Depending on the manufactur date. If it looks like a round actuator, it is a spring, If it has what looks like a piston, it is a spring loaded hydraulic damped device. and they wear out after a few hundred thousand miles. You will not know it is worn out till you change the belt. The tensioner will over extend and only if you are very lucky will it pop back in and allow you to compress it far enough to put the belt back on. Too much force will bugger up the allen bolt you are using to move the tension pulley back. If it does pop back, it will continue to do it's job till the next belt.
Last edited by bherde; 03-13-2010 at 10:12 AM.