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Throttle Valve Cable system 101

PART ONE - EXPLANATION OF STOCK TV CABLE SYSTEM

What follows is our understanding of how the TV control system communicates to the Th-700R4 and Th-2004R transmissions. Part Two will explain where the TV feed fluid goes once the TV cable system tells it to go there. Part Three will discuss TV cable system "illnesses".

Introduced in the early eighties, the General Motors Throttle Valve cable management system has caused more than its share of confusion, misunderstandings and subsequent headaches. The following is an overview of this system as we understand it.

This system was developed as a communication and management link from the engines carburetor / fuel injection linkage to the transmissions valve body for many General Motors automatic transmissions. During this article we will only be describing its use in two of the many transmissions it was used in, the Th-2004R and the Th-700R4. The most important thing to understand about this TV cable system is that it's the ONLY LINK these transmissions have to the outside world. TV cable controlled transmissions cannot operate properly unless the information they need is input correctly through the throttle valve cable system.

The following is a direct quote out of a General Motors service manual for a 1991 C/K truck equipped with a 4L60 (AKA THM-700-R4) transmission

"The TV cable used with the 4L60 transmission should not be thought of an automatic downshift cable. The TV cable used on the 4L60 controls line pressure, shift points, shift feel, part throttle downshifts and the detent downshifts. The function of the cable is similar to the combined functions of a vacuum modulator and a detent cable."

To help you understand this system we will first demonstrate how this system operates in a typical stock factory application. The TV cable management process starts at the vehicles accelerator pedal. As the accelerator pedal is depressed, a mechanical connection (Typically a cable) is connected to the engines carburetor or fuel injection systems throttle linkage. Further references to this linkage will be called the "carburetor linkage". This connection causes the "carburetor linkage" to rotate from its idle position to its wide open throttle position as the accelerator pedal is moved to its fully depressed position.

In Part Two and Part Three we will look at what the "internal" TV system components do and why "setting up" the correct starting and ending relationship of the three TV system components is so critical to these transmissions. Before we do so, one extremely important point needs to be made. You may notice that throughout this article, the term "adjusting" the TV cable will carefully avoided. We will do this for a very specific reason. We have read lots of General Motors manuals explaining how to "adjust" the factory TV cable. While most GM publications used the term "adjust", when you actually followed their procedure, what you actually accomplished was to "set" the system to its correct W.O.T. position which will in turn automatically "set" the correct starting position. This Part One will explain this in specific detail. We are not aware of a single GM publication that advocates "adjusting" the TV cable once it has been properly "set". We are not being "anal" about semantics here but commenting on the wide spread belief that the TV cable system can be "adjusted" to make the transmission "act" better. As far as we know, this system was never designed nor intended to be a transmission behavior adjuster. We further feel this is a very dangerous practice being performed by many "professional" transmission shops and mechnanics. Once have read the remainder of Part One, Part Two and Part Three, we believe you will agree with us that once a correct TV relationship is "set", you cannot re-adjust it without creating a problem of some type. Some of these problems are minor but some can shorten the life of the transmission dramatically! Know what the difference is we will explain in Part Three "illnesses".

The TV cable attaches to the "carburetor linkage" in such a way as to cause the TV cable assembly's inner wire cable to be pulled as the "carburetor linkage" is rotated by the action of the accelerator pedal while it moves from idle to its fully depressed position. The cable assembly synchronizes engine throttle movement with TV plunger movement. (Shown Below)

   
The transmission end of the TV cable assembly is attached to the transmissions case. (Picture #1)
The inner wire of the TV cable assembly is connected to a rigid wire link, which in turn is connected to a throttle lever / bracket assembly that's bolted to the transmissions valve body assembly. (Picture #2)
As the cable is pulled, the throttle lever rotates, pushing in on the TV plunger located in the transmissions valve body.

Pulling and releasing of the TV cable during normal driving, reciprocates the TV plunger back and forth inside the TV bushing. (Shown at left in the fully extended and fully depressed positions)

Whenever the TV plunger (Red arrow) is moved inward inside the TV bushing (Blue arrow), it pushes on the TV spring (Green arrow) which in turn pushes on the TV valve (Yellow arrow). The sole purpose of the TV valve is to direct TV Feed fluid into the orifice shown with the red circle. The volume and pressure of this TV Feed fluid, being directed into this passage, will progressively increase, as the accelerator pedal is being depressed.This increasing TV feed volume and pressure entering into this feed passage is directed to all of the transmissions pressure regulating and shift control circuits.This increase in volume and pressure is not designed to increase in a progressive straight line; it's heavily biased towards increasing pressure and volume more rapidly during the initial movement of the Throttle valve and to have delivered its full influence by slightly over half throttle opening.

