Why won't transmission shift into overdrive?

It is not going into neutral. Your overdrive band is fried on your transmission. Typical problem on these transmissions. Transmission will need to be completely disassembled to get to the overdrive band. Complete overhaul. Recommended when you read the owners manual to not drive these uphill for long periods of time without shifting to regular drive.

Are there any lights lit on the dash ? check engine , overdrive etc..
could be just a shift solenoid ... Try draining trans fluid an replacing filter. Have to drop transmission pan .. what's the fluid look like ? a nice pink color ? does it smell burnt ? Is it full ?

look at fluid levels and color of fluid.. if brown instead of reddish take to transmission service and do fluid and filter change..may have a clogged filter..

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.

It may be your transmission. It is not the cable. I dont know if a shift link would be broke or not. It dont act up any in reverse, 1, 2?

I think rebuild try and service it if you find any lining material in the pan bad news

this is a hard mechanical fault in the overdrive forward clutch, u must overhaul the trans to fix this.

I have that exact same truck and transmission and My Transmission did the exact same thing about a year ago and there was some kind of snap ring inside the transmission that broke. I originally thought it had to be an electirical problem because the truck shifted fine until OD or if i turned OD off. The 4r70w is a light transmission and i would suggest you have a shift kit put in and they make a heavier kit to beef it up.. Sorry its probably not the new you were looking for.

IT IS NOT GOING INTO NEUTRAL. IT HAS LOST THE DIRECT DRIVE CLUTCH PACK INTERNALLY. SOUNDS LIKE IT IS HIGH MILEAGE. IF SO AND YOU CHANGE THE FLUID IT CLEANS UP ALL THE VARNISH IN THE TRANNY AND THE END RESULT IS TRANNY MALFUNCTIONS DEPENDING ON WHAT KIND OF CARE IT HAS HAD IN THE PAST.

First off, I'd try changing the fluid and filter in the transmission. I know a lot of cars claim "lifetime fluid" - it's BS. Believe me, even the lifetime fluids get very dirty. Change out the fluid and filter, and you could even add some Lucas stabilizer to it - that may well help cure the problems.

When the transmission goes completely, it may lock up the drive wheels, sending you into a careening slide, or it may kick out to neutral and coast to a stop. It's also possible that if it flat-out blows, it could fragment inside and you'd hear what sounds like bolts in a blender as you coast to a standstill.

Discounting the transmission blowing up, and just looking at driving in a drive gear other than overdrive, you're looking at more heat in the transmission due to the higher gear RPM, shorter fluid life, possible slippage of the bands down the road due to heat, and of course decreased gas mileage due to the engine running at higher RPM.