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Part Two: TV feed fluid

The purpose of Part two is explain where the TV feed fluid is directed to once it enters the TV feed passage. Once explained, it will become clear that these transmissions need a TV system that buries the TV plunger inside the TV bushing at W.O.T.and positions the TV valve at the correct position inside the TV bushing at idle!

This graphic is a reasonably accurate representation of a Th-700R4 Throttle Valve (TV) system in the correctly set factory position for a Chevrolet C/K pick up with a 350 CI engine with TBI

Quoting GM again, not to be redundant, but to again focus our attention on the functions controlled by the TV cable system!

"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 (TV) cable is similar to the combined functions of a vacuum modulator and a detent cable."

The five activities listed above are influenced by the TV valve movement. As the TV Valve starts feeding TV fluid into the feed orifice (blue circle), it's directed to three places as it enters the passage shown below (area shown in transparent red). TV fluid passes through the spacer plate (green circle in bottom picture) into the transmissions pressure boost circuit where it instantly boosts line pressure. This fluid also feeds to the circuits that control up shift timing and feel called MTV. The transparent red fluid also feeds the port shown (purple circle in top picture) directing TV fluid back into the TV bushing and TV plunger (See heavy throttle upshift and downshift control section below) to assist plunger movement so the pedal pressure isn't excessive. This fluid also feeds the part throttle (Green arrow) and detent (Yellow arrow) ports in the TV bushing.

 

The picture below has a hole (Green circle with explanation in the spacer plate TV fluid flows through after entering the passage in the valve body indicated with the transparent red color in the picture above. This fluid

 

 

There are plenty of "cross over" effects between these three circuits which we will not even attempt to explain since this article is only meant to provide an overview of the TV System and its effect on these transmissions. When you read the following few paragraphs, it should become clear that all activities of these transmissions are effected by the TV valve system. The following information is offered to help the owner / installer understand why the TV valve must have a correct starting (idle) position and a correct wide throttle position, not just one or the other. By the end of Part Two it should be obvious that some aspect of the transmission management will not be correct unless the TV system is managed correctly through its full spectrum of movement!

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The TV feed fluid is directed into three control circuits which subsequently control or effect the following:

 

LINE PRESSURE MANAGEMENT

Line pressure management and upshift and downshift timing and feel are fed directly as fluid enters the feed orifice above.

  • Line Pressure: The TV feed passage (green circle on spacer plate above) directs TV fluid to the transmissions pressure boost system, via the line bias valve. The transmissions line pressure will rapidly rise as fluid enters the TV feed passage and will typically have it's full boost effect by the first 65% of accelerator pedal movement off the idle position! The need for instant pressure rise to off set any increased input of torque into the transmission by the engine is CRITICAL!

 

UPSHIFT AND DOWNSHIFT TIMING AND FEEL

 

  • Upshift and downshift timing and feel: This fluid circuit is called modulated TV up. The MTV up control circuit is designed to appropriately delay upshift timing with increases in accelerator pedal application. The further the accelerator pedal is depressed the later and firmer we want the transmission to up shift. This circuit has its major effects on up shifts timing and feel occur during light and medium accelerator pedal settings and less "noticeable" effects after the TV system passes 1/2 throttle. However, this circuit does effect less obvious things all the way from idle to W.O.T.

    The second circuit is the modulated TV down circuit, this circuit starts having its affects during heavy to full throttle settings and has little or no effect at light to medium throttle settings. This circuit works hand in hand with the next topic; the part throttle and detent ports located in the TV bushing.

 

Heavy throttle upshift and downshift control

The fluid entering the TV bushing (left picture above) comes from the TV feed orifice (blue circle-right picture) which is being fed by the movement of the TV valve over the feed orifice. This fluid progressively increases in volume and pressure as the TV valve is activated. We've tried to illustrate these by color and transparency change.
  • Part throttle port and detent port in the TV bushing. As the TV valve feeds TV fluid into the feed orifice, part of this fluid is fed to the TV plunger through a port in the TV bushing. This fluid increasingly assists in moving the plunger against TV back pressure to prevent the throttle feel from becoming too great. As the TV plunger moves down the bore of the TV bushing it will eventually pass over the part throttle port. The TV plunger must travel 50% to 75% of its full available travel distance inside the TV bushing before it reaches this port. This fluid effects heavy throttle upshifts and under the right circumstances will cause downshifts. These downshifts are normally a single gear.