This feature works hand and glove with the transmissions pressure boost system (controlled by the line bias valve) to more closely match increases in engine torque.

"Engine torque increases more rapidly with early acceleration and increases slower as engine speed increases!"
(This is a direct quote from a GM technicians guide)

To clarify this long winded description, I refer back to another direct quote from the GM service manual mentioned earlier.

"The TV cable used on the 4L60 (AKA THM700R4) controls line pressure, shift points, shift feel, part throttle downshifts and the detent downshifts."

Everything is controlled by the TV feed fluid flowing into the single orifice shown above which is in turn controlled by the positioning of the TV valve! It would be a gross understatement to say that the TV valve and the fluid feed hole it manages are critically important to the Th-700R4 and Th-2004R!

Let's get into the details!

 

The following is a physical description of a typical factory TV cable, plus an explanation describing how to "set" it properly using the factory procedure, followed by an illustration of what actually happens to the TV plunger / TV bushing relationship when the TV cable is adjusted using this stock factory procedure?


TV Cable

 

 

The transmission end TV cable assembly mounts to the transmission case by a single 6MM bolt. A rubber sleeve type seal placed between the TV cable housing and the case hole prevents fluid from leaking out of the transmission.



 

At the "carburetor linkage" end of the TV cable there's a special plastic housing molded onto the end of the TV cable assembly indicated by the red arrow in the picture a right. This "housing" snaps into a metal mounting bracket shown by the yellow arrow. This bracket is typically mounted to the engines intake manifold.

 

The inner wire cable (indicated by the blue arrow above) located inside the TV cable assembly typically has a tear drop shaped plastic connector at one end and a metal connector at the other. The "tear drop" connector (Green arrow above) snaps onto the "carburetor linkage". The metal connector at the transmission end is connected to a rigid wire link (shown in picture # 2 earlier) which is in turn connected to a rotateable throttle lever and bracket assembly (orange arrows at left). A fully extended TV plunger is not able to touch the activating cam surface of this lever when the cable is in its non adjusted full "out" position.

Let's run through the stock TV cable set up procedure!

 

General Motors Corporation is very specific when describing the correct method for "Setting" the TV cable system. The most "common" factory TV cable design has a metal "D" shaped button built into the housing of the cable assembly located where it snaps into the mounting bracket on the intake manifold.

There are other TV cable designs but all of them are designed to establish the proper starting and ending relationships of the TV system components. We will explain what the proper starting and ending points are once we've walked through what happens when we execute the factory TV cable "set" procedure.

The manual states the TV system is set correctly when the TV lever touches the face of the TV bushing at the same time as the engines "carburetor linkage" is at its wide open throttle position and the accelerator pedal is fully depressed.


 

Once everything is connected correctly, the first step of the factory TV "set" procedure is accomplished by first pushing in on the TV cable housings "D" shaped cable slider release button, then pushing the cable slider towards the transmission until it stops as far into the cable housing as it will go. Release the "D" button to hold it in this preset position.

 

 

By sliding the "cable slider" back into the cable housing like this, you're increasing the actual length of the TV cable's outer sheath that's located between the transmission case and the TV cables mounting bracket located on the intake manifold. As the cable sheath gets longer between these two connection points, it will preload the TV plunger into the TV bushing as shown in the picture at the left.

 

 

The manual states you properly "set" the TV system by manually rotating the "carburetor linkage" to its wide open throttle position. However, as you attempt to rotate the "carburetor linkage" towards its wide open throttle position, the TV lever quickly comes into contact with the outer end of the TV bushing as shown by the blue arrows in the picture below.

 

 

The TV lever stops at the TV bushing well before the throttle reaches its wide open throttle position as shown by the orange arrow in the picture above. Notice the TV cable slider position as indicated by the white arrow.

 

 

You must "force" the "carburetor linkage" to its full W.O.T. position as shown by the orange arrow in the picture to the right. This action has "forced" the TV cable "slider" mechanism to "ratchet" out to its correct "set" position! The white arrow in the picture at right shows the TV "slider" in its new extended position.

 

 

We have just established the correct wide open throttle relationship between three key TV system components.

  • First, the "throttle linkage" is at W.O.T. (Orange arrow above picture)
  • Second, the TV cable slider mechanism is correctly positioned. (White arrow above picture)
  • Third, the TV lever is positioned up against the TV bushing as shown by the blue arrows in the picture at the left.

As long as the vehicles accelerator pedal will cause the "carburetor linkage" to rotate to its full W.O.T. position; these three components will always be in their correct relative W.O.T. positions whenever the vehicles accelerator pedal is depressed fully!


