When the idea first came up to write this article, we wanted to call it "Adjusting a TV Cable the Right Way." After digging into the subject and spending some time at Bowtie Overdrives in Hesperia, California, we reevaluated the scope of this story and concluded that our first idea was far too simplistic. This story demanded a brief explanation of throttle-valve function in order to better understand its importance.
If you have a GM automatic overdrive transmission, then you should be familiar with the TV cable. You may also know that an improperly adjusted TV cable can be disastrous to a transmission's life span. First, let's define the term. TV stands for throttle valve, which refers to a small valve in the automatic transmission that determines hydraulic line pressure. This valve is hooked via a cable mechanism to the throttle linkage. As the throttle opens, the cable is pulled out of the transmission, signaling the transmission to increase line pressure to accommodate the additional torque. TV cable position also determines shift quality and firmness and controls all shift timing events. For GM cars, the TV cable is used on the popular TH200-4R and TH700-R4 automatic-overdrive transmissions. This is not the same as the kickdown linkage used on a TH350 transmission.
The bottom line is that the GM overdrive transmissions are inherently good gearboxes. But they seem to have an increasingly bad reputation for failure that can often be traced back to a poor TV-cable setup that leads to an early failure. When this happens, the blame usually lands on the transmission builder when the truth is that the TV-cable system was installed improperly, leading to low line pressure and early failure. If you read and follow the information in this story, it could save you the grief and expense of an overdrive transmission failure.
The TV Feed OrificeThis small orifice in the transmission's valvebody is one of the primary lines of communication to the transmission, aside from the shift-selector shaft that puts the trans in the right gear. The TV feed orifice is a common system that is shared between both the TH200-4R and the TH700-R4 and their variants. The critical issue here is the positioning of the TV valve at idle. The TV valve should sit right at the edge of the feed orifice at idle and completely uncover the orifice at wide-open throttle (WOT), thereby raising the pressure in the transmission.
The throttle-valve spring is what helps determine the position of the throttle valve at idle and therefore controls the pressure. The throttle valve is the virtual gatekeeper. When the spring becomes worn after thousands of cycles, it will not respond to part-throttle inputs, and the pressure may not match the engine's output. No amount of TV-cable adjustment can account for or correct for a worn spring. It must be replaced for proper throttle-valve function. It sounds crazy, but the life of your transmission relies on this small piece of coiled wire. If, for example, the throttle valve does not respond before wide-open throttle, any increase in torque output will probably damage the trans due to insufficient line pressure to handle this torque increase.