I recently picked up a '91 RS with a 305TBI, 700R4 and a limited slip rear diff. I'm working on swaping to a 305TPI, T5, and a posi. rear. I got the engine and trans but I'm having trouble tracking down a posi. rear for a good price so I decided I'm going to pull one from a local junkyard. Only problem is, I don't know which ones have one and which ones don't. I'm looking to stick with one from a third Gen. F-body. Are there any tips or tricks to telling which ones have a posi and which ones have limited slip (ie. All irocs/ z28's/ formulas, gta's, etc.)?

 

Welcome to TGO, grayghost.

Limited slip and posi are the same thing. Ten-bolt info/pics below:

https://www.thirdgen.org/forums/tran...-10-bolts.html

JamesC

 

The easiest way to spot a posi rear in a 3rd gen is to look at the factory option list decal that's usually located in the console glove box. There will be the code "G80" if it's a posi, and the other G code will tell you the ratio if it's a 10 bolt. GU2=2.73, GU4=3.08, GU5=3.23, GU6=3.42. Of course the only flaw to this method of determining the rear is if it's ever been replaced before the car was junked. If it's been changed out, the only way to know for sure is to pull off the rear cover and look to see if the carrier is posi or open, and see what ratio is stamped on the ring gear.

 

Another way to tell if it has posi/limited slip is, if at all possible, get the rear end (both tires) in the air and spin one of the tires. If the opposite tire spins in the same direction then you have found a posi/limited slip, if opposite tire spin opposite direction then it is an open diff and obviously not what you want!

 

Easiest way to spot a posi RELIABLY is to take the cover off and LOOK AT THE PARTS, since you'll be changing the fluid ANYWAY when the rear goes into your car.

 

I'm a fan of this, faster and less messy. Works 9/10 unless posi is blown.