Driveline Slack

[VA_GMC]

I recently pulled the rear drums and axles while doing some maintenance to see the condition of everything - I haven't had the truck for a long time. I lost some gear oil when pulling the axles and added some synthetic blend to top it off. It didn't take a whole lot - maybe a half a bottle. When I got the truck back on the road, I noticed that once I come to a complete stop and take off again I can feel the intial driveline slack (very little if any) but as I increase the torque on the driveline (and am now rolling) it feels as though more slack is being taken out. I also have some driveline vibration at interstate speed - this has been a characteristic of the truck. I checked the u-joints and all feels/looks o.k. - nothing that would cause the definte jolt that can be felt. I can move the carrier quite a bit (at least a 1/4" up and down) but it looks like it is meant to flex. My truck does have a full floating, limited slip differential. Any thoughts, suggestions, similar experiences?

Thanks.
William

[Hye]

This is just a guess. A long time ago, on my '83's with LSD's, a mechanic asked me if I was using an additive GM sells for limited slip differentials. I'd never heard of it. Anyway, it was cheap and he said GM reccommended adding the stuff every 50k??? miles. I bought two bottles. After I put it in the Blazer, a "just feels wrong" problem that I thought must have had something to do with the brakes went away. And the problem in the Suburban of a clunking noise during turns went away, too. Maybe the limited slip just wasn't slippin' like it was supposed to.
Also, I wonder if the synthetic oil you added had anything to do with the change in the way is drives.

[catmandoo]

i have an ext cab sb 2wd with 420,000 and at between 62 and 67 it has a vibration kinda like the back end is hopping,i've also seen this in a 97 1/2 ext 2wd with under 90,000 so its evidently these ext cab trucks quirk,i do have a little play in the hanger bearing and am gonna try that and the rubber to see if that cures it.

[Dvldog 8793]

Howdy
sounds like all your problems can be traced to adding the oil. If it were me, I would pop the cover, let it drain all night and start fresh with new synthetic oil and GM additive.
Just my .02! ;)
Hope it helps!
L8r
Conley

[norm]

I wouldn't use the additive so soon. Some rears are limited slip and some are lockers (with the Gov-Lock setup). If it's a Gov-Lock with no clutches, you don't use additive. With the gov-lock, if you jack up the truck, the wheels will spin opposite ways like an open diff. They won't lock up until one spins at a certain RPM difference. I had (new) a 98 K2500 with this (in a semi-float), and my 95 has it, too (with 10.5 full float).

[sk8rdi16]

I have the same initial jolt after coming to a complete stop. The suburban has the 4.10 and Limited slip. I hope the GM additive will help! Where can I find info about different rear ends, ie. 12/14 bolt floater? I am very new to trucks, this is my first!

thanks

[Dvldog 8793]

Howdy
A full float rear end can be ID by the center hub, it will have 8 bolts that hold the axle in place. a semi float will have no visable bolts on the outside of the hub. You may have to remove your wheels to see this. Most 2500s and all 3500s are full float rear ends. The main difference is that a full float has about double the bearing size and has very little down-pressure stress on the axles. A 10 or 12 or 14 bolt rear diff is ID by counting the number of bolts on the rear cover. The more bolts the bigger the cover and NORMALLY the bigger the ring/pinion gears inside. Any Drivetrain is going to have some slack, with that many moving parts I think it is inevitable. The rear diff additive is a friction modifier that helps the clutches in a limited slip diff work the way they should. Some oil companies say that it isn't needed but I feel better using it. I don't think it will change the slack/take up that you are feeling but it might
I am NOT an expert and I hope that anybody that knows different will correct me! ;)
Hope this helps!
L8r
Conley

[cruzer]

GM made two 14 bolt rearends for trucks. 1 is a full floating rearend which would have a hub extending thru rim, such as dvldog describes. It has a 10 1/2" ring gear. The other 14 bolt rearend is a semi floating rear. No hub extending thru the rim. The axles are held in w/C clips. This rearend has a 9 1/2" ring gear. It was used on light 3/4 ton p/u's and subs.

To ID the 10 1/2" ff 14 bolt w/o taking the hudcaps off, look for bolts a couple inches behind the pinion yoke, similar to a ford 9". FYI,they will not be in a perfect cicle. I can't remember it's either 6 or 8 bolts.

If you have a 8600GVW or more, vehicle you probably have a ff 14 bolt rearend

A locking differential(detriot/gov-lok)will usualy have more play in them, sometimes up to a 1/4 turn.

[Hye]

Mine "thumped" when I came to a stop. Additive cured that on the Blazer.

[VA_GMC]

I looked for a tag on the diff. this evening to see if I could identify what type it is but I couldn't find a tag anywhere.

According to my VIN the truck has the following:
GT4-3.73 Ratio
G80-Limited Slip (Positraction)
C6P-GVW Rating, 8600 lbs
L65-6.5L, Turbo, HO

Is there anyway to tell from this information what type of positrac rear it is?

