Hello. I am looking at a 2004 Crown Vic Interceptor at a Ford dealership. The salesman assures me that the car has a limited-slip rear differential. However, the diff on this car looks (as far as I can tell) just like the one on my girlfriend’s Grand Marquis, which I’m fairly certain is an open diff.
So, my question is, for all of you wise and good-looking people, is the limited-slip differential standard equipment for the P71 and, if not, is there an easy way to tell from a visual inspection whether it has one or not?
Thanks!
Check the axle code on the door jamb. If the code is a letter followed by a number, like “H9” for example, then it has limited slip. If it’s two numbers, like “19” then it’s open.
On a test drive find a gravel area, stop the car, then punch it to accelerate. Look at the gravel in the rear view mirror; two strips of spun gravel is limited slip and one strip of spun gravel is a regular differential.
I am 99% certain that a factory police interceptor Crown Vic will have a limited slip rear end.
Surprisingly, the percentage of P71’s with limited slip isn’t that high. Ford’s Trac-lok LSD was never standard equpment on the P71. It was an option, but based on the fleet sales that I did when I was with Ford, I’d estimate that no more than 25% of the P71’s that I ordered had limited slip. Though from 2005 onward if you ordered the 3.55 rear gears the trac-loc was mandatory.
Yep, I’m surprised. The state troopers I’ve seen cross the medians, and take off after speeders left the tell tale double tracks of a posi rear end.
The presence of two stripes on gravel or dirt isn’t the end-all-be-all way of telling though. I’ve had cars with open diffs that would briefly spin both rear wheels on gavel. My old 1974 F-100 with 390 swapped in and 4.11 gears would spin both rear wheels on our gravel driveway, much to the disdain of my dad 
Checking the axle code is the best way. Another way is that you might ask the “out of work” hair dresser in New York about differentials if you can find her. She would know the answer. (I’m thinking of the movie “My Cousin Vinny” where Vinny’s fiance has the automotive expertise that breaks the case. She explains the limit slip differential to the jury in a way that they can understand).
The old method of getting the rear wheels off the ground an rotating one wheel(by hand)and watching if the opposite wheel rotates in the same or opposite direction has never failed me. An open diff will rotate opposite while a limited slip of some type will rotate in the same direction.
I have always read warnings about making conclusions with this method but it has never let me down.
I agree with all the others. You cannot tell if it is limited-slip from looking at the axle housing. A code number, actually testing the axle, and opening it up are the only ways to tell.
Look to see if there is a metal tag under one of the differential cover bolts, that tag has information that will tell you what differential it is. If it has a tag remove it and go to a Ford dealer parts department and they can look up the specs on the differential.