age

ok.. with the car OFF, and the rear end jacked up.. how do you tell if you have posi or not? Would this also be the same if you had a front wheel drive car?

82 Iron Duke

iTrader: (21)

Good question? Best way to tell is to do a burn out and see if you leave 1 or 2 tire marks? All front wheel drive is posi

koll

Not always true. If you have enought power you wont beable to tell.

Doom86

the way to tell 100% is to pull the cover off and look.

the open rear in my 86z would lock up ever so often.

the whole "jack the rear up and spin the south tire on the summer solctice and if the moon and sun line-up it may be posi" tales arent always true either. digging around in junkyards has proven this one for me.

82 Iron Duke

iTrader: (21)

if I pull the cover off what am I looking for? I did burnouts in the soft dirt and I checked and sure enough there were both tracks.

Apeiron

Umm... no.

Derek The Great

The accurate way to test your rearend is to jack up the rear end with the car out of gear or in nuetral, and spin one of the wheels say counter clockwise, the other wheel should be spinnng counter clockwise also if it is a posi (limited slip) rear end if it is an open rear end it will spin in the oppisite direction.

Doing a burnout wont always tell you the truth, for example, my firebird will lock up both wheels sometimes, but other times it burns just the one
explanation?
-under normal driving conditions, (say driving straight @ 45mph) your car is being pushed by both tires, regardles of differntial type. an open rear end will split the power to the wheel with the least amount of resistance, and when driving straight at a constant speed traction is basicly equal at both wheels, with an open rear end. it isnt until enough power over comes that traction that one wheel will break free and peel on its own, so with our weak suck 305s it is possible to lay down two tracks instead of one. this is most notable on slick surfaces, such as shop floors.

82 Iron Duke

iTrader: (21)

Thanks for the info you 2 Apeiron and Derek The Great , I love this board and I am still learning something new everday

Derek The Great

Doom86

That doesnt always tell you if its posi or not Derek. The only way to be 100% sure is to pull the cover and look. I have pulled countless covers and spun many of tires at the junkyard crawling for gears.

84z28350

jacking it up and spinning the tires will give you a good idea of what you got in there. but to be 100% sure pull the cover, the oil in there is probably half a million miles past due for a change anyways!

GuitarJunki17

iTrader: (1)

I like the burnout method...thats the most fun way

I found out the scary way actually when I nearly lost her in the rain...recovered beautifully, and then the burnout/jack up rear and spin tires, just confirmed it.

84z28350

Burnout method is useless, an open end diff will sometimes lock up and you will burn both tires on dry pavement. On loose surfaces like dirt or wet pavement a open end will bake both tires 99% of the time!

rx7speed

not always accurate either.

one thing that could cause it to spin one way or the other is if it is worn. plus I think a few differentials like a torsen or viscous type lsd will spin the tire the other way

88_Import_Slaye

You can also look at the RPO codes.

sofakingdom

iTrader: (1)

Also not always accurate. All those will tell you, if you can even find them, is whether the car left the factory with posi; they will NOT tell you what rear is in the car NOW, or what's in the rear that's in the car NOW.

Burnouts, "spin the tire" games, codes, all of that crap, is NOT ALWAYS ACCURATE. It's not worth bothering with.

There is EXACTLY ONE method that IS always accurate. Every time, without fail, has a PERFECT track record, always has, always will. Now granted I'm sofakingdom I can't understand much of anything; but I cannot fathom why there is SO MUCH RESISTANCE to doing the ONLY thing that IS 100% accurate.

And that one thing is, TAKE OFF THE COVER AND LOOK. The parts will tell you what they are. Nothing else is accurate. Everything else is a guess. Some guesses might be right, some might be wrong; but regardless, THEY'RE STILL GUESSES.

Same for "what gears do I have". TAKE OFF THE COVER AND LOOK. It's the ONLY way to be 100% certain that you're accurate.

88_Import_Slaye

could you be kind enough to post a POSI and a non POSI photo for us lamers who havent learned the mysterious ways of the rear end yet. I'm only as far back as the tranny Rear end lessons come next tax season

sofakingdom

iTrader: (1)

It's even simpler than photos.

Take of the cover. Rotate the carrier around until you can see into the largest "window" in the side of it. Look inside the carrier. If all you see is 4 small gears and a cross shaft with 2 gears on the inner end of the axles and 2 gears on the ends of the cross shaft, with no additional gears and no clutch plates and no springs and nothing else, then it's not a limited slip. ("Posi" being short for a tradename GM used in the mid 60s for limited slip, that sort of "stuck" in hot-rodder lingo; there are lots of other other trade names for the same thing from other mfrs in other time periods.)

