How do I test for a broken POSI unit?
Thread Starter
TECH Junkie
iTrader: (6)
Joined: May 2005
Posts: 3,185
Likes: 3
From: Highland, Indiana
How do I test for a broken POSI unit? The car's been acting a little weird when turning like one wheel may be slipping or something.
Either way, is there any way to test the POSI unit other than doing a burnout. It just seems like even if the POSI is broke it could still leave two nice long lines of rubber.
Either way, is there any way to test the POSI unit other than doing a burnout. It just seems like even if the POSI is broke it could still leave two nice long lines of rubber.
TECH Addict
Joined: Nov 2005
Posts: 2,379
Likes: 2
Jack up the rear, open up the differential cover and inspect the unit. A broken rear is not something you want to "discover" on the road. A catastrophic failure could lead anywhere from simple teeth mashed/broken to axles coming off to the whole unit seizing depending on what broke in there. The last thing you want to find out is that while you are driving.
Since you have a 98+ car, you have at the least a zexel torsen differential. When jacked up in the air, the wheels should turn in the opposite direction. Also that diff is a it either works or it doesn't as it has a bunch of small worm gears in its design. If they are damaged, you WILL get a lot of noise out the rear like a bunch of rusty nails rattling in a can. I
Since you have a 98+ car, you have at the least a zexel torsen differential. When jacked up in the air, the wheels should turn in the opposite direction. Also that diff is a it either works or it doesn't as it has a bunch of small worm gears in its design. If they are damaged, you WILL get a lot of noise out the rear like a bunch of rusty nails rattling in a can. I
Thread Starter
TECH Junkie
iTrader: (6)
Joined: May 2005
Posts: 3,185
Likes: 3
From: Highland, Indiana
Originally Posted by SladeX
Jack up the rear, open up the differential cover and inspect the unit. A broken rear is not something you want to "discover" on the road. A catastrophic failure could lead anywhere from simple teeth mashed/broken to axles coming off to the whole unit seizing depending on what broke in there. The last thing you want to find out is that while you are driving.
Since you have a 98+ car, you have at the least a zexel torsen differential. When jacked up in the air, the wheels should turn in the opposite direction. Also that diff is a it either works or it doesn't as it has a bunch of small worm gears in its design. If they are damaged, you WILL get a lot of noise out the rear like a bunch of rusty nails rattling in a can. I
Since you have a 98+ car, you have at the least a zexel torsen differential. When jacked up in the air, the wheels should turn in the opposite direction. Also that diff is a it either works or it doesn't as it has a bunch of small worm gears in its design. If they are damaged, you WILL get a lot of noise out the rear like a bunch of rusty nails rattling in a can. I
Edit: ok its quiet during normal driving, but I just jacked it up to inspect some brake noises and the wheels do turn in opposite directions when you turn one. But it also makes a rattling noise, is any noise at all normal? It is not VERY loud, but it is noticable.
Last edited by Luna; May 23, 2006 at 04:40 PM.
TECH Addict
Joined: Nov 2005
Posts: 2,379
Likes: 2
I have 2 zexel torsen posi differentials on 2 cars and neither make noise. This is not to say yours is the same. GM has released a technical service bulletin on the issue indicating "some" zexel torsen posi differentials tend to make the "rusty nails in a can" sound under low speed turning. ie turning while reversing say into a parking spot. The soultion is to use GM friction modifier to "quiet" down the worm gears. I personally just stick with standard gear oil for the rear, my latest is havoline 80w90 I think. No noise, 2 strips of rubber is fine by my book. But as far as a little noise, not a problem. Like I said, inspect the rear, its a bit on the messy/smelly side, but a little goes a long way in terms of piece of mind.
Things to have on hand.
