Does the 04 town car signature sires come with pos traction rear end?
I am sure it was an option. If you jack up both back tires and put the transmission in neutral, try rotating the rear tires by hand. If they both turn in the same direction you have a posi-traction diff. If they turn in opposite directions you have an open differential. The tags on the differential should also tell you if it is open or limited slip.
They actually don't make a posi trac anymore, too many cars actually shoved themselves right off a slick road due to the posi setup, now they use limited slip differentials, the internal clutches detect when'll slippage and then they apply themselves to transmit power equally to the other wheel. HTH
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