Differential / 4WD general knowledge questions

6716

Original poster
Member
Jul 24, 2012
830
So, it looks like I've got the 3.42 gearing, according to the glove box code.

That also happens to be an "open" differential on these vehicles, right? I spin the right rear in the wet grass, and the left rear doesn't ever engage.

So, what happens when I switch the selector to AWD or 4HI? Do I really get ALL wheel drive, or just one rear and one front?
 

CaptainXL

Member
Dec 4, 2011
2,445
6716 said:
So, it looks like I've got the 3.42 gearing, according to the glove box code.

That also happens to be an "open" differential on these vehicles, right? I spin the right rear in the wet grass, and the left rear doesn't ever engage.

So, what happens when I switch the selector to AWD or 4HI? Do I really get ALL wheel drive, or just one rear and one front?

Look in your glovebox door you should have a G80 code on the RPO sitcker. If so then you have the locking diff. If it's not working then it needs service.

All wheel drive is a full time option on the Bravada and SS but not on your vehicle.

In 4X4 mode if you are in wet grass or mud and both axles are slipping (no traction to all 4 wheels) then 3 wheels should spin (only two should spin if there is no locker). The front is not a locking diff so one of them just sits there.

In 4X4 mode under normal operating conditions all 4 wheels propel the vehicle if they all have traction.

[video=youtube;q-rQTHMVAuw]http://www.youtube.com/watch?v=q-rQTHMVAuw[/video]
 

6716

Original poster
Member
Jul 24, 2012
830
No G80 code, so I guess it's an open diff.

What does the "RPO" stand for?

I kept searching after I started the thread. Between Roadie and Hardtrailz it looks like the front diff only powers both wheels when they have equal traction (if I understood correctly) which Hardtrailz seems to say is effectively never, anyway. And the rear (on mine) is not a locker or limited slip.

So I guess maybe a better way to ask that question might be: if I have the open differential in back, switching to 4HI (because don't get Roadie started on A4WD) doesn't do me any more good for the rear, it just adds power to effectively one wheel in the front --- right? Or no.

And my original post was not super clear. In 2HI, on wet grass, I hit the gas and spun one tire, which makes sense since I don't have the G80.
 

6716

Original poster
Member
Jul 24, 2012
830
Out of curiosity, do you happen to know which of the front wheels is the one that would be powered?

Seems like on the rear it's the right ....
 

6716

Original poster
Member
Jul 24, 2012
830
Er, wait ... both wheels are powered, but if one of them breaks traction it will spin while the other sits still?

So, when I had the right rear spinning on the wet grass, it was just the wheel that broke traction first?
 

tblazerdude

Member
Dec 4, 2011
321
unfortunately, the tire with the least traction (least resistance) will spin. This applies to front and rear axles/differentials. Could spin on same side or opposite sides.
 

tblazerdude

Member
Dec 4, 2011
321
6716 said:
Er, wait ... both wheels are powered, but if one of them breaks traction it will spin while the other sits still?

So, when I had the right rear spinning on the wet grass, it was just the wheel that broke traction first?

Correct, whichever breaks traction first, spins.
 

CaptainXL

Member
Dec 4, 2011
2,445
6716 said:
Out of curiosity, do you happen to know which of the front wheels is the one that would be powered?

Seems like on the rear it's the right ....

Both get power with repect to how the differential splits it up. But the wheel with the least amount of traction gets the most power. If any one wheel is on ice it will just spin and you go nowhere. Thats where limited slip or lockers come in handy.
 

CaptainXL

Member
Dec 4, 2011
2,445
You know you can add a G80 if you want. I got a like new G80 pull out from eBay about 7 years ago for my GMC Sonoma. All that's required is that you remove the carrier and swap over the ring gear. Make sure everything goes back the way it came out. You might need some extra shims in case the backlash is not perfect with the shims that came with the open carrier.
 

6716

Original poster
Member
Jul 24, 2012
830
Yeah, I saw that I could get a different differential. That brings me back to the gearing, though, right? I'd have to make sure I had the same gearing as the one that comes out so it matches still with the front, wouldn't I? On Rockauto.com I didn't see that gearing was a point of selection for the parts, so I didn't understand that, either.

I'm going to have to do a lot of study first before I am going to be confident in pulling off the differential swap-out. My mind is always three repairs ahead of my budget and shop time.

Mostly, this all started earlier today when I was looking at a truck to buy for snow plowing, and I noticed what looked like oil all over the pumpkin. And the cover was all rusty, and there was a drop of oil at the bottom of the cover waiting to fall. So I looked at a different truck and it didn't look like that, and then I looked at my TB, and not only did my TB not look like that, I realized I didn't know as much about differentials as I probably ought to if I thought I was going to buy and run a plow truck through the winter.

As I looked at parts on-line to try to figure out what my financial exposure for the differential would be if it failed (or was obviously crap and that's why the truck was so attractively priced) I kept realizing I still had more to learn.

AND THEN when I went to look for internet forums for that vehicle type, it became clear that GMTNation is a rarity among forums. Not much of a community out there on other platforms, that I could see. Maybe I just didn't hit the right ones, I don't know.
 

CaptainXL

Member
Dec 4, 2011
2,445
6716 said:
Yeah, I saw that I could get a different differential. That brings me back to the gearing, though, right? I'd have to make sure I had the same gearing as the one that comes out so it matches still with the front, wouldn't I?

