Diagnosing Throttle-Valve FunctionBoth the TH200-4R and TH700-R4 are equipped with an external pressure port (see photo) located on the driver side of the transmission case. A pressure gauge can be fitted to the port once the plug is removed to see if your TV is functioning properly. At idle, the gauge should read between 55 and 65 psi if the throttle valve is properly adjusted, assuming that your transmission is healthy. Any slight movement of the TV cable should produce an instant response. If not, the TV cable needs adjusting. Fortunately GM makes adjusting the TV cable quite easy. The cable should then be tightened until there is an instant response on the line-pressure gauge to cable movement. This is by far the simplest manner in which you can diagnose throttle-valve function. But it is crucial to note that if the TV spring is worn or broken, you will never be able to properly adjust the throttle valve.
Let's say that the throttle valve is adjusted properly but the transmission's performance is sub-par. This may be acceptable for a mild daily driver since the transmission will not hurt itself. Transmission behavior changes require changes in the geometry of the cable system, and you are basically at the mercy of the factory-designed system because the geometry is built-in. What if the early soft shifts just don't inspire you? What if you want to change the behavior of your transmission? What if you've changed intakes and are now using a Holley or Demon carburetor with your TH700-R4? Enter: Bowtie Overdrives and its throttle-valve system.
Changing The Behavior Of Your TransBowtie Overdrives has developed a complete carb mounting system for not only ensuring proper TV function, but also for changing the behavior of your trans to your liking using an adjustable cam-system mount on the carburetor. The cam system is unique in that no matter where it is positioned by the user, the cable pull distance never changes. Once the TV is adjusted, changes made to the cam system will not affect the TV function, but it will change the behavior of the trans. The cam only changes light/part-throttle shifts, because once WOT is achieved, transmission pressure increases to the maximum. Bowtie Overdrives offers a number of different applications to cover a wide array of different carburetors. Since the TV-cable bracket is mounted underneath the carburetor or throttle body, intake-manifold selection has no effect on the geometry of the cable in relation to the throttle. Each Bowtie Overdrive kit has a matching cam system and throttle-valve spring unique to a specific carburetor.
The number one failure in TH700-R4 transmissions, according to Steve Holmes of Bowtie Overdrives, is the loss of the overdrive clutches. This is usually caused by a torque increase without a pressure rise in the transmission. It all boils down to an improperly positioned throttle valve. Most people (ourselves included) are more concerned with the behavior-how the trans acts and reacts-than about pressure. But, the truth is that pressure is very important to trans life. If you have doubts about the TV function of your trans, we would suggest immediately taking time out to diagnose your system. It could mean the difference between a trans that lasts the life of your engine and one that self-destructs. The choice is yours.