Is it possible to test my positraction differential in the absence of oil - to check if it still works correctly? I've opened it up and now I want to see if my diff is still ok. It is supposed to turn both wheels in the same direction with the drive shaft locked, but does there need to be oil in it? Still I don't really get how that should work. I understand there are clutch plates present which make it possible for each shaft to have a different rotational speed. But when the drive shaft is locked; it should be impossible to rotate both wheels in the same direction right? Because the differential case is then locked as well. And due to the differential pinion the shafts should only be able to rotate differently. I have it with springs, but in the image below, when one side gear turns, the pinion gears also turn, and therefore the side gear on the other side must rotate opposite. I can imagine that because of clutches one side does nt turn, but I'm not able to imagine it turning in the same direction.. And according to Eaton the driveshaft must be locked. So apparently my reasoning is wrong. Can anyone explain this to me, or have a good informational link for me?
That does not make sense at all to do a no load test and you must have oil if it is the OEM rear. A quick test would be to lock the drive shaft and attempt to turn one wheel. If I am remembering the manual correctly, locking the drive shaft and using a torque wrench on one axle to measure the breakout force is done to determine if it is in spec. It should not turn unless you're the bionic man and are able to exceed the breaking force of the clutch pack/or cones depending of rear type. If it does turn the other wheel will turn in the opposite (wrong) direction like an open differential and means you need to rebuild/freshen up the clutches. Another quick check is place the car in neutral, and spin one wheel the other should spin in the same direction. I know nothing about Eaton rear-ends lockers and/or LSDs. I might some how be wrong.
Still, I don't see how both wheel should be able to move at the same direction. (I do have a Canadion 12 bolt POSI-rear, perhaps that makes a difference) If you look at both images: "Unlike the Truetrac, most limited-slip differentials like the one shown above operate like open differentials but have spring-loaded clutch packs to mitigate the otherwise unrestricted slip between the two sides of the differential." (expeditionexchange.com) When one shaft turns (and the driveshaft is locked), the other shaft has to turn the other way around, due to the side gears and spider gears.. So I don't think my wheels should move into the same direction. When using a LSD with a pressure ring I cn imagine it, because the spider gears are not connected to the case as mine are: https://youtu.be/PEdnH7_7_yc?t=25s
Try these videos and see if that helps. The springs and clutches in a posi or LSD grab when under load, which makes the wheels turn together. https://www.youtube.com/watch?v=F40ZBDAG8-o https://www.youtube.com/watch?v=RaxMchRXAEs
IIRC, with the trans. in park, if you have a limited slip (posi) clutch type rear, it only takes about 40 ft.lb. of torque to slip the clutches if one wheel is locked, THATS if I read it correct years ago
Ok first off about the posi Clutch or cone If you have the drive shaft in the PARK selection gear... Your posi unit will turn like an open differential as far as DIRECTION goes. IE If you turn your drive side wheel forward the passenger wheel will spin opposite direction because of the planetary spider gears. If you have the trans in nuetral or in any gear BOTH wheel will spin in the same direction as the clutches/ cones are locked and planetary gears are not spinning in the carrier cavity. SO really only in turns does the posi unit ever wear. If you drove straight forever the clutches and cones would never wear NOR would you need plantary gears in the first place...LOL. NOW on to the load test. THIS is a terrible way to test a posi unit for wear. JUST does not work ... IT will give you a general idea how good or bad a unit MIGHT be but really does not give you what you need to lay down money at a swap meet with confidence. If a cone type posi unit is bottomed out it will has the same if not more preload break away pressure ... BUT constant spinning force it will totally fail which can not be recreated in slow speed operation. Spring pressure isd one one aspect of posi unit workings. Initial spin force is where most of the engagement force happens. YOU really do not need preload springs fr a positive traction effect to happen. coil and other preload springs are really only working at low speed operation and make your posi react quicker to the torque load applied instantly. There is alot more to talk about in the workings in posi units. I am sure I will be here for a while... Jim
Thank you for your comprehensive post. So I was right; POSI diffs react as an open diff when the driveshaft is locked, but both wheels turn forward (or backwards ofcourse, but have the same direction ) when the transmission is in neutral. I m going to check how much torque it takes to turn a wheel when the other wheel and driveshaft are locked, just to have done it once. But I understand it won't tell me that much. Is there another way to test it properly, without having the car to drive? Because that will take me at least one more year :grin:
Honestly I would not even worry about it for now... Get it up and running and then do a burn out and you will find out QUICK if it is working properly.
The clutch units that came from the factory were not set up very aggressively at all.Most of the time,it was just enough to make it work. You can tighten them up by adding clutch shims in each side. I've added as much as .070" on each side before.
My car has a factory unit and it is pretty loose. When on the lift I can effortlessly break the clutches and spin the wheels in opposite directions like an open diff. It has never once slipped or put down a one legger. with or with out the sticky mickeys...
It's a lot easier for cornering that way. That is where the springs take their function.They help push the clutches together.The looser setups will have a lot more play between the side gears and spider gears.Basicly,they are making contact with each other farther out on the tips of the teeth.This is where you see chipped & damaged side gear/spider gear teeth on a performance application. By shimming the unit tighter,it also brings those gears closer together for a deeper mesh contact.A nicely tuned unit does not need the springs in there at all,and it can actually work as good or better than it did from the factory,but with less chance of failure.There was a nice article about this,written by Tom's Differentials,published in Hot Rod,in the early 1980's. Very informative stuff.
What oil should go in actually? 75W90 or 80W90 with posi additive? And I suppose mineral? I can't find it in my manual, but I don't have an official Buick manual.
Most people use 80/90. I use 85/140. Use conventional oil. Do not use synthetic. Any new clutch style posi unit you get will also have a little card inside,with the same warning.
Your confusing how the planetary gears work... Posi and open planetary gears are the same... Of coarse they will spin the same way. It is just the torque bias in the left and right tire to the ground is controlled with clutches or cones etc. In a open diff the planetary gears quickly default to the wheel that is easiest to spin turn which is why one wheel goes up in smoke.
