Rear end issues

[mntb]

I'm having issues with rear end of my tb. I believe it has a limited slip diff and 3.73's. The rpo codes are gt4 and g80.

When trying to make a sharp left turn, the left rear will drag like its locked to the right rear wheel so they won't turn at their own speed. I can make sharp right turns no matter what no problem. It only does this after leaving a slippery/low traction surface like it ice or sand. It doesn't happen all of the time but it's becoming more frequent. There is a slight vibration at high way speeds. Others don't feel it but I do. I can't tell if it's coming from the rear end or not however. So I don't know if it's related to this issue or not.
I was going to change the rear diff oil/lube and hopefully that is the issue otherwise maybe the diff is shot? Wouldn't surprise me at this mileage.
What gear oil/lube can I use? I believe 75w-90 is what I need, correct? Some info I've read said only gm/dealer stuff to be used while other info says mobil1, amsoil, or royal purple can be used. Which is it?

Also, is there anything that I should be looking for while changing the diff oil?

How much will it take to refill the rear diff?
Any other tips/suggestions are welcome.

Thanks in advance

 

[Paul Bell]

GM posi rears have a centrifugal weight that engages the posi parts. Sometimes they stick locked after it's no longer needed.

How many miles on this rear? When was the fluid last replaced?

 

[mntb]

The TB it's self has 306,xxx miles on it. Doesn't look like it though. We have owned it since 290,xxx. We have owned it for a little over a year. I assume the rear end is the same mileage. We were not given any service records or anything. It was a small used car lot we bought it from with a good reputation. I asked for service records and they said they didn't have any from the previous owner.

 

[Denali n DOO]

Edit. No ls additive required.

 

[Mooseman]

First off, you have a G80 locker, not an LS posi (G86). Additive won't do anything.

Sometimes they stick locked after it's no longer needed.

This is likely the scenario. You should open it up to check things out and change the fluid, 70w90 synthetic gear oil.

 

[mntb]

How much oil will I need?
Do I need a new gasket or is the current one reusable (assuming not ripped)?
I've read about guys using hand pump to add gear oil, is that the same as a transfer pump with two hoses or is there one that connects directly to the oil bottle?
After opening the diff, is there anything specifically I should look at?
Basically just wipe as much of the old stuff out as possible?
Should I use anything to clean the cover or internals?

Thanks for the responses

 

[Mooseman]

If it still has the original rubber gasket (which I doubt), you can use RTV like in the video. 1.8 qts. A pump does make it easier for filling.

 

[Chickenhawk]

Always remove the fill plug first. They can freeze to the diff because of the dissimilar metals, and then you are screwed. The factory fill plug is a 3/8" square socket extension end. Clean out the hole and find an old (or cheap) 3/8"-to-1/2" adapter, and grind the 3/8" end until its flat. Hammer it into the hole. Use a long 1/2" ratchet and hope for the best. Strip the hole. Curse. Have a beer. Take it to a shop. They use heat to get it off; weld a socket extension to the fill plug in worst case scenarios. (They have done this thousands of times.) Pay the bill. Curse. Get home. Have a beer. Vow to use a better fill plug. (My shop used a plug from a Vette. Same thread, but uses a hex socket.)

For filling, I use an outboard motor gear oil hand pump. It screws directly to the top of the gear oil bottle. Works great.

I always use the best gear oil I can find. Synthetic. Here, Royal Purple is expensive but available everywhere. I don't buy into the "boutique" oil scam, but my shop sells it and I can buy it at Canadian Tire. It is overpriced snake oil, but at least I know it is top quality.

You don't need limited slip additive. However, unlike the others who tell you (quite correctly) that our diff does not need LS additive, I found that a squirt helps IF you have occasional problems like you describe.

That being said, the design of the diff is such that what you experience is normal. Change the fluid though.

And, PLEASE, don't forget the transfer case fluid MUST be changed every 50,000 miles. DO NOT NEGLECT THIS. Most owners do.

As for your slight vibration at speed, if it seems to come on at certain speeds (such as 80 to 90 KPH) then look at your outside mirrors. If they are vibrating slightly, it will be noticeable. If so, it is usually driveshaft U-joints. If it is more of a "feeling" in your butt, it is likely the beginnings of a wheel bearing going. Don't worry about it for now, and see what symptoms seem to be getting worse. If it's a wheel bearing (which is likeliest) then you will start to hear some faint rumbling noise too.

By the way, if you think it's a wheel bearing and you hear it more turning in one direction and it goes away when turning in the other direction - trust me on this - do not assume the bad side is the side where the noise goes away when turning toward it. This is the traditional way of diagnosing which side is bad, but it doesn't work on our platforms. We have had dozens of members - myself included - who heard the noise go away when turning right and assumed it was the right side that was bad ... and were wrong. (I bought two, just in case.)