Proper tools, 13 mm(I think is the size), torque wrench, jacks and jack stands, and drain pan, lots of rags, a hand pump (to get the new fluid into the rear)
Brake cleaner
New 10 bolt gasket (gm stock or whatever is cheaper)
2 bottles of gear oil 80w90 or whatever you desire
friction modifier (either from gm)
The easiest way to tackle this is to jack up the rear from the "pumpkin". Once high enough, place jack stands on the frame IN FRONT of the lower control arms. Release jack to allow rear to drop but the car remains in the air with the rear now exposing the majority of the differntial cover.
Unbolt the cover, use a screwdriver to "pop" open the seal. Ensure the drain pan is underneath to catch the mess of fluid. Take cover, inspect the cover magnet for any matierial LARGER than small flakes/powder. Check the bottom of the differential pumpkin with your hand for ANY large pieces. Rotate your tires or your driveshaft in order to turn the gears and inspect the teeth on the ring and pinion. Look for significant damage aka missing teeth, chunks, cracks, bluish marks (indicating high heat). Clean off the cover and the magnet with brake cleaner and rags, ensure the old seal is off the cover and the differential. (if it was gm stock, it should come off easily as a whole piece else it will need a scraper of sorts to clear off. Put the cover back on with the new gasket and tighten the bolts in a criss cross pattern and torque to spec. If torqued correctly there should be no leaks. Fill the rear using the hand pump. The motto is fill till spill. Ie fill it till it leaks out the fill plug. The fill plug is on the passenger side of the housing. Check for leaks, drive it around, check for leaks again and done.
Things to have on hand.
Proper tools, 13 mm(I think is the size), torque wrench, jacks and jack stands, and drain pan, lots of rags, a hand pump (to get the new fluid into the rear)
Brake cleaner
New 10 bolt gasket (gm stock or whatever is cheaper)
2 bottles of gear oil 80w90 or whatever you desire
friction modifier (either from gm)
The easiest way to tackle this is to jack up the rear from the "pumpkin". Once high enough, place jack stands on the frame IN FRONT of the lower control arms. Release jack to allow rear to drop but the car remains in the air with the rear now exposing the majority of the differntial cover.
Unbolt the cover, use a screwdriver to "pop" open the seal. Ensure the drain pan is underneath to catch the mess of fluid. Take cover, inspect the cover magnet for any matierial LARGER than small flakes/powder. Check the bottom of the differential pumpkin with your hand for ANY large pieces. Rotate your tires or your driveshaft in order to turn the gears and inspect the teeth on the ring and pinion. Look for significant damage aka missing teeth, chunks, cracks, bluish marks (indicating high heat). Clean off the cover and the magnet with brake cleaner and rags, ensure the old seal is off the cover and the differential. (if it was gm stock, it should come off easily as a whole piece else it will need a scraper of sorts to clear off. Put the cover back on with the new gasket and tighten the bolts in a criss cross pattern and torque to spec. If torqued correctly there should be no leaks. Fill the rear using the hand pump. The motto is fill till spill. Ie fill it till it leaks out the fill plug. The fill plug is on the passenger side of the housing. Check for leaks, drive it around, check for leaks again and done.
Thread Starter
TECH Junkie
iTrader: (6)
Joined: May 2005
Posts: 3,185
Likes: 3
From: Highland, Indiana
Originally Posted by SladeX
I have 2 zexel torsen posi differentials on 2 cars and neither make noise. This is not to say yours is the same. GM has released a technical service bulletin on the issue indicating "some" zexel torsen posi differentials tend to make the "rusty nails in a can" sound under low speed turning. ie turning while reversing say into a parking spot. The soultion is to use GM friction modifier to "quiet" down the worm gears. I personally just stick with standard gear oil for the rear, my latest is havoline 80w90 I think. No noise, 2 strips of rubber is fine by my book. But as far as a little noise, not a problem. Like I said, inspect the rear, its a bit on the messy/smelly side, but a little goes a long way in terms of piece of mind.
Things to have on hand.
