TV Time

Throttle Valve Tuning For TH700-R4 And TH200-4R Automatics

Jeff Smith Mar 1, 2002 0 Comment(s)

Step By Step

This is the simple Bow Tie Overdrives TV Made EZ kit that sells for $95. The system comes with an under-carb mounting plate, two gaskets, a cable mount, an adjustable TV cable, all the fasteners, a drill bit, and—most importantly—three plates that position the cable end to the linkage. A transmission pressure gauge is recommended and is an additional $28.

The first step is to mount the under-carb plate with the two gaskets with the cable mount positioned behind the carb’s throttle linkage. Then find the three slotted throttle-cable mounting brackets (shown). Note that these pieces sandwich the linkage.

The kit supplies the stainless fasteners to bolt the linkage mount together. Note that the top hole needs to be drilled with the supplied 0.144-inch drill bit. Don’t drill all the way through or you’ll mess up the threads in the rear mount. Start drilling, then remove the linkage pieces before finishing the hole.

Note that part of the carb’s linkage arm protrudes into the slotted area (arrow). This will need to be removed with a die grinder. You may have to test-fit the pieces several times to ensure a proper fit.

Now place the cable attachment stud in the slot and move it all the way forward, which is the most aggressive setting (shown). If you place the pin in the rearmost position, this is the least aggressive setting. With the pin the farthest forward, bolt the carb to the intake manifold.

Snap the cable into the mount and connect it to the carb linkage. The TH700-R4 and TH200-4R transmissions offer a line-pressure tap in the driver-side of the case just above the shifter location. With the engine running, unlatch the TV cable’s slider lock and pull the slider (arrow) out toward the front of the engine until transmission-pump pressure stops decreasing. Next push the slider back in until the pressure starts to rise. Then lock the slider in place.

Line pressure for our test TH700-R4 ended up being 80 psi at idle with the pressure increasing to 125 psi at part-throttle.

The most aggressive setting on the carb linkage places the cable lowest on the carb mounting plate. If you want the least aggressive setting at the rear of the slot, space the cable mount higher. The kit comes with four spacer shims (arrow) for this purpose.

This is the entire kit bolted on an Edelbrock Performer carburetor. The system will work on the Carter AFB-style carburetors as well.

This is the TV Made EZ kit on Tim Moore’s ’67 Chevelle using a Demon carburetor. Moore prefers the most aggressive setting for his Chevelle with the Hot, Hot, Hot 355ci small-block. He also discovered that he would have to modify his throttle linkage if the shims were used since the cable mount would interfere with his existing linkage.

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.

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