Hi everyone!
I got my 88 iroc 3 years ago and i don't know about what the previous owners did in the lasting 19 years.
so, to set my mind at ease, i want to change the rear axle fluid;
it is borg warner POSI 2.77 ratio
questions:

- what fluid i have to use?
- any additive?
- is there any part to replace after 23 years?clutches, bearings?

thank you all!

ps: the car is runnig fine but sometimes axle doesn't lock

 

80w90
and you need the posi additive.
don't know if the auburn is rebuildable.

 

75W-90 is also acceptable.

Get the additive from the GM store.

 

The GM posi additive is the best. It really stinks though.

80W90 is regular GL-5 gear oil. The 75W90 is synthetic and will work just as well but both still need the posi additive. You're going to need 2 quarts/liters of oil to fill the diff.

 

Read your labels carefully... Many synthetic gear oils (and brands geared towards high performance) come with the limited slip additive already mixed in.

 

Only Auburn can rebuild an Auburn.

Auburn will not warranty the unit if you have used synthetic gear lube in it. Neither will Eaton. Really chafes my hide that they insist on living in the past.

 

asfalto,when you drain the oil pay attention to the
amount of fine iron dust in the oil-place a magnet in
the drain stream if possible,if there is a lot of iron
that means the posi housing and cones are likely worn
-very,very common problem with the B/W 9 bolt.
If all is well,i would drill and tap the housing for a
drain plug and change the oil every 7-10,000 miles
as any particles in the oil will wear and gauld the
posi cones in the B/W rear.

 

thanks a lot!
you guys rule!!!

ciao

 

Working for a gear design company has its perks. I now know why, from a gear design engineers standpoint, that we insist on GL-4 or GL-5 with MT-1 straight Dino Lube.

Heres what we found out.

Auto Gear makes custom 4 speeds and transfer cases. Several of our customers had a problem with our Muncie 4-spd based unit shifting into 2nd gear under load. The only way to get one of these problem boxes to shift into second was to drop the rpm's by 50%, then the trans would slide into second like butter. Several grand in research later; our chief engineer George realized that the bronze synchronizers in the Muncie require a certain amount of friction in the oil. In essence, the oil acts as a friction brake when you shift. These super lubricants are TOO slick. they can't slow down fast enough, there isnt enough resistance in the film. Take one of those problem transmissions, disassemble and clean in a degreaser, fill with 75w90 GL-4 (say Valvoline in the white bottle) and the transmission can take 540" bigblock dragstrip abuse for decades. We were in talks with several oil companies who had trouble with our situation. Candidly we were told by someone in the industry that the API "MT1" sub-spec is essentially a friction modifier.

What does this mean for POSI's? If you need to add a friction modifier, it requires a specific friction as a baseline, before you add it. any rear that uses friction to lock...is not necessarily living in the stone age. Its 60yr old tech that requires 60yr old tech lubricants.

BTW just draining the synthetic out of problem transmissions....doesnt work, its too "clingy" at a microscopic level. It needs to be flushed out. One overseas customer drove and drained his fluid 5 times before it would shift normal.

Synthetics are great for some things, but when using it in a vintage application, you need to remember the needs/requirements could be different

 

Not a good idea to use synthetic gear oil in the rear diff on my 1987 Iroc?

 

What Gear Oil Should I Use?
Standard 80-90 weight and a friction modifer (GM P/N 1052358) if the limited slip is an Auburn.* A Gov-Lock or open/standard differential doesn’t require the additive. The rear holds approximately two quarts of gear oil.

*Though some TGO members use a full synthetic, such as Mobil 1 75-90, along with the above friction modifier, both Auburn and Eaton do not recommend synthetics for usage with their limited slips.

JamesC

 

I'm not sure if it's Auburn or not. I know it has the borg-warner 9 bolt posi with 3.45 gears. Does that mean it has the Auburn?

 

I have an ‘87 IROC with a POSI rear end, which doesn’t lock up. Possibly someone changed the fluid but didn’t add the friction modifier? Hate to have to rebuild it.

