up-shift speeds so the shifts are not stacked to where the transmission shifts almost immediately into second and then third gear. A proper TV cable adjustment will delay second gear until roughly 15 to 20 mph and third gear will be engaged at around 25 to 30 mph and overdrive at 45 to 50 mph.
Keep in mind that this is merely an upgrade to the transmission and in no way will it improve a transmission suffering from worn clutches or damaged parts. This upgrade assumes the transmission is in decent shape.
It’s also important to note there are two different valve bodies depending upon the age of the transmission. The early, small-input shaft 700-R4 versions use a different valve body than the 1987- and later transmissions. (TCI part numbers are in the chart near the bottom of this article.)
My friend Jimmy Galante at RaceTrans has built a couple of street-driven 700-R4 automatics with the TCI constant pressure valve body and he is very enthusiastic about how well it works. The ideal situation would be to rebuild the trans using a few other essential performance additions from TCI or Sonnax that should make even a stock 700-R4 a decent street transmission.
Ask Away! TCI Constant Pressure Valve Bodies