I have an 87 iroc z with 305 t5 and a 9 bolt rear with I believe 373 gears. shouldnt it have a posi? Only 1 wheel spins so??? would there be an option code to find out if its posi?

Its not 3.73s. Most likely its 3.27s or maybe 3.45s.

Check the rear for ''Posi Tags''. It will be on the lower left bolts of the cover. If not then just pop the cover, you will find out the gears and see if it does have posi and might be just worn out.

it does have 2 tags on it but I couldnt read them, rusted too bad. why do you say its not 373s. it seems like its geared kinda low just because I go through gears fast

I made out out 1 of the tags it says slip resistant diff. So then what is causing it not to work right? What parts do I need to fix it And thanks for the earlyer reply

Well for one its not 3.73s since those are gears for a 10 bolt. Only thing close would be 3.70s. 3.70s 9 bolts are very rare and from my research are found in 85 and 86 years.

Most likely the cones are wored out. Check out this thread:

https://www.thirdgen.org/forums/faq-...now-about.html

I agree. There is test to check over in the tech articles and excellent write ups on the internal design. 9 bolts use 2 cones with a spring and gear setup to build friction to lock the carrier. Overtime cones wear (case where cones sit wear too) and slack happens and it doesn't build friction properly. It's moderate to easy job to check it yourself if you have tools.

10 bolts and 9 bolts parts aren't swapable. Different pumpkin sizes and internals.