If it is a gov-lock locker, would the additive hurt anything or would it just be a waste of money?

[Hye]

This link might help for differential identification.

http://www.coloradok5.com/axleguide.shtml

[ANXIOUS-SUBMAN]

I have the 14 bolt full floater with the govloc. When I first bought the truck with 25K miles the rear end would knock quietly when turning at the end of a ramp coming off the expressway. Several posts I read somewhere suggested changing to a syn gear lube which I did. It took a while for the syn to work in but it took care of the noise. At 92k the sound is coming back so I guess it's time to change the lube again. I use Mobil 75w90 syn.

I forgot to add that I have always had noticable slack in the drive train. I read somewhere long ago that some slack is a natural characteristic of the govloc.

[VA_GMC]

It looks like the 10.5" picture on the web.

I called AutoZone for a gasket - they have all the sizes in stock = I plan take a look at the gaskets to find out for sure which one it is.

I will probably get the Mobil 1 75-90 while I'm there. I'll probably get the get the friction modifier while I'm in town.

Maybe there's a way to tell what type it is once I get the cover off?

[norm]

Good question, VA_GMC. I doubt if the additive can do harm to a gov-lock, but as Mike points out it can be sensitive to lube quality. The option in my '98 (8600 lb/350 K2500 p/u with semi-float 9.5" 14 bolt) was a G80 "locking rear diff." according to build sheet - and had the gov-lock. The option in my current truck(K3500 6.5 with full-float 10.5" 14 bolt) is listed the same way, and is a gov-lock. You can tell a gov-lock by the rotation method, removing the cover and viewing the fly-weight mechanism, or feeling it kick in when one tire loses traction on the slick -- smooth it ain't, but when it's locked it's locked.

[Dvldog 8793]

Howdy
I'veseen ALLOT of rear diffs leak with a gasket and not many with just silicon. Get a tube of "the Right Stuff" and use that instead of a gasket. Make sure you get both surfaces CLEAN.
I normally take off the cover and let it drain for as long as possible with the nose of diff jackd up slightly, then when I'm ready to put the cover back on let the nose down so I get a good dry surface to work with.
Have fun!
L8r
Conley

[catmandoo]

also wipe the surfaces with lacquer thinner to get all the grease off the sealing edge.and i too haven't had very good luck with gaskets either.

[VA_GMC]

Thanks for the replier/pointers.. I did get the gasket but may take it back. However, it did allow me ID the size - 10.5".

I bought the Mobile 1 75-90 and a tube of Trans-X Limited Slip Gear Additive (GM#1052358 - listed). I looked through my owners manual and didn't see anything about an additive for the gear oil. Reading the Trans-x bottle, it says it works with conventional or sythetic oils. It doesn't appear that it would hurt anything to add it anyway. -?

[VA_GMC]

I just read norm's post on the "front & rear diff oil, change or not" topic. I will take a picture when I have the cover off tomorrow to see if it will help with identifying the positrac type.

[lineman]

Howdy,Use the silicon,it's the best and cheap.There's also another way to I.D.the 14 bolt.I was taught to look at the pumpkin from the rear.See if it comes to a point at the bottom or if it's nice and rounded. The 14 comes to a point. Which means less ground clearance here around the farm and woods.But it is some kind of stout and dependable!...... Lineman

[Dvldog 8793]

Just read my post from the other night and thought I better clarify. "The Right Stuff" is a gasket making product made by Permatex (I think)
It is the same stuff that GM uses on the line. When I pulled my motor apart the valve covers were pretty much one with the heads because the "stuff" was INCREDIBLE! It comes in a pressure can and does not get hard in the tube like other silicon. It ain't cheap but it DOES work!
L8r
Conley

[VA_GMC]

Here is a photo of the diff.

It looks like a limited slip - but I have no idea of what gov-lock looks like.

http://home.earthlink.net/~themonroes/GMC/diff2.JPG

[VA_GMC]

Changing the fluid did help some with the "jolt" but it is still there at times - maybe not as bad.

Changing the fluid did change the characteristics of the 65+ MPH interstate vibration. I haven't decided if it is better yet or not. The vibration used to last for about 3 seconds and then it would be gone for 3 seconds - it would continue this oscillation. Now is seems to be more drawn out and possibly reduced, although, the vibration is still there. At least I know that it is related to the diff. - maybe it's normal for this type of diff. I know others have mentioned a higher speed vibration as well.

[VA_GMC]

Almost forgot to mention that I used the "great stuff" - pretty good stuff smile.gif . No leaks so far..(300+ miles since Saturday) and some moisture had come in around the outside edge of the existing gasket and pitted it some.

Anyone had to remove it? Is it tough to clean up when taking something apart later?

The only thing I didn't care for was that our local parts store had it in a caulk type tube. It wasn't the easiest to use.