1996 Cadillac Seville started shifting hard all of a sudden

Lets start by checking your brake light switch.
Your car has a switch on the brake pedal that not only makes the brake lights come on, but also tells the transmission it can shift out of park.
This really may be 2 separate switches, but should be under the dashboard and both should "open" when you push the brake pedal down.
Take a good flashlight and look.
The switches are small, about the size of your thumb, and both should have 2 wires attached to them.
Be blessed.

It can be shift mechanism issue or its faulty worn out gears.On some cases like these its also seen that problem relates to transmission rebuilt.But that is rare,if these problem is suddenly occurred without any previous problem.To troubleshoot these issue,i suggest you to refer the solved helplinks with appropriate solutions.-----------Click the link below:-----
2007 Chrysler 300: Transmission Stuck in Park?

http://repairhelpcenter.blogspot.in/2012/06/2007-chrysler-300-transmission-stuck-in.html

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1996 Toyota Corolla: Transmission Shift Interlock will not Release?

http://technoanswers.blogspot.in/2012/04/1996-toyota-corolla-transmission-shift.html

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Lexus LS400: Transmission or Torque Converter?

http://technoanswers.blogspot.in/2012/04/lexus-ls400-transmission-or-torque.html

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Gear indicator Shows "3" Instead Of "D"? http://whoisbyme.blogspot.in/2012/07/gear-indicator-shows-3-instead-of-d.html --------- 2006 Chrysler 300c: Shifter Stuck in Park? http://technoanswers.blogspot.in/2012/06/2006-chrysler-300c-shifter-stuck-in.html ---------- 1998 Ford Expedition: Will not Shift Out of Park? http://schematicsdiagram.blogspot.in/2012/06/1998-ford-expedition-will-not-shift-out.html --------- Car will not move when in gear? http://repairhelpcenter.blogspot.in/2012/02/car-will-not-move-when-in-gear.html ----------- Mercedes-Benz transmission sticks in first gear? http://technoanswers.blogspot.com/2012/01/mercedes-benz-transmission-sticks-in.html ---------- Mercedes Benz not engaging in first gear? http://technoanswers.blogspot.com/2012/01/mercedes-benz-not-engaging-in-first.html ---------- 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
------- 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 -------- 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 -------- Nissan Sentra stuck in park? http://technoanswers.blogspot.in/2012/02/nissan-sentra-stuck-in-park.html --------- Car will not come out of Park?

http://howtobyme.blogspot.com/2011/11/car-will-not-come-out-of-park.html
----------- 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
-------- Buick Park Avenue: Transmission will Not shift into drive?

http://schematicsdiagram.blogspot.com/2011/12/buick-park-avenuetransmission-will.html
------------ No First gear Starts In Second Gear http://www.howtobyme.blogspot.com/2012/07/no-first-gear-starts-in-second-gear.html ------- Ford Taurus won't shift from park? http://schematicsdiagram.blogspot.com/2011/12/ford-taurus-wont-shift-from-park.html --------- 2006 Ford F150: Will Not Shift out Of Park? http://technoanswers.blogspot.in/2012/06/2006-ford-f150-will-not-shift-out-of.html -------- Car Transmission will not go into Park mode?

http://schematicsdiagram.blogspot.com/2011/12/car-transmission-will-no-go-into-park.html
---------- 1998 Chrysler Voyager: Transmission Problem? http://howtobyme.blogspot.in/2012/06/1998-chrysler-voyager-transmission.html ------ Shift indicator directly goes to 2nd when key is turned ON?

http://schematicsdiagram.blogspot.com/2011/12/shift-indicator-directly-goes-to-2nd.html
-------- Third gear missing and engine losing power?

http://schematicsdiagram.blogspot.com/2012/01/third-gear-missing-and-engine-losing.html
-------- Chevrolet silverado has whining sound in gear?

http://technoanswers.blogspot.com/2012/01/chevrolet-silverado-has-whining-sound.html
---------- 2002 Chevrolet Avalanche Z71: Gear shift Issue? http://whoisbyme.blogspot.in/2012/06/2002-chevrolet-avalanche-z71-gear-shift.html ---------- Hard to shift gears on Manual transmission?

http://technoanswers.blogspot.com/2012/01/hard-to-shift-gears-on-manual.html
-------- Car hard shift at slow speeds?

http://technoanswers.blogspot.com/2012/01/car-hard-shift-at-slow-speeds.html
--------- Transmission-Hard To Down Shift http://technoanswers.blogspot.in/2012/07/transmission-hard-to-down-shift.html --------- Automatic transmission shift lever hard to move on cold mornings?

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

may need a transmission flush , new filter and fluid or a solenoid shifter which requires a tranny shop to repair.

If you need to lower sub frame, to remove side cover on trans.

get a transmission flush. and add a bottle of "LUCAS BRAND" transmission stop slip

it sounds like the problem started with the brake shift iterlock switch by forcing it he now broke the trans shift cable mech at the column

CHECK BRAKE LIGHT SWITCH FUSE OR BRAKE - TRANSMISSION SHIFT INTERLOCK SOLENOID FUSE.IF BOTH FUSES OKAY.THE PROBLEM COULD BE A BAD BRAKE LIGHT SWITCH OR A BAD BRAKE TRANSMISSION SHIFT INTERLOCK SOLENOID.YOUR IGNITION SWITCH IS BROKEN.MAKE SURE YOUR BATTERY IS 12.5 VOLTS.IF NOT YOU CANT SHIFT INTO PARK.

on top of transmission where final drive housing is.

The pressure control solenoid needs to be replaced. It"s under the side cover on the transmission on drivers side. Not a simple fix at home. You will also need a side cover gasket. The transmission does not need to be rebuilt.

A bad IAC (idle air control)or vacuum leak. Check for bad vacuum hoses first, the IAC is around $50