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1989 Brougham d'Elegance, 1985 Fleetwood Brougham *Coupe*
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4,256 Posts
Go to an empty parking lot. Leave engine running. Place gear selector in "DRIVE." Place left foot firmly on brake pedal. Slam right foot onto the gas pedal. Hold brake until vehicle begins to move forward. Release brake pedal, while still holding gas pedal down. Bring vehicle to a complete stop. Observe tread marks, burned into pavement.

If there is only one, you have a standard differential. If there are two, you have a limited slip.
 

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Super Moderator
2014 ELR
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9,693 Posts
Sven's technique works if you have a WORKING limited slip. After thirty years, it might be worn ;)

Also, with the high gears in his car, there may not be enough torque multiplication for the rear tire(s) to actually spin on a brake stand. And to further complicate things, it is possible to spin both tires without a limited slip.

The best way to check is for the tag, or to actually visually verify by pulling the cover.

I guess the real question is... why do you care? No snow in Brazil. Doubt you'll do much drag racing or road racing with the 78, too...
 

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84 Coupe w/500
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5,665 Posts
Isn't it if you have disc brakes, you have posi? Disc brakes only came with posi, posi only came with disc?
 

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Super Moderator
2014 ELR
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That's one of the problems. He has disc brakes on a car that never came with them. A DeVille. The disc rear was standard on the 77-79 FWB. I don't believe that posi was standard, however. A quick check of the 77-79 options should confirm.
 

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1978 Coupe DeVille
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313 Posts
Discussion Starter #7
i removed the wheels, driver side have the calipers on the back, passenger side have the caliper on the front of the disc. still the height sensor is there, everything looks factory... i really dont know how it get there, but they are 4 wheels discs, looks like the brougham system... i will take some picutres.
 

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Registered
Past: 95 Fleetwood, 91 Brougham. Now: 92 Lexus SC300
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5,418 Posts
Go to an empty parking lot. Leave engine running. Place gear selector in "DRIVE." Place left foot firmly on brake pedal. Slam right foot onto the gas pedal. Hold brake until vehicle begins to move forward. Release brake pedal, while still holding gas pedal down. Bring vehicle to a complete stop. Observe tread marks, burned into pavement.

If there is only one, you have a standard differential. If there are two, you have a limited slip.

Or if you want to be a little less harsh on the car, go to the empty parking lot when its raining. Make a sharp right hand turn at about 5mph, then stab the gas about midway through the turn. If the inside tire just fries to nothing, open diff. If the whole rear of the car starts to rotate and does a donut on you, limited slip.
 

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Cadillac Technician
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11,051 Posts
Jack up both rear wheels off the ground and grab one side and try to rotate the tire.
If the wheel spins and the one on the opposite side rotates the opposite direction it is not limited slip.
If you can't turn it then it is posi.
 

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Registered
1989 Brougham d'Elegance, 1985 Fleetwood Brougham *Coupe*
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4,256 Posts
^ Actually, some limited slips function like a normal differential when you do that.
 

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Super Moderator
2014 ELR
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9,693 Posts
The correct test for a working limited slip is to measure the breakaway torque required to turn one wheel while in in the air. The procedure should be in the FSM. I know it is for the later B bodies...
 
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