Posted 30 March 2014 - 07:25 PM
Torque bind can be found in both auto and manual transmissions. The causes are similar in nature, but the parts affected work very differently and exhibit different symptoms.
The general symptom is the car lurches or jerks when trying to make tight turns on asphalt, and you generally have to use more throttle to overcome the forces causing the bind and complete the turn. Manual transmissions generally have to be warm for this to occur, such as after driving for several minutes on the highway.
In the manual transmissions the viscous coupling unit in the center differential overheats and locks up. This forces the front and rear driveshafts to turn at the same speed when they need to turn at slightly different speeds.
Normally these differences in speed between the front and rear axles will be absorbed by the center diff. But when the diff locks it forces those differences to be absorbed by the tires, which they do by slipping on the road surface. On dirt or gravel its not noticeable, but because asphalt has a higher friction co-efficient the tires grip and cause the drivetrain (axles, driveshaft) to soak up the difference until it binds.
Early stages could be described as a clunk feeling, or you may even hear a clunk if you have any worn out suspension parts like ball-joints or tie-rod ends.
The primary cause is uneven tire wear, and mismatching tires front to rear. I.E. 2 New tires in the front, 2 old worn tires on the back. Or buying 1 new tire because of a blowout, and the other three are 50% worn. This causes the front and rear driveshaft speeds to differ ALL of the time, which causes the viscous coupler to have to work ALL of the time, and leads to overheating of the fluid in the unit. A difference in axle speeds causes the viscous fluid to heat up, which is what makes the unit work, but when the difference is too great it causes the fluid to overheat, which will make the unit lock up entirely.
Fixing this first requires making sure all 4 tires match in size, brand, and wear within 1/4" of circumference from least worn to greatest worn.
Sometimes just getting new tires will alleviate the difference in speed between front and rear axles and prevent overheating of the fluid in the unit, which will keep it from locking. This will only happen if caught at a very early stage of torque bind.
The next step is to replace the center differential unit. The viscous coupler is not serviceable by itself, though the unit can be replaced. It's just much easier to pull the center diff and put a whole new center diff back. These don't go bad often so used is a good option. New units are in the $400+ range last I checked.