Lock-out, missed shifts - Mystery solved?

[hawk]

Disclaimer - The following is simply a theory. Treat it as such, i.e., devise your own experiments to prove or disprove it.

Note- I refer to Maximum Motorsports instructions below, but they generally apply to any brand. These parts are nearly identical between various suppliers, and in fact I used UPR components. But MM has the best online instructions I have found.


I have had occasional problems since I bought the mach with shifting - the infamous "2nd and/or 3rd gear lockout" at the track, and sometimes sometimes difficulty going into reverse in daily operation. I have driven standard transmissions for 30 years on cars, trucks and bikes, and have never had these kinds of problems.

I had been blaming the TR3650, and had taken steps to work things out, namely installing a Hurst Billet Plus shifter and changing the fluid to RP Synchromax, filling to 3.2 qts.

These improved operation, but I still had the same problems periodically. I was getting really tired of blowing track runs because of the car not wanting to go into gear.

I bought an aftermarket clutch quadrant and firewall adjuster a while ago, thinking one day if the stock plastic pieces broke, I would have something on hand to fix the car.

After missing 2nd gear twice last time at the strip, I was fed up and decided to try the quadrant and adjuster, figuring it can't hurt. Several people had said that this mod would eliminate the shifting problems, and gave reasons that generally amounted to "the plastic quadrant flexes" making clutch operation inconsistent.

While I was doing the upgrade and studying the parts I removed, the real cause of the problem became apparent to me. My analysis follows:

There are two factors - first, stock clutch operation is vague because of the firewall cable mount. It has a 1" long rubber bushing between the cable end and the plastic cup that mounts to the firewall. This bushing is designed to compress, and apparently soften the feel of the clutch. I am convinced that it is designed to reduce NVH (noise, vibration, harshness), but it also contributes to vague clutch pedal feel. This bushing is eliminated when the aftermarket firewall adjuster is installed. For a very nice description and photos of this part of the mod, see the Maximum Motorsports adjuster instructions (PDF)

The second (and most important) factor is the self-adjuster mechanism built in to the quadrant assembly. Again, I refer you to Maximum Motorsports for the best photos of the quadrant install in their installation instructions (PDF).

It is helpful to understand how the stock mechanism works. Basically when the clutch pedal is up, i.e., clutch engaged, the adjuster pawl hits the stop and is rotated away from the quadrant. The quadrant is pulled against the cable with a large spring. When the pedal is first depressed to disengage the clutch, the pawl moves up and off the stop, and can then rotate so the teeth of the pawl can mate with the teeth on the quadrant. The quadrant doesn't move before the pawl locks into it. As you push the pedal further down, the quadrant is then rotated against the force on the cable due to the diaphragm springs in the pressure plate, and the clutch is disengaged.

As the clutch wears, or the cable stretches, the quadrant will come to rest further back, but since the pawl is pulled away at the top of pedal travel, it should theoretically contact lower teeth on the quadrant, when the pedal is depressed, thus preserving the same pedal action as the components wear. This is fine and works ok for typical street use when "granny shifting". For a mass-produced car, it is a decent design that eliminates one little maintenance item for folks whose single use of the car is to drive to school, work, or Piggly- Wiggly.

Enter the enthusiast - When the pedal is stabbed quickly, the pawl may travel further along the arc of the quadrant before it can rotate in and lock teeth with the quadrant. It may also skip over a tooth or two before it settles in. The result of this is inconsistent clutch engage/disengage height. Note that if the pawl does hit higher up, the pedal will need to go further down in order to release the clutch, but it can only go so far until it hits the stops or the firewall.

Even if it disengages before the pedal stops, it is still lower that the last time, so if your muscle memory in the left leg is expecting the clutch to be disengaged at a certain point, and only pushes the pedal to that point, you may find the tranny won't go into gear because the clutch is still dragging due to the inconsistent pawl-quadrant engagement.

If you are having trouble visualizing this stuff, I encourage you to just climb under the dash and observe the action of this mechanism. It is far easier to see than it is to explain.

I believe anyone who drives their mustang with any kind of performance in mind should do this mod immediately. I wish I would have done it the day I brought my car home.

The major downside to changing to a firewall adjuster is that you will now have another maintenance item to perform, i.e. you will need to periodically screw the adjuster out to compensate for clutch wear. There is also a bit more noise from the pedal mechanism - it clinks or clunks when the pedal returns to the top of travel.

The upside is greatly improved pedal feel, no transmission "lockout", consistent shifting, and better ET's. Oh yeah, and you will never have to worry about the plastic crap breaking someday. Darn sure worth an hour's work and less than $100 worth of parts!