The kickdown linkage in my 1968 Mustang coupe worked fine when I test drove the car, but not for long after. Minutes after money changed hands, the first time I put my foot to the floor on the road north out of Carolina, the kickdown linkage popped.
I drove the car that way for a few months. Unlike later overdrive transmissions that burn up when their throttle valve cables are disconnected or out of adjustment, missing or maladjusted kickdown linkage won’t damage an old three speed Ford C4 automatic. The C4’s vacuum modulator will shift the transmission ok during normal driving. Without a properly installed and adjusted kickdown however, the transmission will not drop from third to second gear under hard acceleration, and during normal driving the two-three upshift comes very quickly. You can still manually downshift for a passing gear, or for a spirited run down a country lane, but engine vacuum alone doesn’t give the transmission enough information to operate at its best.
Rather than a bulky factory rod or pricey aftermarket cable running from the kickdown lever up to the carburetor like many old cars, the Mustang’s C4 kickdown lever is connected to an accelerator arm under the driver’s side toe board via a short cable. Complete factory style replacement kickdown kits are available for less than forty bucks. We installed a Scott Drake kickdown kit, which comes with everything you need, except instructions. No kidding. So, here’s how to install and adjust the kickdown cable linkage on your classic Mustang.
- Pull your Mustang up on a set of sturdy ramps so you have easy access to the driver’s side of the transmission from under the car. Be sure to chock your rear wheels to prevent the car from rolling back on you. Remember, you are messing with the transmission.
- Preassemble the cable. On the transmission end, poke the stud on the arm in the kit through the lower hole in the cable end. Drop the supplied washer over that and snap the c-clip on to hold it together. On the other end, screw the threaded ferrule down about 2/3 the length of the threads on the rod.
- This next part is a little tricky. Underneath the car, locate the accelerator arm near the driver’s side exhaust downpipe at the toe board. Straddle the accelerator arm with the supplied clip, lining up the holes. Sandwich the wavy washer inside the clip and the arm, and insert the stud on the threaded ferrule through the holes. Now rotate the clip down and snap it over the threaded rod to hold everything in place.
- Push the arm on the opposite side of the cable over the rectangular stud coming out of the center of the neutral safety switch. Tighten it down with the supplied lock washer and half inch nut. Do not over tighten it. The arm should be pointing up.
- Move back to the servo cover and remove the lower right bolt. Install the spring anchor bracket from the kit using the servo bolt, placing the bracket’s locating tab inside the groove in the servo cover.
- Install the return spring with one end in the bracket and the other end in the empty hole in the cable end.
- To adjust the cable, with the car off and cold (you probably didn’t need us to tell you that, right?!) have a helper open the throttle all the way (if you’re working solo you can push the accelerator arm if you have a free hand) and determine if you need to adjust the threaded ferrule. Ideally the kickdown lever on the transmission should engage just before WOT.
- Go for a test drive and readjust as necessary.
- While adjusting the ferrule I found it easiest to unbolt the cable from the neutral safety switch and use a pair of pliers to twist the threaded rod in or out of the ferrule. Every time I messed with it the other way the clip and washers would come apart and were cumbersome to reassemble…especially with a hot exhaust in the way after a test drive.
- It’s better to start with the cable a little loose, and adjust it tighter. A too tight cable will pop with an aggressive stomp on the gas…which is how I ended up having to do this project in the first place.
- You’ll likely need to do a few test drives and come back for more adjustments under a hot car. Be careful around the hot exhaust!