[Ravalli Surfer]

Hey! New to the forum. I inherited a 2003 Envoy from my grandpa when he passed this last summer. I’m fairly confident he swapped out the factory rear end with something else. Is there an easy way for me to identify if it has an LSD or the Eaton locker without referencing the RPO codes in the glovebox? Thanks!

I'm sure whatever he put in the vehicle won't be referenced by the RPO code any longer, especially when you state: "he swapped out the factory rear end with something else".

Testing it could create a problem if he used a Gov-Lock type, although I'm pretty sure with the <false> ideal that it is bad design, not many would consider going TO it.

Did he want to simplify and go with an open differential? Who knows!

You can confirm if it is a limited slip type, but no guarantee what system he bought. If you decide to "test" it by doing burnouts with one wheel in the dirt and the other on asphalt ... you can damage any of the limited slip driveline by trying that process to diagnose what you've got.

Pull the cover to confirm it ... are our TBs "C-type"?

I dunnow yet ... haven't had to open mine up so far ... I think they are though.​

​

Jacking the rear wheels off the ground and manually trying to spin one wheel or the other ... won't work, not conclusively.

You SHOULD find out, for sure, 'cause you need to use whatever secret sauce it needs when you service it every once in a while.

.

 

[Ravalli Surfer]

The stock axle was an 8.5 or 8.6; the Trailblazer SS and perhaps others got a 9.5.

All of these have aftermarket replacement differential cases with various forms of "posi" or lockers.


Yes, it could.

The 8.5.8.6 factory locker (Gov-Lock) is often called "Gov-Bomb". The units used in the 8.6 and smaller differentials do not have a good reputation. The 9.5 and 10.5 versions seem to be fairly robust.

Don't be so quick to call a Gov-Lock out ... I have one as the original unit in an '86 K5 with a stroker 350/383, a Crane cam and some early FI heads (small cc's) in front of a very highly modified THM700R4 (manual valve body) and 265/75R17s that I have never treated kindly and yet it works as it should every time.

That's over 30 years of my abuse on it and it hasn't failed yet hauling logs and 3-cord trailers (over 8k lbs) full of firewood in the snow - over stumps and into "oops" holes and trenches and swales and on the highway all summer and winter long.

 

[Ravalli Surfer]

What axle?

I think we decided it was the larger one -- not sure which ...

 

[Ravalli Surfer]

If you know how to use it, the G80 grenade-loc is a great diff. Mashing your foot to the floor before it locks up is a great way to explode it...

If you have zero traction on one wheel and lots of traction on the other, ease into the gas and let it lock up first. If you mash the gas it will have so much inertia when it locks it just goes bang.

True dat!

I just "blip" the throttle a little bit and it hooks softly --- I can feel it go IN .... and from that point it's gonna work very well.

Age and mileage tells me that I've been doing it right all these years (cumulatively, about 25 years or so by now)

 

[Ravalli Surfer]

HERE'S AN INTERESTING TIP FOR LOW-TRACTION SITUATIONS .....

MOST 4L60s will allow you to make a "2nd gear startup" if you put the shifter quadrant into the "2" position.

The transmission will start in 2nd gear, keeping wheelspin to a minimum; it is designed to do this at no harm to the transmission --- and CERTAINLY A LOT LESS STRESS ON OTHER DRIVETRAIN PARTS TOO!

This is MOST 4L60s --- there are some that won't do this. Test it first even on dry surfaces and you'll feel it immediately.

 

[Ravalli Surfer]

That goes back a long time and isn't just the 4L60's. Many transmissions have that feature, even the old TH350/TH400's back in the day. I think the old hydro-matic did too but I never owned anything with one. The 2 speed Powerglide was basically like using 2nd and 3rd gear and that always did fine.

Ford and Chrysler did it and I think many foreign manufactures have that. It's not all models, but many do and it's definitely worth trying if you're in a low traffic situation.

Great tip.

Yeah --- I wasn't going for Ford or Mopar wisdom nor the 7R4s either as they're not in our purview here on a TB/Voy site.

But that's fine --- you're basically right ... BTW: the Ford Version of 2nd gear startups was the Ford Green Dot Cruis-o-matic.

GM's Hydromatic in slant-pan and flat versions had a 2nd gear position, but most started in 1st and then upshifted to-and-held 2nd gear in that position unless you drilled a small 0.030" hole in the valve body or had a Stick-Shift Kit in it.

