Throttle Valve Cable Bracket
This project was an outgrowth of my original Cruise Control Throttle Bracket. It was required when I decided to swap in a TH700R4 transmission in place of the TH400 transmission in my 1975 GMC Suburban to gain the benefits of a modern overdrive transmission. This page started life as a small section on my 1975 GMC Suburban page, but quickly took on a life of it's own. This was the final problem I had to solve so I could complete the installation of the new TH700R4 transmission and hook up the throttle valve (TV) cable to the carburetor linkage.
What is a TV cable? The TV cable goes between the throttle linkage on the carb and down to the transmission to control the overall pressure inside the transmission which in turn drives shift firmness and various other details. It is how the transmission knows how much throttle you are giving the engine so that the transmission can do the right thing. Do not make the mistake of thinking the TV cable is just a simple kickdown cable as used on other transmissions! They are a critical and precision piece that must be properly hooked up properly. Failure to get the TV cable installed and adjusted right will result in your transmission failing to shift properly and can even cause the transmission to burn up ($$$) in very short distances - as in less than 100 miles. The transmission is calibrated to work with the TV cable working a certain way, and it is very precise. Getting this installed and adjusted correctly will be one of the single most important steps to completing my transmission swap.
Background and Research
I currently run a Holley carb with a custom throttle linkage bracket so I can hookup my cruise control, and the only TV cable bracket I could find for a Holley carb did not have a provision for a cruise control cable. To complete this swap, I had to figure out how to add a TV cable hookup to things, and it would involve some custom bracket fabrication.
I first found out that I needed a bracket to mount on the carb linkage to hook the TV cable up to - the original Holley carb linkage was designed for use with various kickdown linkages and cables, and the geometry was not correct for the TV cable. TCI makes a "Cable Geometry Corrector" (Jeg's part #890-376715 for Holley carbs, part #890-376710 for Edelbrock carbs) that does exactly this and it's a simple bolt-on part. From what I can gather, the distance the TV cable has to move is not quite as simple as it seems, and it has to travel some pretty precise distances with slightly more distance being covered from idle to 1/4 throttle than from 1/4 throttle to full throttle. Figure 16 in the second page shows this clearly, the mounting pin must be located such that at 1/4 throttle there is a 90 degree angle between the TV cable and the line formed between the throttle shaft and the TV mounting pin. Think about the geometry long enough and it makes sense - the mounting pin travels in an arc as the throttle is opened. The TV cable converts this to a linear pulling motion. Since the initial 1/4 throttle is almost in line with the TV cable, it pulls the cable more that the final 3/4 of the throttle travel where the mounting point moves steadily up and away from the straight line of the TV cable on an arc. I believe this has the net effect of raising the transmission pressure more quickly right off idle - that's why any mis-adjustment in the TV cable can be so bad for the transmission - the pressure stays too low, clutches will slip, shifts will be slow, and a lot of heat will be put into the transmission in very short order. Read the two sets of instructions in the pictures below for more details, but basically, you need to get the right adaptor from TCI to make this all work or fabricate one that does the job for you. Remember, my goal was not to fabricate everything from scratch, it was to get a TV cable hooked up with something similar to my existing throttle cable and cruise control cable bracket. So, I opted to buy the TCI Cable Geometry Corrector and focus on fabricating the bracket to hold the cables.
Next, I had to figure out how to get the TV cable mount point integrated with my throttle linkage. For that, I decided to purchase TCI's "TV Cable Bracket" and measure it for reference points to use in making my own bracket. If need be, I can use this bracket while I am still fabricating my own - I just won't have the ability to use my cruise control until I finish building my custom bracket. I bought and measured Jeg's part #890-376705 which is for Holley carbs, if you have an Edelbrock carb you will need to get part #890-376700 and do your own measurements.
The critical dimension I needed to know was where the TV cable mount piece was located in relation to the rear mounting bolt for the carb so that I knew where to position it on my new custom throttle bracket. That is one of the two bolts I use to mount my bracket, and the TCI bracket uses the same bolt as it's mount mount on the carb. By taking measurements from there, I can accurately position the mounting pieces on my bracket and it should all just work. The TV cable itself is adjustable, so it can take up small differences in the mounting bracket, so I believe that if I can get down to 1/16" accuracy in my bracket, everything should be OK in operation. Here are the dimensions I measured on the TCI bracket using a ruler and a carpenters square.
The trick will be that I need two adjustable mount points for my throttle and cruise control cables, plus a non-adjustable mount point for the TV cable, and a mounting point for the throttle return spring. I think I can accomplish that by adding the TV cable mount to the bottom of the existing bracket design with a small "U" shaped slots in the areas where the adjusting bolts for the throttle cable mount points need to pass through. If needed, I can also add small pieces at the edges that are welded to the main bracket to provide stability.
At some point, I need a TV cable to do sizing and measuring on the end of the TV cable where it mounts to the bracket. I may need to put a specific shape and size on it to make it all work properly. I also need to make sure you can access the adjuster button on the TV cable once the entire assembly is installed on the car.
Page last updated 01/02/2009 01:51:39 PM