    Detent port fluid feed. The TV plunger must be moved all the way to the end of its available travel to allow TV fluid to enter into the detent port of the TV bushing. Fluid entering this port does a number of things but we feel one of the most important is its effect on two gear down shifts (sometimes three gear)! We are continually amazed at how many people with these transmissions don't realize their transmissions are capable of doing double gear downshifts because theirs never has. TV fluid can only enter this port when the TV plunger is completely buried at W.O.T. For the part throttle and detent down shift fluids to have their proper effect on downshifting and upshifting, the TV plunger must move from the systems 50% throttle position to its W.O.T. position!

 

Line pressure management has its greatest effects during the first 50 to 65% of TV valve movement. Light and medium throttle up shift timing and feel is managed during the first 50 to 60% of TV valve movement. Wide open throttle upshifts are heavily influenced during the last 30% of TV valve travel. Part throttle port in the TV bushing only gets fed after 50% to 75% TV plunger movement. The fun two gear downshifts are initiated by TV fluid directed through the TV bushings detent port. This port is only fed when the TV plunger is completely buried inside the TV bushing at W.O.T.

 

Each of the three TV fluid feeds described in the preceding paragraphs has its own specific effect on these transmissions during a designed range of movement as the TV plunger moves from its idle position to wide open throttle position. Line pressure is controlled during the first half of movement, up and down shifting during the full range of movement and the part throttle / detent ports effects only occur after half throttle to W.O.T. Once you understand that "all positions" have effects on specific transmission management activities, you will then understand how important it is to establish a correct full movement TV cable system.

 

All the information provided in Part Two is to make one point crystal clear. A correct starting position of the TV system and a correct ending position of the TV system must be established for all functions to perform correctly! Getting the correct (W.O.T.) ending position is very easy to do using the factory TV cable adjustment procedure. The trick is to get the correct starting position of this system. The correct starting position is determined by two factors, length of the TV spring and the distance the TV cable is allowed to return once the correct TV cable W.O.T. relationship is "set". A method to fix the relationship between these two factors in stock applications will be offered at the end of Part Three.

 

Summary of Part Two

For one of these wonderful overdrive transmissions to operate properly, the TV system must start with the TV Valve positioned at the edge of the TV feed orifice with the accelerator pedal in its idle position and end up with the TV plunger completely buried inside the TV bushing when the accelerator pedal is fully depressed!

Using the following procedure, the installer can easily check to be sure the TV plunger is positioned correctly relative to the feed orifice, moves instantly with even slight applications of the accelerator pedal and confirm that the TV plunger is mechanically positioned against the face of the TV bushing at W.O.T.

This procedure will require the use of a second person. Connect a 0-300 PSI pressure gauge to the transmissions pump diagnostic port on the drivers side of the bell housing. Perform the TV "set"procedure, start the engine and check for the following:

Click here for an explanation of how to hook up a 0-300 PSI gauge

  • First; your pressure gauge should read the same with the TV cable connected or disconnected. A slight pre-load of 2 to 4 lbs at idle will not normally produce any adverse effects.

 

  • Second; when the TV cable is connected and adjusted correctly, even slight movements of the TV cable where it exits the cable housing should produce instant pressure rise.

 

  • Finally; have your assistant depress the accelerator pedal fully and hold it there. Using a set of pliers, try to pull the TV cable further out. If you can not, then this demonstrates the TV Lever is firmly against the face of TV bushing at this W.O.T. position.

 

  • If your system doesn't pass these tests, they can be fixed! The fix process may require the removal and reinstallation of the transmissions pan a few times but it is not that difficult to do. Delayed pressure response can be very serious and needs to be addressed A.S.A.P.

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Part Three will explain how to determine which "Illness" your transmission has and how to cure the problem.

© Bowtie Overdrives 2004