How can you tell if you have limited slip diff? I just purchased a used 99 F-150 and it has a lot of options, but don't know about LS.

Philip

 

On the rear end cover is a metal tag on one of the bolts mine reads 3L55 9 75 9M19 ,the first L is for limited slip, the 3 55 is the gear ratio 3.55, the 9 75, is the 9.75" rear end. A truck that peels out with one tire all the time especially in the rain is the more common and standard "open differential".

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
4-wheel disc/ABS,Chestnut/Parchment,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.

 

Look at the ID tag on the driver's door jamb. Under T there will be a 2 digit code. If the first digit is the letter "H" you have an LS. If the first digit is the number "1" you have an open.

 

Chock both front wheels and jack up the rear end. Get both back tires off the ground, then with the tranny in neutral spin one of the rear wheels. If the opposite rear wheel rotates in the same direction you have a limited slip differential, if it rotates in the opposite direction you don't.

 

actually if you jack up and both wheels spin the same way it's a locker or gm's version positrac, and if one wheel spins the other way its a limited slip,and they must have changed the info on the tags on the cover cause l is locker just like on my 90 f-150 my 83 bronco and and four 5 liter mustangs......

 

There are no factory lockers from Ford anymore. The standard rear end is an open (jacked up wheels spin in opposite directions), the optional Traction-Lok is a limited slip. It is a clutch plate unit, virtually identical in design to the older (non-cone) GM/Eaton type.