With all four wheels on the ground.....Depress the accelerator pedal as far as you can, as quick as you can. Look on the ground behind the car after the smokes clears. You have posi-traction if there are two black rubber stripes on the pavement. No posi or points for one stripe. LOL
If you are checking a tired, high mileage axle the only certain method of determining positraction is to remove the cover and examine the carrier. A unit with badly worn clutches effectively becomes a non-posi axle; wheels will rotate in opposite direction.
William is right. Pull the cover and look for the two plates with four springs between them, or the "s" spring in the later 10 bolts. This is the only way to be sure. Some standard rear ends will weld themselves together after many one wheel burnouts. I've seen bent housings cause both wheels to turn the same direction too. I almost got burned by that one twice.
'69 RS/SS396 pro street
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