If you see any of those other things, then it is (or was at one time) some kind of a limited-slip system.

Presence, or absence, or parts; that's what you're looking for.

phess11

Best way is to open up the rearend and look. The "burnout" will work every time IF you put one rear wheel on grass and one on pavement. If the car barely moves (i.e. grass wheel is spinning and the pavement wheel is not) you don't have posi. If the car starts to move out then the posi is transferring torque to the pavement wheel.

rx7speed

holy shizzam my wigga. we agree on something for once.


amen this world is comming to an end


but it does get over anything.
worn lsd can act like an open diff under some situations. open diffs can act like lsd's at some times. codes don't mean it's there now. your jack can't do it. spinning them can do it. nothing can really tell you for sure unless you pull the cover off. this way you know for sure what you have and this way also know if you need to replace it or not as well

rx7speed

ok so that means on my friends vehicle that comes with posi from the factory about 200k miles ago that he doesn't have it even though he is the original owner and has taken the rear cover off to verify this?

AGAIN worn lsd still means you have lsd it just isn't working as though it should. viscou style lsds I also think have some weird things going on with them as well and think a lot depends on the oil there.


here is another one for you. my mazda does not have lsd yet I can still spin both tires sometimes with one on slick one on good grip.

Derek The Great

i am against that "open the cover" to find out what you have, if it was a posi and is now burned out, who gives a darn bc you will end up replacing/rebuilding that rear end to make it a posi again, which is close to swapping a limited slip in the car... i see poping the cover as a waste of time, unless you plan on other maitenence, and or you need to change the diff oil, or replace bearings. For most ppl (myself included) we just want an easy 5 min way to try and figure out what we've got, sure rpos are one way, but when your at a scrap yard and or the car is 10+ yrs old thers no tellin whats in there now...
-GM cars, however, usually have a metal tag on one of the lower cover bolts to identify a factory posi...

84z28350

OK guys, how hard is it to crawl under your car, pull 10 bolts off to know 100% what the heck is in there. Their is absolutely no other way to know for sure what you have and its pretty darn easy to pop the cover off...

phess11

I guess mazdas are good enough to defy physics. They invented lsd for a reason. If you can have even torque to both back wheels with significant differences in traction between the drive wheels, then you don't need it. I seem to always notice that the wheel with the lesser traction slips and the other is almost still. I guess they invented lsd for me.

xplane

not only that but if ya dont know whaty ya have 99% of the time you have yet to change the Diff fluid and ya probably should. ill bet that if ya have had the diff open to change it you have taken a peek in there. plus the gasket is only like $1.00 at the most and ya might get to $10.00 in new fluid if ya get a moderatly good kind.

Derek The Great

its not that hard, but now you need a drain pan, and need to eventually throw that crap away and now you need new diff oil, gasket, or rtv and posibly limited slip additive... Some people are just interested in seeing what the car does as of today, and dont necessarily plan to change anything with the rear end..

xplane

well ill say what i just did again if ya have yet to open the rear you should just to know that it has fresh fluid and such. if you have not opened the rear in your car since you have owned it it would be a VERY good idea just so ya know it has been changed. its not that hard and even if ya have to buy a catch pan and such you should change the diff fluid on a regular basis. i have always done it every 12months or if it sees heavy use more frequently.

flrtin1

The other thing that has not been addressed is the fact that in the mid 80's the factory was known to have too much additive in the diff on new cars causing the limited slip to only work sometimes or not at all

rx7speed

nobody said anything about defying physics

and with what your saying if a car is able to spin both tires with slicks on then you don't need slicks either right?

never said it did it all the time but at some times it did do it

HellWolf04

pretty damn hard when ur car sits so low u have to drive it up on two by fours and then jack up the front a couple inches and jack up the back a couple inches and do this over and over until its high enough to get a jack stand under it, i myself hate jacking up my car, drop kit makes the handling sweet and makes it look even sweeter though.

88_Import_Slaye

that is in your case only. My car has eibach 2" drop springs in the front so my 3 ton jack doesnt fit, but I can fit my big a$$ under the back without the jack. (stock springs in the back)

anyway, the guys are right, you SHOULD open the diff cover if for nothing more than to change the fluid and look at how shinny your ring and pinion are. At least, mine were.

HellWolf04

yea ive got two inch drop in the front and the back so my 3 ton doesnt fit in either place i have to use the little travel jack to jack it up enough to get the 3 ton jack under there