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It Happened One Night

It takes a dame to get some help. Some guy I know was driving his 99 Mercury Sable Le about 30 mph when the speedometer suddenly died. Why did the speedometer suddenly die? The car continued to run albeit a bit sluggishly. He then stopped and turned off the ignition. He had been driving the car for only 5 minutes and the heat gauge was at the normal midpoint where it remained for almost an hour before declining. Why did the heat gauge take so long to decline? There also is a precedent to this puzzle. A couple weeks ago, he was driving the car 55 mph when the speedometer fluctuated wildly from 0 to 55. He stopped and after 10 minutes resumed driving without a hitch. Now he’s had the 30 mph speedometer problem. In neither instance did the engine repair light appear. What’s going on with the speedometer and the heat gauge? Where are those Magliozzi brothers when you need them? Please help me solve this Puzzler.

Did the transmission shift normally during all this? If not, then everything points to a problem with the vehicle speed sensor; The transmission doesn’t know how fast you’re going and so it doesn’t shift properly, which makes the engine run faster than normal, which makes it heat up faster than normal. And of course, a bad VSS would make the speedometer display strangely.

Yup, this is not really very mysterious at all.
It simply sounds like a case of a bad VSS.

The temp gauge in some cars is designed to be less sensitive in the normal operating temperature range, which could be why it hung around the midpoint so long if the engine was fully warmed up. An engine is made of a few hundred pounds of metal and can take a while to cool!

My 98’ Taurus acted the same way on me several months ago.

I can’t rememeber the name of the part that was replaced though