Hi,

I have a problem with my 79 Vette - the rear side yokes are probably worn and when the rear end gets warm after 30 minutes of driving or so, it begins making some screeching/whining (like whouwhou) noise on turns. I've checked that there are no signs of the differential housing being damaged from the outside by the side yokes.

I've bought a 2.5 ton hydraulic jack to my garage and jack stands. It gives plenty of room to work underneath.

I'm wondering whether it's a good idea to go under there myself and is it complicated to get to the housing? How heavy is the differential itself?

I haven't done drivetrain jobs myself yet, but I've done stuff on the engine, electrics, interior etc. I can also get some friends to help when it gets too complicated.

Thanks for all the help!

 

Find a big parking lot and drive in circles in both directions for a while, this may help silent the clutches in your diff.

 

Oh, I'll try that. I guess I have to do it once they're already warm and making noise.

 

If you feel like the car is waddling around corners the clutches are probably stuck. My advice would be to change the fluid in the rear end and instead of one bottle of the posi additive put in two.

Once you have the fluid changed, and then do exactly as Toolman said. Find a large parking lot and turn some circles. I’ll bet you’ll free up those clutches in no time. . . If this is the problem!

If you’re not feeling the duck waddle on turns, then you might have a different problem. Are you sure the noise is coming from the rear end and not one of the trailing arms?

How much does a rear end weigh, about 75 lbs without the cover so I’d say maybe 95 with the cover plus the fluid and axles.

Can you rebuild this rear end yourself? Sure you can! You have the will to learn and there is some great info on this forum on how to do it yourself. GTR1999 is the resident rear end expert and there was a wicked write up on here about six months ago from another individual.

Willcox Inc.

 

Thank you Willcox. We changed the rear end fluid but it wasn't a GM fluid or anything. Still, the place I have my cars fixed at said that it's already a special mixed LSD diff lube. I'm eager to replace the fluid again with the right stuff but please tell me some guidelines on how to find the proper lube. What could be the different names or titles the lube + additive might be sold at?

Thanks so much for your effort, all of you.

What the car is doing is that I can take normal corners at speed with no noise, but when it has gone warm, I turn and accelerate from halt or very low speed, it begins grindling (it doesn't sound like a gear noise to me). It also sometimes wants to shake the rear end a bit and when I give more gas, the noise converts into tire squeeking and I can almost feel it coming from the inside tire of the turn.

I might add that I've done numerous hard accelerations, changing into 2nd from city speed and giving full throttle, there have been NO evidence of issues with the gears. If those were almost worn, I bet they would have already given up...

 

But adding the 40 lbs or so for the crossmember makes it really flippin' heavy. Not 100% necessary to drop the crossmember, but it's way easier than trying to get at the mount bolts with it in the car. And getting the crossmember loose is no picnic.

 

First you need to check the end play of the yokes. As much as 1/8 inch movement is normal....but no more. Do the rear wheels point in at the top or are they fairly straight? If they obviously lean in at top the yokes are worn and need to be replaced. I don't think the '79 has a drain plug so you may need to suck out the fluid with a suction device. If the yokes are OK, get the fluid out and get your gear oil and additive from the Chevy Dealer. The ONLY other product I would use would be Royal Purple, but I prefer the GM product.
You can rebuild yourself, but if you do, replace all the bearings and replace the yokes too. Rebuilt yokes are OK. Set up the gears carefully just like the book says. May as well replace the clutches too while you are in there. Good luck. You can probably do it.

I don't know the current prices, but an exchange, rebuilt unit might be about as cheep and will give you the opportunity to change ratio if you want to.

 

My '78 with 50K was doing that chattering starting from a dead stop and turning. I pulled it out cause I was doing a new spring and the diff side seals and cover leaked. The side yokes were worn to the point of the C clips almost falling off. I read that they were made of poor metal those years. The oil was a mess cause of the metal wearing off. I talked to Gary GR1999 about rebuilding it and I know that would have been the best thing to do, but at the time I just didn't have the money to ship it out for a rebuild. So I got new, I think the rebuilt, yokes from Willcox, seals, cleaned it out as best I could and bolted her back up. Make sure shims go back as they came out. The side yoke play may not be perfect, but it's what I was able to do myself at the time and it runs darn nice now with no noise at all.
After reading up here on the forums I used Lucas 80-140 gear oil and 1 bottle GM posi additive.
With the car secure on jack stands, I put concrete blocks with wood on top or 2 6x6's as a added measure under the frame and with a big car jack, make a craddle for the jack to lower the diff with the cross member attached.