Pete

 

The friction modifier REDUCES the friction of the clutches, to keep them from chattering when going around corners. Lack of it WILL NOT prevent the posi from working.

Your "posi" was probably an Auburn if your rear is a 10-bolt. Well and widely known to be a total POS. Equally well known to almost NEVER still be working after around 75k miles or so. Yours is probably no different from everybody else's in this respect. It is most likely now a pile of metallic sludge laying in the bottom of your pumpkin. Again, no different from everybody else's. It is not rebuildable. (this being the icing on the multi-layer cake that defines that POS as a POS... one time use ONLY) Throw it in the trash and start over. I recommend a worm-gear type of carrier rather than a clutch type (after all, it IS the 21st century now) such as a Torsen or a True-Trac.

If your rear is a 9-bolt, it has basically the same problem, but can be temporarily partially somewhat "fixed". In those, the clutches are cones, that fit down into a tapered hole; as they wear, they sink farther and farther down into the hole; eventually they bottom out and at that point quit doing anything. It's possible to machine off the bottom surface of them, and allow the "sides" to once again contact the hole. You can even get now cones, but the new ones, being for a slightly different rear, don't fit exactly right; but can be easily modified to work. And, I do believe you can get a worm-gear carrier for it, as well, rather than dinking around with the Stone Age clutch systems.

All of this is unrelated to "rebuild", except to the extent that any sensible person that takes apart something that wears out, and that has enough use on it that it's already worn out, would replace all the "wear" parts like bearings. Doing that is what we call "rebuild". You can "rebuild" the rear and not fix the clutches, or fix the clutches and not do a "rebuild"; but neither of those makes sense. If you're gonna tear into it, might as well do it all and do it right.

 

Thanks for the info! Still trying to figure out what to do...trying to keep it as stock as practical, but want it to be right. My IROC only has 30k miles on it; she's been sitting in storage most of her life (previously owned by a collector) but I want to drive her, not just stare at her.

Pete

 

Without going too far down this rabbit hole I'm in the middle of recone-ing my 9 bolt. I've been going over everything I could find in the FSM and online and I saw your previous comments about wasting time on a 9bolt. Just tonight in re-reading the FSM for the 10th time there is a specific note that says "do not replace the cones separately from the differential." This really struck me, as the cones and the differential body are mated wear surfaces, replacing just one does not make sense. Well anyways I waited 3 weeks to get the cones from AUS and I will put it back together with them and some dial gauge measurements and wear pattern look-sees. If she blows then I will be moving on.

 

Your differential uses metal clutch cones, and not clutch plates... (2.77:1 gears are a Borg-Warner Diff).
Replacement parts for the carrier assembly have never been good... but are available if you HUNT for parts.

Bearings and seals can always be replaced after all this time.

If the Diff. assembly has no leaks; and the POSI operates properly... consider yourself lucky (most of these have bottomed out clutch cones).

There are some very good threads on this forum about rebuilding/ fixing the carrier.
I like the lighter weight gear oil for a stock drive-train and the heavier stuff for a performance build.

There are also ARP carrier cap studs and multiple cover girdles available to try and put a little power through them.

I really like the tiny BW carrier, with its 4 little spider gears.
I ran one of these with 31-spline side-gears, cones, and Moser axles for a very long time with over 600 HP to the tires...
Too many trips to the track on slicks, is what eventually killed it.

The Dana 44 rear (warranty upgrade, only after multiple failures) was the best OEM rear for 3rd Gens...
But it was never installed from the factory (some dealers up-fitted them) and some aspects of the BW rear are actually better.

 

A stock 87 Camaro 350 with a stock 5-speed,.......... Are you sure about that ???

FYI: If your car has a 5-speed it's almost certainly a 305 engine and it's probably got a 9-bolt rear end. The fluid you'll need to use in the 9-bolt is GM 1050010 (75W90) and NO additive.

https://www.thirdgen.org/forums/hist...er-9-bolt.html


The friction additive was recommended for Auburn type ( 10-bolt ) "POSI" rear-ends.