Differentials
Most of our vans came equipped with open differentials. As you make a turn the outside tires travel a greater difference than the inside tires, to prevent binding/tire scrubbing an open differential allows both left and right wheels to turn at different speeds.
Front Differential:
GM started offering the AWD models in 1990, using the aluminum housed GM 7.2 IFS front differential, there are a few differences in shafts/bearings/seals/housings over the years, make sure you get the right rebuild parts.
You can also upgrade to the cast iron alloy housing that was used in some s-10 models(94-04?), this is recommended if you are going to install any traction adders to the front differential.
Rear Differential:
10-bolt cover, 26 spline axle, 7.5inch ring gear through 89
10-bolt cover, 28 spline axle 7.6inch ring gear 90-up
Gear Ratio:
by RPO code(sticker on door/glovebox/purchase sheet):
GU5 Rear Axle: 3.23 Ratio
GU6 Rear Axle: 3.42 Ratio
GT4 Rear Axle: 3.73 Ratio
GT5 Rear Axle: 4:10 Ratio
Here are the differential options offered on my 03 Astro as an example:
G80 Differential, locking, heavy-duty, rear
GT4 Rear Axle: 3.73 Ratio
GU6 Rear Axle: 3.42 Ratio
Read the stamp on the ring gear or count the teeth, then divide:
41 teeth on the ring gear, 10 teeth on pinion = 41/10=4.10 gear ratio.
41 teeth on the ring gear, 11 teeth on pinion = 41/11=3.73 gear ratio.
41 teeth on the ring gear, 12 teeth on pinion = 41/12=3.42 gear ratio.
Jack up one of the rear wheels:
Block the front wheels then jack up the rear on one side so that only one tire is in the air and put the transmission in neutral.
Using the valve stem as a guide, rotate the tire that is in the air 2x times and count how many times and how much the rear driveshaft spins.
(insert video here)
What is the difference between an Open, Locking and a Limited slip differential?
An open differential allows both axles to rotate at wildly different speeds(think one wheel peel). AKA standard rear differential available on all of our vans.
A locking differential forces both axles to rotate at the exact same speed when engaged. G80 RPO code on our vans.
A limited slip differential allows the axles to spin at slightly different speeds. A limited slip was never an option from the factory with Astro/Safari vans. Unfortunately GM decided to use the G80 RPO code for different things on the cars versus the trucks/vans, so that can cause some confusion. The G-80 on our vans is not a limited slip as it doesn't allow the axles to spin at different speeds when engaged.
Factory Traction Adders:
The only factory available traction adder on our vans is made by Eaton and is called the G80 Mlocker - Mechanical Locking Differential, GM Order/RPO Code G80(on trucks and vans) AKA Gov-lock. Normally it acts as an open differential, though as long as you are under 20mph and have a wheel speed difference over 100 RPM it then locks up behaving as a locker forcing both wheels to turn at exactly the same speed. It is not a limited slip as it does not allow a different axle speed while engaged.
Wheel speed difference (left to right) in excess of 100 RPM will cause a flyweight mechanism to open and engage a latching bracket.
Stopped flyweight triggers a self-energizing clutch system until both axles turn at the same speed (full lock). Automatically unlocks.
Latching bracket swings away from the flyweight mechanism above 20mph preventing lockup.
Requires no lube additives
LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109 - (Aug 15, 1991)
SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)
VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS
Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849). The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.