 

I feel a bit embarrassed now since we've already gone through the potential issues of my diff in another thread, but thanks a lot for the help.

Here it isn't very easy to find the GM lubes but I was able to order a bottle of perhaps the right additive with GM original part no. However, they had a similar Ford bottle in stock which they knew to say is the exact same thing. I took pictures of the two different oils and the additive. Can I get the right liquid combined out of those?

http://www.georgescarblog.com/2009/08/31/oil-photos/

 

With cover and yokes they are 100 lbs even, at least on our digital scale they are.

Most yoke endplay is a combination of yokes wear( 74-79 mostly) and production built posi's. I just had one come in because of this and the problem wasn't the yoke wear, those only had 003 wear on them over 37 years but rather the case hole. Instead of a slip fit this one had about 030 wear so be sure to check it good. I was able to machine the case as well and install a 12 bolt cross shaft to repair it rather then replace it.

 

Alright guys, there is an update.

I went through at least 3 automotive stores today and I finally found the 75W-140 fully synthetic gear oil. My personal logics told me that if my diff is chattering and making noise once it has driven around 10-15 miles, the issues rise due to heat (equals thinner oil). That's why I specially was eager to try the -140 oil in it. I also bought Ford Racing Posi Additive which was told to be the exact same thing as GM has. I also confirmed it from another page I found from Google.

I went to my mechanics and we sucked all the old fluid out with pressurized air. Then we filled it with almost 2 Qts of the 75W-140 oil and one bottle (4 oz) of the additive.

I went driving, took it calm the first miles, then went to a parking lot to drive 8's a bit. There was no noise whatsoever or chattering at that moment.

Then I took off with hard throttle to the highway, rode it at about 80 mph for some miles and went to another parking lot to try the 8's. No noise whatsoever, no chattering.

I decided to have a drive through downtown and then after going through huge traffic jams I ended up on a country road where I could drive at 60-70 mph for dozens of miles. At the end I had driven around 70 miles after the oil change and the rear end is behaving PERFECT SMOOTH. Before it used to be this smooth perhaps only when it was fully cold. In addition to the fact that none of the noises appear anymore, I used to have a very noticeable chatter going around corners and the inside wheel wanted to drag with tire screech. It's now all gone.

I don't know if this fixed it or whether it's a temporary solution, but at least my car feels NORMAL again and I don't have to be embrassed for the terrible noises. I will eventually test it again once it has fully cooled down.

Driving 70 miles at high speeds with the previous setup the rear end would have been showing all its faults.

These are the products I used:

2 bottles of this


1 bottle of this

 

Your problem was merely that the proper Posi-Trac additive was not installed when the lube was changed. Without that additive, the clutches in the differential unit will chatter. Some will say that the synthetic lube you put in will be a problem. I'm of the opinion that, if your differential is running well now [with synthetic + additive in it], it will likely run that well for many years to come.

 

That was me.

http://forums.corvetteforum.com/c3-t...ild-paper.html

 

I'm glad to see that this turned out well and cheap! Especially since you don't have easy access to all the parts suppliers we have here in the states.
Now drive it like you stole it! Winter will be here before you know it!
Glenn

 

Haha, true! Before today, I had the oil changed in the beginning of Summer and they added 80W-90 semi-synth oil meant for LSD diffs. It's some pre-mixed "crap" I guess. I believe I should just forget listening to those local mechanics and continue fixing things YOUR way.

P.S.

I still have too big movement with the side yokes but there are no signs of wear on the outside of diff and I've been reading here that sometimes when rear-ends are rebuilt, these c-clips are not even installed. Perhaps that's what was done in my diff by somebody. This is just a big guess though. I think I should still open the diff this Autumn or at least Winter and if necessary, replace the side yokes and whatever else might be needed.

 

Little update again for those who are seeking for info on this issue:

Second day of driving for HOURS wasn't any different from yesterday - works perfectly. I haven't been able to enjoy this car so much EVER BEFORE, because it has always had either a cooling system issue which is fixed by now or this rear end issue. Now it seems rather OK, although there are many items I need to deal with over the winter.

At least one thing is sure - I had a damn great time today with a nice girl sitting next to me, driving through the city, having lunch at an American restaurant, then driving some more