Proper tools, 13 mm(I think is the size), torque wrench, jacks and jack stands, and drain pan, lots of rags, a hand pump (to get the new fluid into the rear)
Brake cleaner
New 10 bolt gasket (gm stock or whatever is cheaper)
2 bottles of gear oil 80w90 or whatever you desire
friction modifier (either from gm)
The easiest way to tackle this is to jack up the rear from the "pumpkin". Once high enough, place jack stands on the frame IN FRONT of the lower control arms. Release jack to allow rear to drop but the car remains in the air with the rear now exposing the majority of the differntial cover.
Unbolt the cover, use a screwdriver to "pop" open the seal. Ensure the drain pan is underneath to catch the mess of fluid. Take cover, inspect the cover magnet for any matierial LARGER than small flakes/powder. Check the bottom of the differential pumpkin with your hand for ANY large pieces. Rotate your tires or your driveshaft in order to turn the gears and inspect the teeth on the ring and pinion. Look for significant damage aka missing teeth, chunks, cracks, bluish marks (indicating high heat). Clean off the cover and the magnet with brake cleaner and rags, ensure the old seal is off the cover and the differential. (if it was gm stock, it should come off easily as a whole piece else it will need a scraper of sorts to clear off. Put the cover back on with the new gasket and tighten the bolts in a criss cross pattern and torque to spec. If torqued correctly there should be no leaks. Fill the rear using the hand pump. The motto is fill till spill. Ie fill it till it leaks out the fill plug. The fill plug is on the passenger side of the housing. Check for leaks, drive it around, check for leaks again and done.
Things to have on hand.
Proper tools, 13 mm(I think is the size), torque wrench, jacks and jack stands, and drain pan, lots of rags, a hand pump (to get the new fluid into the rear)
Brake cleaner
New 10 bolt gasket (gm stock or whatever is cheaper)
2 bottles of gear oil 80w90 or whatever you desire
friction modifier (either from gm)
The easiest way to tackle this is to jack up the rear from the "pumpkin". Once high enough, place jack stands on the frame IN FRONT of the lower control arms. Release jack to allow rear to drop but the car remains in the air with the rear now exposing the majority of the differntial cover.
Unbolt the cover, use a screwdriver to "pop" open the seal. Ensure the drain pan is underneath to catch the mess of fluid. Take cover, inspect the cover magnet for any matierial LARGER than small flakes/powder. Check the bottom of the differential pumpkin with your hand for ANY large pieces. Rotate your tires or your driveshaft in order to turn the gears and inspect the teeth on the ring and pinion. Look for significant damage aka missing teeth, chunks, cracks, bluish marks (indicating high heat). Clean off the cover and the magnet with brake cleaner and rags, ensure the old seal is off the cover and the differential. (if it was gm stock, it should come off easily as a whole piece else it will need a scraper of sorts to clear off. Put the cover back on with the new gasket and tighten the bolts in a criss cross pattern and torque to spec. If torqued correctly there should be no leaks. Fill the rear using the hand pump. The motto is fill till spill. Ie fill it till it leaks out the fill plug. The fill plug is on the passenger side of the housing. Check for leaks, drive it around, check for leaks again and done.
Is there anything visually with the posi unit that I could see whether its done for or not ? or should the fact that only one wheel turns be proof enough?
11 Second Club
Joined: Feb 2003
Posts: 5,524
Likes: 1
From: The South
Originally Posted by SladeX
Since you have a 98+ car, you have at the least a zexel torsen differential. When jacked up in the air, the wheels should turn in the opposite direction.
They went to torsens as standard in 99, though SSs still had an Auburn option
Originally Posted by Luna
Well here's an update for ya... when I said before that they do both turn that is with the trans in park. Once i change it to neutral only one wheel will turn. I am going to drain the rear tomorrow and make sure there aren't any chunks in there. I changed the rear end fluid about 4,000 miles ago and it looked fine, but I also had no problems with the POSI then.
Is there anything visually with the posi unit that I could see whether its done for or not ? or should the fact that only one wheel turns be proof enough
Is there anything visually with the posi unit that I could see whether its done for or not ? or should the fact that only one wheel turns be proof enough
I'd say you lost your POSI.
You definately have something broken back there.