IIRC -- the Slat Pan version was only available in the Chevy and GM PIckup trucks - typically the up-trim versions like the Cameos ...

Funny in that Reverse was also Park when the engine was off. The pattern was PARK/REVERSE - Neutral - 1 - 2 - 3 - 4 - with a lockout gate on Reverse once the governor sensed that the driveshaft was turning. You couldn't accidentally hit Reverse while going forward at all.

ET built my Hydro (Earl Trapp Industries in Huntington Beach, CA) and he at first had the Reverse Lockout working until I protested that B&M had two kits - one with --- and one without the Reverse Lockout.

I wanted that safety removed ... and he begrudgingly did it for me after a few months of me hanging around his shop - looking sad.​

Earl actually gave me my first job as a mechanic, changing GM Single-Range Hydros to Dual-Range versions as they were stronger and lasted longer. This was on the big ol' Pontiacs, Buicks and Oldsmobiles of the day. Huge behemoths of vehicles.​

That man was so-o-o kind and forgiving to me and after all these years I can say he was the one singularity who sent me into the world of things that go 'round and make noise and smoke.​

I had a flat pan ET Stick Hydro and a 301 Duntov .30/30 solid cam with an Engle Rev-Kit and dual C-style AFBs with a '59 Pontiac 3:71 Posi rear end in a 1959 Chevy 210. (210 was the solid B-pillar, no gauges (idiot lights instead), no chrome (polished aluminum instead), no-roll down rear window - salesman's version - also extremely lighter than the other 4,000 lb versions of the 1959 Chevy body). Mine was 3660 lbs, as weighed at Lion's Drag Strip in SoCal.

My best 1/4 mile was 13.04/106.66 - and this was in 1964. I drove this car on the street for some serious street drag money and lots o'pinks.

Some weekends I'd come home with 2 or 3 cars from LA or Santa Ana or their $$$ equivalence; I always took a couple of driver-buddies with me when I went a-racing so they could drive my winnings home.

That Hydro was just really - if you think about it - a 4-speed granny automatically shifted transmission. It matched shift points with the 4-speed manual dump truck transmission, RPM-to-RPM.

 

[Ravalli Surfer]

Addendum ..... thinking about it here --- I don't remember the THM350s nor the THM400s having 2nd gear start possibilities --- but for sure the THM350Cs and perhaps the THM 3L125C, the THM3L180C, THM3L200C, THM3L325C, the Eurobox THM3L330C/4L430C, and MAYBE the THM3L425C might have had it.

I believe that with a FEW exceptions and builds, when GM put out the Lockup "C"-units, that they all were capable of 2nd gear startups. Mebbee.

It's been a lo-o-ng time since I got stinky on those old clutchpacks and burnt fluid ... fer sure!

 

[Ravalli Surfer]

Addendum #2 --- I remember changing those Slim Jim transmissions out - but the driveshaft was a curiosity to me ...

... and although this is a GM truck I believe, the Big -3 GM battle cruisers had the same sealed driveshaft - in a tube that also doubled as the fore-aft differential location - as a giant torque tube.

Here's almost the same as it was on the transmission end ...

.... and a slim driveshaft went all the way through it. It wasn't very high-horsepower capable --- but it simplified the rear suspension quite a bit. Servicing it was a big bugger though!

Here's a typical pix of what the front end of the driveshaft used --- that big ball is a swivel joint for the driveline and it also kept the front-to-rear dimension as the next support was the leaf springs which could not hold the horsepower from moving the differential when you accelerated.

The third point (you've gotta have at least 3 points to keep driveline alignment and power under control) was either a Dual Beam Watts Linkage or a single pannier rod.​

At an uncertain-to-me date, GM went with coil springs, upper and lower control arms and a single pannier with an open driveshaft.​

The Corvettes and Cadillacs have gone back to Torque Tube drivelines, since they simplify and add a lot of dimensional strength to the rear ends ...

 

[Ravalli Surfer]

IIRC --- the 409 ended about that year too --- 1964.

Contrary to popular belief, the 409 was NOT originally a 348. They were entirely different from the block face-angle to the intake manifold and valve covers not being interchangeable.

AND --- again IIRC --- the 348 only made it in the 1958 1/2 year, all-models up to about 1962 or so. The 348 might've been available in the truck lineup though, for a few more years.

I forgot about the 1958 GMC shorty pickup I had with the 348/Powerglide ... but I only owned it for a few months.