Experience has taught us to go one step further and check to be certain the vehicles accelerator pedal will rotate the "carburetors linkage" to the wide open throttle position. 60% of the vehicles that come into our shop do not!

 

 

When the General Motors procedure is followed, it establishes the correct wide open throttle position of the TV plunger when the vehicles accelerator pedal is fully depressed. Nothing else is supposed to be done! This wide open throttle position "set" is designed to calibrate the relationship of the vehicles accelerator pedal mechanism to the TV system. This position will then automatically establish the correct working relationship between the TV system components as the "carburetor linkage" rotates back to its idle position.

What happens when the "carburetor linkage" is permitted to rotate back to the idle position is at the very heart of understanding these transmissions TV cable systems! As the "carburetor linkage" is allowed to rotate back to idle, the distance the inner wire of the TV cable is allowed to travel back out is the sole determining factor of where the "starting position" of TV valve will position in the TV bushing.

Watch how this happens in the following pictures


 

When we left off last in our effort to "set the TV cable up, we were holding the "carburetor linkage" at is wide open throttle position. As you allow the "carburetor linkage" to rotate back to its normal "idle" position, this movement also permits the TV cable's inner wire cable to "follow". This in turn permits the TV lever to rotate away from the face of the TV bushing and the TV plunger to also follow to its starting position inside the TV bushing. The TV plunger will end up partially "preloaded" in the TV bushing at an exact point which was "pre-engineered" by General Motors when the system was designed. This starting "point" is determined entirely by the distance that the wire cable was allow to return!

 

 

The picture at the left and the picture above show this particular factory TV cable adjusted to "its" correct starting point. Pay particular attention to the picture at left which clearly indicates the TV valve is being position to a part way in the available travel inside the TV bushing. It is not longer at the full out position. Once you understand what just happened, you will then understand that the starting position of the TV plunger will always be determined by the amount of return cable movement from the previously set correct W.O.T. position! "Carburetor linkage" design determines the distance the inner wire cable is allowed to travel on its return to idle.

 

 

Once the TV cable is "set", the TV plunger start position also positions the TV spring which in turn positions the TV valve.

Now would be a good time to explain that when the transmission is operating, there's a certain amount of hydraulic pressure preloading on the three components, TV plunger, TV spring and TV valve. This does effect their relative starting positions so our pictures and illustrations are positioned as they would be in the valve body when it's operating. We will not spend time in this article discussing hydraulic pre load because it will just confuse the issue. This article was written to help people understand how the manipulation of the TV plunger, TV spring and TV valve by the TV cable system, controls all of the vital functions of these transmissions. However, we will explain latter in this article how to use a pressure gauge to guarantee that the TV valve is positioned correctly at its starting idle position and offer solutions how to fix it if it's not.

Everything written in Part One is written for the purpose of making the following point: The actual starting position of the TV valve in the valve body is determined by two variable factors, only one of which can be controlled by the builder of the transmission, no matter who the builder is! The actual position of the TV valve at idle is determined by the length of the TV spring and the distance the TV cable travels as the "carburetor linkage" is rotated from its correct W.O.T. position back to the idle position.

A TV spring with a specific length will be installed when the transmission is remanufactured. The distance the TV cable is allowed to travel while going back to idle is determined by the "carburetor linkage" in the specific vehicle the transmission gets installed into. For any TV system to be "correct" there must be a very specific relationship created between these to elements.

Only the installer can determine whether the system is calibrated correctly. This can only be done after the transmission gets installed into the vehicle and only the installer can adjust this relationship if it's not correct!

 

 

Question: So what is the correct starting position of the TV valve?


Answer: We've never seen anything specifically published about this critical
relationship but years of working with this system has convinced us that the TV valve needs to be positioned right at the edge of the TV feed orifice when transmission and the linkage is at idle. This is our opinion, but logic and years of experience keep confirming this starting position to be ideal
for most vehicles.



Summary of Part one

"The TV cable used on the 4L60 (transmission AKA THM-700-R4) controls line pressure, shift points, shift feel, part throttle downshifts and the detent downshifts."

Everything up to this point in the article boils down to one specific point: The TV valve and one orifice feed passage control everything about operating pressures and transmission behavior!

The position of the TV valve is controlled by the length of the TV spring and the distance the TV cable is allowed to return while the "carburetor linkage" returns from the correct W.O.T. setting to the idle position.

 

Once the factory "set" procedure has been followed, the installer can easily determine whether the TV cable system is set up and responding correctly using the following methods and tools.

