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Originally Posted by MurphyMax
I have a "thunking" noise issue in my '97 Windstar 3.8L (71k miles) that presents itself only above 40 mph. The "thunk" in the drivetrain can be felt and heard, sort of like hitting a small bump in the road. It repeats once a mile or two, and sometimes it will thunk two or three times in a row. I was thinking it might be the overdrive gear cycling in and out, or maybe the A/C compressor clutch. While the RPMs do change slightly during the thunk, the RPMs don't jump nearly as much as when the OD is manually shut off. After reading some of the comments in this forum, could this be coming from elsewhere in the drivetrain, like motor mounts or CV joints? Is anyone familiar with this problem? The engine runs fine and the suspension seems solid. (P.S. I forgot to mention that the speedo fluctuates fairly wildly above 40 mph as well, buit not sure this relates.) Thanks!
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This is a guess as I don't even know if it covers 97s
It might be the VSS (Vehicle speed sensor). I believe the transmission uses this data to decide what gear it should be in. It may also feed the speedo.