I have a chevy 12 bolt on my 55.2 (that came off a '78 blazer). I'm going to pull the rear differential and put in new fluid (I don't think it's been done since ...1978). I know you need to put additive in the oil if it's a posi rear end, but I don't know how to tell. There is no tag on the rear end. I did the test of spinning one wheel, and the other wheel spins the opposite way, but some of the reading I've done says that is NOT 100% accurate. Any one have tips on how to tell if the rear end is posi or not?
Jack up one wheel, and GENTLY accelerate the throttle with the transmission in gear. If the vehicle tries to pull ahead you've got a posi. Don't go much above idle, just enough to try to spin the jacked-up wheel. An "open" differential will allow the lifted wheel to spin freely. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Easy to tell w/ cover off. In center of diff you will see side gears (splined to axles), diff. pinion gears (spiders) w/ pin going thru them & the case. This is an open differential. A posi, limited slip or locker will either have a spring pack, locking latch, ect between side gears in center of diff.
BC 1960 Chevy C10 driver 261 T5 4.10 dana 44 power loc 1949 GMC 250 project in waiting 1960 C60 pasture art Retired GM dealer tech. 1980 - 2022
