TV Time

Most people think they spend too much time with the TV, but when it comes to Bow-Tie hot rodders, the exact opposite is true. If you have a carbureted engine backed with a TH700-R4 or TH200-4R automatic overdrive trans, you probably have not spent enough quality time with your TV cable.

No, we’re not talking about some new automotive channel on cable TV, we’re talking about the throttle valve (TV) cable system on those overdrive transmissions. In the days of GM automatics like the Powerglide, TH350, or TH400, engineers needed a way to modulate transmission-line pressure to adjust shift points based on engine load. What they came up with was a simple device called the vacuum modulator valve. A vacuum line connected engine-manifold vacuum to this valve on the transmission that instantly signaled a load change to the transmission via the change in engine vacuum. As the throttle opened, the manifold vacuum dropped, signaling an increase in load to the transmission.

This decrease in engine vacuum increased the line pressure in the transmission as the vacuum dropped off. This worked especially well as long as the vacuum connection between the engine and the trans was intact. If the line was damaged, the vacuum dropped to zero and the transmission assumed the engine was at wide-open throttle (WOT), which increased the transmission line pressure to the maximum amount. This is why the shifts are firmer and at a higher speed if the vacuum modulator fails or the line is damaged or removed.

This system worked well, but engineers being who they are, decided a cable connection between the engine and transmission would be a better system. This change took place with the design of the TH700-R4 and TH200-4R automatic overdrive transmissions. The throttle valve inside the trans takes the place of the vacuum-modulator valve and accomplishes the same job. The TV cable is attached to the throttle linkage, and the outside of the cable snaps into a bracket at the rear of the carburetor. Opening the throttle pulls the TV cable and moves the throttle valve >> in the transmission to progressively increase the line pressure. As the cable is pulled farther open, the part-throttle shift points increase.

Even though the cable looks similar to a kick-down linkage system like you’d find on a TH350 trans, the TV cable controls an entirely different system. Since the TV cable directly controls transmission line pressure, it is highly recommended to accurately adjust the TV cable. To prevent transmission damage, you must always install the TV cable on the carburetor or throttle body before driving the vehicle.

So far, this is pretty simple. What complicates this system is the location of the TV cable mount and cable pivot point on the throttle linkage. GM engineers established the proper location of the cable mount and the pickup point on the throttle linkage with each new engine combination including both carbureted and fuel-injected engines. But what if you want to install an overdrive trans like a TH700-R4 or a TH200-4R on an engine equipped with a Holley, Demon, or Edelbrock Performer carburetor?

Bow Tie Overdrives has created a very slick TV Made EZ kit designed to solve that performance-carburetor dilemma. Bow Tie Overdrives has separate kits for each of these popular carburetors to allow you to accurately position the TV cable on the carburetor linkage for optimal transmission operation. The system is also tunable so you can establish the TV cable position and shift firmness that best suits your driving style.

Bow Tie Overdrives sent us kits for all three carburetors so we could experiment with the system. We spent about an hour modifying a Holley carb to use the TV Made EZ system, and it works very well. The beauty of the system is its adjustability. If you think the shift firmness of the transmission is too soft or too abrupt, you can make easy changes to the linkage to accommodate that part-throttle feel.

Follow along as we bolt one of these systems onto a Holley carbureted small-block. Originally, Bow Tie Overdrives had intended to build a kit for Rochester Quadrajet carbs as well, but soon discovered there are way too many throttle-linkage variations for a universal Q-jet kit to work. So for now, Bow Tie Overdrives has chosen to concentrate on the three other popular carburetors. According to Bow Tie Overdrives, each kit will sell for $95. It’s the simplest and easiest way we’ve seen to custom-modify the TV cable mount and adjust transmission line pressure.