Yes, unless you have a center differential used in AWD vehicles. In which case you would need a different transfer case. But even then you would still want the same front and rear final drive ratios so you put down equal amounts of power to all wheels.

In your case all you need to do is swap the ring gear over and that is about the hardest part. Once you have the rear on jacks, remove the wheels and drain the fluid. Remove the center pin and c-clips holding the axels in. Then remove the bearing caps and it will come out. Then swap the ring gear over, put back in and adjust backlash using correct thickness shims on either side of carrier bearings.

When shopping for a new carrier such as a G80 you need to make sure you have the correct number of splines in the side gears and also the correct size carrier so the old ring gear will fit.
 

tblazerdude

Member
Dec 4, 2011
321
It has been discussed before that the easiest/least opportunity for mistakes is a junk yard rear end with the G80. Here in Illinois, you can get one shipped freight for $150, and the axle itself is around $150-$300 depending on year and mileage. Then it's plug and play. Just make sure you match ratios and if possible, get one from an XL/EXT since they are stronger.

then you will have three wheel drive
 

CaptainXL

Member
Dec 4, 2011
2,445
Oh, almost forgot. If you do go ahead and do this (since the axles will be out) it would be a prefect time to replace the rear wheel bearing and seals if needed. If you are up in the miles category I would recommend new Timken seals. Also, wouldn't be a bad time to do a complete rear brake pad/rotor service on both sides as well. Just thinking ahead for ya.:smile:
 

CaptainXL

Member
Dec 4, 2011
2,445
tblazerdude said:
It has been discussed before that the easiest/least opportunity for mistakes is a junk yard rear end with the G80. Here in Illinois, you can get one shipped freight for $150, and the axle itself is around $150-$300 depending on year and mileage. Then it's plug and play. Just make sure you match ratios and if possible, get one from an XL/EXT since they are stronger.

then you will have three wheel drive

I can see how you might save money getting a used axle but I would skip the junk yard when looking for a G80 unless you plan on rebuilding it. I've heard too many horror stories about people ending up on the side of the road or worse after grenading a used one.

The best option is to search around for like new pull-outs from dealerships or shops that specialize in performance rear end upgrades. Many times performance enthusiasts will have shops replace their G80 for Zexel Torsens or some other locking diff like a Detroit Locker or ARB for off road use. I got mine on Ebay for about $150 including free shipping. When I got it, it was brand new with no visible signs of wear. So whichever way you go, just be aware that there are good pull-outs available in the market for excellent prices.
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
I usually use the torque = water analogy. Differentials are essentially T-fittings in plumbing. Torque flows in, and gets split. If the resistance (traction) is equal on both sides, the pressure can build up. If one wheel is slipping, all the torque leaks out that side and the pressure can't build up on EITHER side. The amount of torque applied to the terrain is equal to the lowest torque tire. Which with open diffs can be pretty darned low.

In contrast to some AWD vehicles, our transfer case, when locked up in 4HI or 4LO, can be a pretty effective front/rear locker, so the RPM of the front and rear driveshafts will be the same.

The G80 (or other aftermarket locker) in the rear is what makes rock crawling possible at all. I've been on trail rides where a 4wd/no G80 truck needed multiple strap assists while a 2WD/G80 truck got through, admittedly with some intermittent wheel-spinning drama.
 

6716

Original poster
Member
Jul 24, 2012
830
tblazerdude said:
then you will have three wheel drive

Three-wheel drive :rotfl:

See, I'm learnin' stuff here. I'm always amazed by what I used to believe before I got into the details. Like "I've got four-wheel drive!" means all four wheels are always powered directly and independently when the selector is on 4HI, so I will never have a traction problem and can therefore go out in any conditions and drive anywhere over anything and nothing bad will ever happen to me. And everyone lived happily ever after.

And if I really can upgrade for about $150 and the chance to take the truck apart and see if I can get it back together again, that's pretty tempting.
 

6716

Original poster
Member
Jul 24, 2012
830
CaptainXL said:
Oh, almost forgot. If you do go ahead and do this (since the axles will be out) it would be a prefect time to replace the rear wheel bearing and seals if needed. If you are up in the miles category I would recommend new Timken seals. Also, wouldn't be a bad time to do a complete rear brake pad/rotor service on both sides as well. Just thinking ahead for ya.:smile:

See Cap'n, that's the problem. One ball joint has already turned into struts, UCAs, shocks, springs, links...

It's going to take me to 300,000 miles to get all my "want-to" "may-as-well" "while-we're-at-it" stuff done, but by then it'll be BETTER than new, right?

Fortunately pads and rotors are only 8 months old.
 

Jkust

Member
Dec 4, 2011
946
Just since it hasn't been mentioned, the slightly newer 360's got traction control in the 4wd truks as it used to only be on the 2wd and they also got a separate system called Stabilitrac. Of course you'd shut these all off when you rock crawl but everything mentioned above takes a turn with these two systems.
 

navigator

Member
Dec 3, 2011
504
Jkust said:
Just since it hasn't been mentioned, the slightly newer 360's got traction control in the 4wd truks as it used to only be on the 2wd and they also got a separate system called Stabilitrac. Of course you'd shut these all off when you rock crawl but everything mentioned above takes a turn with these two systems.

I turn mine off anytime I think it might be possible to get stuck.

it works great on gravel roads if you don't want to fishtail but going through mud I have had it basically stop because it was auto-braking every wheel.
 

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