Hope this helps.

 

[mntb]

Awesome info. I'm guessing with my luck the plug will be seized inside the hole especially because I live in Minnesota.

As far as the vibration goes, I've had both wheel bearings go bad. I could usually feel them in the steering wheel. I always assumed it was the side I was turning away from. Since it was the side being loaded with the weight/pressure of the turn. I feel this vibration in my seat and I can see it in the mirrors.

Right now o'reilly has royal purple synthetic max-gear high performance gear oil for $17/quart which is only a few dollars more than the house brand.

The front diff, transfer case, and Trans are all on my list for fluid changes.

Thanks for the info/advice.

 

[smt 59]

If you go with Amsoil you will not require any additives at all, best quality oil, as for your vibration it is likely the front u-joint starting to go. Replace with Moog greasable joints. For the cost I would recommend replacing both while the shaft is out. As mentioned "Change the TC fluid" asap.

 

[Wooluf1952]

Many of the 2003's have a drain plug. Helps keep the mess down. When I changed the fluid in my 2003 Envoy I used Mobil 1 with LS. They say it's not needed, but, it was the same price and I figured it wouldn't hurt.

 

[Mooseman]

I'd still open up the cover even with the drain plug. And as mentioned, open the fill plug first. Use an old 3/8" extension and grind down the square end flat, as most are rounded, for a better grip. Soak the plug in PB Blaster prior.

 

[mntb]

When you guys say grind down the square end, do you mean the tip of the extension so its a flat top as opposed to rounded?
I have more than a few extra 3/8s extensions laying around. Will the ball bearing that holds the sockets be a problem?
Would amsoil severe gear 75w-90 be a suitable gear oil to use in both the front and rear differentials?
Can the same gear oil be used in the swb TBs?
I use my tb for work. Delivering newspapers to residential customers 7 days a week. I don't know how much weight I carry necessarily but on Sundays (about 450-470 papers give or take a few) it's enough to make the rear level with the front as opposed to the normal stance that all TBs usually have. It's a lot of stopping and going.
For the transfer case, I have to get that stuff from the dealer, correct? How do I need?
I just want to make sure I use the right stuff.
Thanks in advance for help.

 

[Sparky]

Exactly. If you don't do that, it doesn't grip as well. I didn't have a spare extension to do that with and it sucks! I need to do that before my next gear oil change.

That oil choice is fine for both front and rear. Nothing special between LWB and SWB differentials, just the LWB (and the V8 models) have a little larger gear set in the rear.

The Autotrak II fluid you get from the dealer. 2 bottles worth.

Definitely take the cover off and look at the rear end gears condition, if you're having issues with it locking randomly. You might have a damaged piece in there or something causing that, and just a fluid change wouldn't necessarily tell you much.

 

[Mooseman]

Will the ball bearing that holds the sockets be a problem?

It might fly out but since you are purposing that extension for the fill plug, it won't be a problem.

 

[HARDTRAILZ]

I found its best to use an impact on the fill plug cause the vibration really helps it pop free. I have never round the extion down, but it does make sense. I just put an ext on the impact and hit it a couple times. spray some PB and hit it w impact a gain and they pop right out.

 

[smt 59]

Yes, use Amsoil on both.

 

[mntb]

So I crawled under my tb today. And looked for the fill plug to see if I can in fact get it out before I went and got the rear gear oil. I didnt have a lot of extra time though. I couldnt find the damn thing. Or a drain plug. Didn't figure I'd find a drain plug.
All I see is in the location of the fill plug in the video is not a plug but a nipped with a hose connected to it. The hose is secured to the frame with a plastic clip. There is a plug on the end of the hose.

Am I supposed to use this hose to fill? If not what is it for? A vent hose of some sort?

 

[Sparky]

That's the vent hose.

The fill plug is on the passenger side of the pumpkin, tucked up close under the gusset halfway up the side, near the axle tube. It tends to blend in. It doesn't have a hex head on it, just a square hole for the 3/8" drive.

 

[Blckshdw]

Yeah you found the vent hose. On the passenger side, just in front of the axle, is where my fill plug is... Just snapped this.

**scooped by @Sparky **

 

[mntb]

OK. Thank you. I will Take another look tomorrow after a "car/under body wash". I hate using the car wash at the gas stations. I prefer the coin/self service washed where I can bring my own soap and bucket.
Like I said, didn't have a lot of extra Time today. And EVERYTHING was white and caked with salt and dirty. Everything blends in. I thought the plug would at least stick out a little and not be almost flush with the housing.
Thanks for the pic.