VEHICLES/COMPONENTS INVOLVED: ----------------------------- Some light duty trucks equipped with locking rear axles, RPO G80
Aftermarket Traction Adders:
Limited Slip Rear:
26 spline 3.23 & up
Auburn Gear High Performance Positraction 542057
Auburn Gear Pro series Positraction 542044
Eaton Detroit TruTrac 911A319
Eaton Posi 19663-010
Yukon Dura Grip Positraction YDGGM7.5-3-26-SM
28 spline 3.23 & up
Auburn Gear High Performance Positraction 542040
Auburn Gear Pro series Positraction 542046
Eaton Detroit TruTrac 912A317
Eaton Posi 19599-010
PowerTrax GRIP LS LS247528
PowerTrax GRIP PRO GT247528
Yukon Dura Grip Positraction YDGGM7.5-3-28-SM
Lockers Rear:
26 spline 3.23 & up
Detroit Locker 162C58A
PowerTrax No-Slip 92-0775-2605
PowerTrax Lock-Rite 1930-LR TRUE 7.5” CARRIER WITH EQUAL WINDOW OPENING FRONT AND BACK
PowerTrax Lock-Rite 1932-LR 7.625” CARRIER WITH UNEQUAL WINDOW OPENING
Yukon Gear Minispool YP MINSGM7.5-26
28 spline 3.23 & up
Detroit Locker 162C59A 3.23 & up
PowerTrax No-Slip 92-0776-2805
PowerTrax Lock-Rite 1931-LR
Yukon Gear Minispool YP MINSGM7.6-28
Limited Slip Front:
Mfactory Torsen ~$750 Was developed to support the power hungry Sy/Ty guys https://www.teammfactory.com/mfactory/helical-lsd/MF-TRS-05SYC
Engineered Performance clutch type LSD, $600 if you send good core, Egnineered Performance(pontiacjeff on some forums) 404-394-5776 (8:30am-5:30pm, M-F, also does PCM tuning if you are local to Atlanta) http://www.engineered.net/eplsd.htm and https://www.s10forum.com/forum/f13/new-limited-slip-diff-for-the-7-25-front-diff-514090/
Locker Front:
Powertrax 1935-LR (Discontinued?)
Can I install an aftermarket traction adder myself?
The "lunchbox" lockers that only replace the spider gears don't require you to reset the gear mesh, so those can be installed by virtually anyone. Others that replace the entire carrier, will have to have the gears set up again, and that takes a little skill and a bit of patience. I fully recommend that if you have a reputable rear end shop near by, talk to them about your install/rebuild and see if that is the route you want to take. Otherwise, DIY is definitely an option. https://www.astrosafari.com/viewtopic.php?f=108&t=129450
Which one do I want for my application?
Everyone has different wants and needs, however in short a limited slip differential is best for on road performance and a locker is best for off-road performance. For most people the G80 M-Locker that was optional on our vans is a great choice, not heavy duty for serious off-roading, performs like an open diff = good road manners until it starts slipping when it locks up and gets you through the snow/mud/ice. Torsen units don't have clutches to wear out and are great for on-road performance. Lunchbox style lockers are the cheapest and easiest to install as it doesn't require resetting the gear mesh but can me kinda noisy when they unlock.
There are a few traction adders that really start to blur the lines as well, for example, the Auburn ECTED Maxx-Locker is normally a limited slip differential, but once you apply 12vdc it instantly locks up 100%, truly the best of both worlds. Unfortunately the Auburn ECTED Maxx-Locker is not available for our differentials.
What is a Gov-lock differential?
Gov-lock is an unofficial term for the Eaton Mlocker listed above.
Need more traction, get a set of these: https://amzn.to/2BamHe0
Can I repair/rebuild my differential myself?
Of course you can if you have basic mechanical skills, the proper tools, and patience to do it right. There are plenty of videos and manuals out there that detail how to set up a rear end. Often times you only need a couple of parts replaced, not every part needs to be replaced, but every questionable part should be. Bearing is loose/damaged = replace it, missing teeth on your gears = replace them. CLOSE ENOUGH should not be your state of mind when setting up gears though, shoot for as close to perfection as you can get and have a long lasting repair/rebuild.
This manual from USA Standard Gear covers all of the tools, tolerances, patterns, etc that you need to be able to fix your differential yourself. https://www.usastandardgear.com/install-manuals Down at the bottom of that link is the USA Standard Kit Instructions.