Saturday, September 5, 2009

Older Holley 4150 and 700R-4 TV Cable Problems


I'm in the process of mocking up my re-built Chevy small-block to find unanticipated problems and have been very successful: I've run into a big problem.

I'm using a Weiand 142 blower "kit" P/N 6500-1 and Weiand's recommended Holley carb, P/N 80572 (it's a boost-referenced Holley 4150; tech info can be found here and a HOWTO on modifying one can be found here).

The problem is that I'm also using a 700R-4 transmission and the Holley carb, according to the instruction manual, " ... is NOT designed for use with ANY automatic overdrive transmissions." (Of course, they only disclose this in the installation instructions; it's mentioned nowhere else. Farging bastages.) This incompatibility with an AOD is my big problem.

Obviously, what they are really referring to is not a Hatfield-McCoy (or PC-Mac) kind of incompatibility, but the lack of any appropriate bracketry associated with the throttle cable attachment point so that a TV cable can even be attached. This also means that Holley's "solution" (P/N 20-121, their 700R-4 kickdown throttle bracket) has NOTHING to mount to. Even if it did (and have one), the geometry is all wrong, only pulling the TV cable through a 30 degree arc instead of the required 78 degrees. That may be why others have had a problem with their 700R-4s even when using the Holley bracket.

Working from info at Sumner Patterson's site, I found I could modify/construct a bracket to add the 700R-4 functionality that Holley left out. You'll need to construct a diagram of the sweep of the TV cable stud following the drawing, but Sumner's directions weren't clear to me at first. You'll need a compass, a protractor and a ruler that reads in 1/10ths of an inch. The TV cable stud moves in an arc whose center is 1.094 " to 1.125" from the throttle shaft center. From closed throttle to WOT (wide open throttle), the arc is 78 degrees. First, mark a point on some card stock; this is the center of the throttle shaft. Draw a vertical line through this point. Set our compass between 1.094 " to 1.125"; Sumner used 1.1" which is about the middle of the two and draw a circle. This circle represents the arc that the TV cable stud follows. Using your protractor, make a line 55 degrees from the vertical line just like in the drawing. Now make another line 78 degrees from that just like in the drawing. One reason the drawing is confusing is that you'll never see the cable sitting on that vertical line; that's just used as a reference to get the other lines drawn in the correct location. To finish, make a horizontal line at 90 degrees to that vertical line about 1.25" below the shaft center point. That horizontal line must be held parallel to the bottom of the carburetor. Cut the card stock like Sumner shows here and read his detailed version of the process here.

To work through the details, I constructed a piece of card stock with the correct geometry , located the correct placement of the TV cable stud, and mocked up a solution. I noticed that the Holley bracket, if installed according to their directions on a newer 4150 carb that has an extended bracket will not track the correct geometry. I will simply cut up the otherwise-useless Holley 20-121 bracket and welded it to the carb in the appropriate place. Based on my mock-up, the TV cable stud will move through the correct geometry from closed to WOT. In theory, I've solved my problem, but there's a lot more to understand.

It's also important to position the TV cable bracket at the back of the carb correctly in relation to the stud on the carburetor. The distance will vary because there are different length cables in use. The way to measure yours is detailed here. The cable needs to run parallel to the base of the carb as well. My next step is to modify the bracket I have to get the correct location. No one said this was going to be easy, but and improperly installed cable will ruin your 700R-4 in short order.

There's a good overview of how the 700R-4 throttle valve works here. A nice FAQ about the 700R-4 here. And a speedometer gear calculator for both the 700R-4 and 200-4R here. It seems the best way to check if your 700R-4 is adjusted correctly is to hook a pressure gauge up to it and observe the readings from idle to WOT. BowTie Overdrives provides a PDF document describing the installation and setup of a 700-R4 or 200-4R transmission plus info on measuring for a driveshaft and wiring a lockup switch and brake relay switch. They recommend a 0 to 300 PSI gauge and 7 feet of hose and a 90 degree 1/8" NTP fitting. The pressure gauge is attached to the direct pump pressure port on the driver’s side of the transmission which is about 3-1/2" above the manual shifter shaft. They don't provide the "full Monty" of the test (or the pressures), but do offer a briefer "field test" on pages 16 and 17. The suggested pressure is 65 to 80 lbs at idle for either transmission; too high a pressure at idle will start you in second gear; too low a pressure will cause it to slip. The pressure should spike when you leave a stop light and if it doesn't, the tranny is slipping. A good discussion is here.

I used the "standard" method:
  1. Depress the adjustment button and collapse the adjustment sleeve.
  2. Releasing the button, move the throttle to WOT; the cable self-adjusts.
  3. Attain adjustment Nirvana.
You raise the pressure by pushing the cable adjuster back into the cable, preferably doing it a click or two at a time. What seems to be critical is the distance the cable travels from closed to WOT and doing it at a steady rate. That's why the geometry is so important; bad geometry moves the cable at an uneven rate and so varies the pressure at an uneven rate causing improper shifting and resulting damage. From 73-87.com: "To raise throttle pressure (and raise shift points, and make "kickdown" more responsive) move the cable housing towards the firewall (away from the throttle linkage), as you simultaneously depress the button on the cable housing, move the cable housing away from the carburetor or (throttle body) to increase throttle pressure. Move the cable housing adjustment a small amount at a time (1 click or 1/16" or so), a small adjustment can often make a world of difference. Naturally, to lower the pressure (and lower shift points, and make "kickdown" less sensitive), move the cable housing towards the front of the truck.

Here is a good discussion of not only adjusting the TV cable, but measuring critical distances including the length of the cable itself.

You should always use a transmission cooler with a 700R-4 because heat is a killer for transmissions. The stacked-plate coolers are superior to the serpentine coolers. I always use the B&M #70264, rated at 14,400 BTUs; it's their biggest one. You can find stacked-plate coolers on Volvos in the wrecking yards. B&M also makes a fan-cooled remote-mount version, # 70297, but you should be able to fab something up for less that the $250 they sell theirs for. Ugh.

Problem solved; the transmission works great!

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