I have a 1999 Ford Taurus that has two problems that I hope are not a transmission going bad. First, the overdrive light goes on and off. Second, the speedometer has stopped working. Third, the shifts are jerky sometimes (not always, though). Any ideas? Thank you.

You can go to
youtube.com
Search "Torque Converters Explained "

You will see all about it.
Hope this helps.

Taurus are notorious for tranny problems,get rid of it

Check the help link mentioned below:----

Overdrive light in the dash blinks?

http://repairhelpcenter.blogspot.com/2012/01/overdrive-light-in-dash-blinks.html

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OD light flashing and transmission fluid leaking?

http://repairhelpcenter.blogspot.in/2012/02/od-light-flashing-and-transmission.html

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Also i suggest you to refer these gear shift issues, which are mostly noticed. These are direct links with appropriate solutions to the specific problems of gear ****.

GEAR SHIFT ISSUES

Gear indicator Shows "3" Instead Of "D"?

http://whoisbyme.blogspot.in/2012/07/gear-indicator-shows-3-instead-of-d.html

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2006 Chrysler 300c: Shifter Stuck in Park?

http://technoanswers.blogspot.in/2012/06/2006-chrysler-300c-shifter-stuck-in.html

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1998 Ford Expedition: Will not Shift Out of Park?

http://schematicsdiagram.blogspot.in/2012/06/1998-ford-expedition-will-not-shift-out.html

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Car will not move when in gear?

http://repairhelpcenter.blogspot.in/2012/02/car-will-not-move-when-in-gear.html

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Mercedes-Benz transmission sticks in first gear?

http://technoanswers.blogspot.com/2012/01/mercedes-benz-transmission-sticks-in.html

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Mercedes Benz not engaging in first gear?

http://technoanswers.blogspot.com/2012/01/mercedes-benz-not-engaging-in-first.html

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Chrysler PT Cruiser only goes in First gear?

http://howtobyme.blogspot.com/2011/11/chrysler-pt-cruiser-only-goes-in-first.html

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Car is stuck in park and the shift will not move?

http://howtobyme.blogspot.com/2011/08/car-is-stuck-in-park-and-shift-will-not.html

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2007 Ford Expedition: How to shift car in neutral with Dead Battery

http://www.howtorepairguide.com/2012/11/2007-ford-expedition-how-to-shift-car.html

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2002 Nissan sentra will not go out of park mode?

http://technoanswers.blogspot.in/2012/02/2002-nissan-sentra-will-not-go-out-of.html

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Nissan Sentra stuck in park?

http://technoanswers.blogspot.in/2012/02/nissan-sentra-stuck-in-park.html

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Car will not come out of Park?

http://howtobyme.blogspot.com/2011/11/car-will-not-come-out-of-park.html

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Automatic transmission will not go into high gear?

http://howtobyme.blogspot.com/2011/11/automatic-transmission-will-not-go-into.html

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Car won't downshift sometimes?

http://schematicsdiagram.blogspot.com/2011/12/car-wont-downshift-sometimes.html

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Buick Park Avenue: Transmission will Not shift into drive?

http://schematicsdiagram.blogspot.com/2011/12/buick-park-avenuetransmission-will.html

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No First gear Starts In Second Gear

http://www.howtobyme.blogspot.com/2012/07/no-first-gear-starts-in-second-gear.html

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Ford Taurus won't shift from park?

http://schematicsdiagram.blogspot.com/2011/12/ford-taurus-wont-shift-from-park.html

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2006 Ford F150: Will Not Shift out Of Park?

http://technoanswers.blogspot.in/2012/06/2006-ford-f150-will-not-shift-out-of.html

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Car Transmission will not go into Park mode?

http://schematicsdiagram.blogspot.com/2011/12/car-transmission-will-no-go-into-park.html

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1998 Chrysler Voyager: Transmission Problem?

http://howtobyme.blogspot.in/2012/06/1998-chrysler-voyager-transmission.html

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Shift indicator directly goes to 2nd when key is turned ON?

http://schematicsdiagram.blogspot.com/2011/12/shift-indicator-directly-goes-to-2nd.html

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Third gear missing and engine losing power?

http://schematicsdiagram.blogspot.com/2012/01/third-gear-missing-and-engine-losing.html

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Chevrolet silverado has whining sound in gear?

http://technoanswers.blogspot.com/2012/01/chevrolet-silverado-has-whining-sound.html

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2002 Chevrolet Avalanche Z71: Gear shift Issue?

http://whoisbyme.blogspot.in/2012/06/2002-chevrolet-avalanche-z71-gear-shift.html

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Hard to shift gears on Manual transmission?

http://technoanswers.blogspot.com/2012/01/hard-to-shift-gears-on-manual.html

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Car hard shift at slow speeds?

http://technoanswers.blogspot.com/2012/01/car-hard-shift-at-slow-speeds.html

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Transmission-Hard To Down Shift

http://technoanswers.blogspot.in/2012/07/transmission-hard-to-down-shift.html

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Automatic transmission shift lever hard to move on cold mornings?

http://technoanswers.blogspot.com/2012/01/automatic-transmission-shift-lever-hard.html

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2000 Nissan maxima shifts hard?

http://schematicsdiagram.blogspot.in/2012/02/2000-nissan-maxima-shifts-hard.html

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Vehicle noticed sluggish delayed shifts while driving?

http://schematicsdiagram.blogspot.in/2012/03/vehicle-noticed-sluggish-delayed-shifts.html

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2007 Hyundai Santa Fe: Will not Shift out of Park Mode?

http://schematicsdiagram.blogspot.in/2012/06/2007-hyundai-santa-fe-will-not-shift.html

These details will help.

Thanks.

your overdrive shift solenoid is stuck open or closed . need to replace it

Your transmission is toast. Smell the fluid on your ****** dipstick. It will smell really burnt.

Sound like a torque converter problem; also check valve body and fluid quality/levels.

this is the drive gear position with the overdrive being part of the transmission drive gear when the car is driven over a pre-determined speed,it goes into the overdrive function.

The 1st thing you need to do is go to an Auto zone or Kragen auto and have them run a trouble code test, the OD light is blinking to let you know there is a transmission related fault, but it sounds like the may be other issues like the MAF sensor and passably a low fuel pressure concern.