  • Have a helper depress the accelerator pedal all the way and hold it there. You should attempt to rotate the "carburetor linkage" further by hand. If it rotates further, your accelerator pedal cannot activate the TV system to its wide open throttle position and needs to be fixed.
  • Manually hold the "throttle linkage" to the wide open throttle position and confirm that the TV cable cannot be pulled further. This confirms the TV plunger is fully buried.
  • Temporarily connect a 0-300 PSI pressure gauge to the transmissions pump diagnostic port located above the manual shifter shaft. (Insert a picture) We have found seven feet of hydraulic hose should allow the installer to have the pressure gauge in his hand while working at the carburetor or fuel injection location. Make sure the engine is warmed up and off the fast idle cam or circuit. With the engine at normal idle, the pressure should be at minimum regulated pressure or slightly higher. You can determine whether the TV valve is positioned correctly or not with the next two tests. Write down the minimum observed pressure at idle, disconnect the TV cable. If the pressure drops, if is normally safe to drive the vehicle. If drops a lot (over 3-4 lb.), something is not correct in your system. This condition is normally safe to drive but the transmission may perform unsatisfactorily. We call this condition, Long Spring Syndrome (LSS); its cause will be discussed in depth in Part three "illnesses". If pressure didn't drop when you disconnected the TV cable, reconnect the cable; gently pull the inner wire cable without moving the throttle linkage. If you can gently pull the cable any distance without instant pressure rise, something is seriously wrong and you should identify the source of this condition which we call Short Spring Syndrome (SSS). This is potentially a very dangerous condition that can lead to rapid clutch failure. The causes of this condition will be discussed in detail in Part Three - "illnesses"

Installers responsibilities

  • The TV system controls line pressure, shift points, shift feel, part throttle downshifts and the detent downshifts through one orifice
  • Everything the transmission needs to know is transmitted through the TV cable to a single orifice which is controlled by the TV valve. Determining whether this is correct can only be done by the installer!
  • The factory "set" procedure establishes the correct wide open throttle position relationship between the "carburetor linkage", the "slider" position on the TV cable and the TV lever poitioned against the face of the TV bushing. This can easily be determined by the installer, just hold the linkage in its W.O.T. position while pulling on the TV cable, if the TV will not pull further, it's correctly set at W.O.T. position.
  • What the factory "set" procedure (Manually rotating the 'carburetor linkage by hand) doesn't confirm is whether depressing the accelerator pedal fully also causes the "carburetor linkage" to rotate to its W.O.T. position. Rotating the "throttle Linkage by hand is not the same as causing W.O.T. to occur by depressing the accelerator pedal fully. If the accelerator pedal doesn't cause the "carburetor linkage" to fully rotate to its W.O.T. position, the TV Lever will not fully bury the TV plunger, even it the factory "set" procedure was absolutely performed correctly. Proving this relationship is correct can only be done by the installer. This "check" normally requires two people, one to depress the accelerator pedal, the other to check if the "carburetor linkage" is completely rotated to W.O.T
  • The distance the TV cable travels as the "carburetor linkage" is allowed to rotate back to idle, determines the "starting" position of the TV valve.
  • There are to "illnesses" that the TV system can have which will have a direct affect on these transmissions. Whether one of these "illnesses is present or not can only be determined by the installer after the "set" procedure has been performed and can only be determined by the use of a pressure gauge screwed into the pumps diagnostic port. The builder of the transmission has nothing to do with determining whether one of these "illnesses" are present. This can only be determined by the installer.

 

Our company highly recommends that our customers allow us to stay closely involved all the way through the installation, set up test driving and warranty authorization. We do this so our customers can feel very comfortable with getting this transmissions installed correctly. With Th-700R4 and Th-2004R transmissions, the installer is every bit as responsible for the successful operation of these wonderful overdrive transmissions as the builder of the transmission! No amount of finger pointing, screaming denials, claims of ignorance or quoting of other sources, (like well meaning brother in laws or your favorite mechanic at Bubba's transmission shop) by the installer will change this fact! In all stock applications, especially in custom applications, these transmissions are not Plug and Play! These transmissions require the installer to set, check and correct the TV cable system (if needed)!

If your transmission behaves inappropriately, exhibits delayed pressure response or has elevated idle pressures, only you the installer, can fix these issues!

Your transmission builder has nothing to do with the distance the TV cable travels as it returns from wide open throttle to the idle position! For this relationship to be correct the TV cable travel distance must be matched to the length of the TV spring by the installer!

Part Two will explain where the TV fluid goes once the TV valve directs it into the TV feed passage!



© Bowtie Overdrives 2004