While going thru drive-up at a local burger joint,,, the engine made a loud thump, power steering and air conditioner quit. I pulled out of the line and pulled over to a parking spot , and found the outside half of the harmonic balancer laying on the subframe. The belt had somehow re-installed its self around the rest of the pulley allowing me to at least drive back home, slowly. I got it to the ford dealer the next day and they installed a new one for way too much money.
Ever I have owned the car,, there has been a slight vibration beginning at 69 mph, and lasting till 71 mph. I didnt notice this when I purchased the car because,, well,, city police dont like people doing 70 thru town for some reason..
Anyway, I had hoped that the harmonic balancer was to blame for the vibration, but unfortunately, it wasn't , so now I am looking at a new driveshaft..
It is strictly speed related not rpm related. I have downshifted out of overdrive , keeping rpms the same and vibration goes away, tires have been rebalanced and rotated, no help.

 

sorry 08 convertible 4.0l auto

 

how do you know for certain it's the driveshaft and not the motor or trans mounts or a tire/wheel that is slightly out of balance?

 

short answer, been there done that
Tranny service in April with new mount also

 

driveshaft fixed it,, mostly...which proved my diagnosis...