how to tell gear ratio and posi?

[Ricks1971]

Guys, I have a 78 4X4 and was hoping for help finding out what gear ratio and if it has posi? I know you can tell by the codes in the glove box? thanks Rick

[cor86]

Your right! It should list the ratio in the spec sheet, and there is a code for posi, but I cannot remember it. The not-so-easy way is to mark the driveshaft, put it in neutral, while jacked up, and count how many times the tire turns before the driveshaft does a full turn. For posi, just pop the diff cover off and look.

[camshaftgsxr]

Or roll into a water puddle and powerbrake it... It also says the gear ratio on the ring gear and a posi is EASY to spot with the diff cover off

[iroc12valve]

Posi- jack up one wheel turn, if it turns easy no posi

Gears- do the driveshaft counting method

Word to the wise: due to parts interchangeability most old square bodies have been changed/modified/swapped/played with/upgraded so your glove box might not have the codes that match your current rear end, it might not even be your original glove box... lol

[68 TT]

Originally Posted by Ricks1971

Guys, I have a 78 4X4 and was hoping for help finding out what gear ratio and if it has posi? I know you can tell by the codes in the glove box? thanks Rick

The option code for posi is G80. You can decode everything on the sticker in your glove box. Just do a search for RPO codes.

[68 TT]

Originally Posted by iroc12valve

Posi- jack up one wheel turn, if it turns easy no posi

Not necessarily true about the posi check with a wheel in the air. With the gov-lock posi carriers that are very common in trucks you have to have several hundred RPM difference between the wheel speeds for it to lock and engage both tires and then they are mechanically locked together like a spool. Either wheel will spin freely if one is in the air. They feel like an open carrier when you do this test. Only way to tell for sure is to pull the cover and look. With a clutch type posi this test will work. If the rear end is in the air like at a pull it yourself wrecking yard both tires will spin in the same direction on a clutch type posi and in opposite directions with an open carrier.

Originally Posted by iroc12valve

Word to the wise: due to parts interchangeability most old square bodies have been changed/modified/swapped/played with/upgraded so your glove box might not have the codes that match your current rear end, it might not even be your original glove box... lol

Exactly what I was going to say.

[LONGHAIR]

Originally Posted by 68 TT

With the gov-lock posi carriers that are very common in trucks you have to have several hundred RPM difference between the wheel speeds for it to lock and engage both tires and then they are mechanically locked together like a spool.

Not so....."Gov-Lock" is not a "Locker", that is a mis-leading name. (It was probably intended that way) It is just a "clutch-type" positraction unit that has a variable pressure system. When the governor unit senses one wheel spinning faster than the other, it adds pressure to the clutches.

The whole mess is a way over-complicated timebomb.
Any kind of horsepower gains or tire size increase will spell it's end pretty quickly.
Spin the tires while pulling your boat up the ramp and hit the dry pavement.......boom.

[camshaftgsxr]

Any Links to a full and detailed description of a gov-lock longhair?

[LONGHAIR]

Yup, right here

Demo video

They audio part still refers to it as "locking" which it does NOT do. It just applies pressure to the clutch packs. There are no springs between the side gears like a traditional Posi unit. So the Gov-lock functions much like an open differential, until the centrifugal mechanism ramps up....but even at that point, it's still just a clutch-type posi, with some little "clock works" parts to overcomplicate things.

[Keith Seymore]

Just FWIW - even at the factory the terms "locking" and "limited slip" are used interchangibly - in error.

There are even two separate RPO codes: the aforementioned G80 but, in addition, there is a lesser known G81 code as well.

The locking (usually with a moveable pawl) vs slip designs (usually done with clutch packs) are dramatically different in the way they manage how torque is distributed. The specifications people would often mix and match the release codes, either because they didn't fully understand how the device worked, or simply out of convenience (or habit).

Don't ask me to recall which is which (....further proof of my original point...)



K

[motomaster99]

Does anyone know about the process of rebuilding a G80 Posi? I have the factory one in my truck and I'm pretty sure the clutches are worn out because it only works half the time and not that well.

[68 TT]

Originally Posted by motomaster99

Does anyone know about the process of rebuilding a G80 Posi? I have the factory one in my truck and I'm pretty sure the clutches are worn out because it only works half the time and not that well.

I have rebuilt many of the old Eaton clutch type posi carriers. Pretty easy job. Hardest part is getting the preload springs & retainer plates back into the carrier.

Get a bottle of the GM posi additive from the dealer and use it to lube up the new clutch plates before you put it together. This will prevent a whole lot of damage to the brand new clutches and make it last longer after the rebuild.

Buy new preload springs too. The old ones have lost much of their original strength over the years and new ones are pretty cheap.

[LONGHAIR]

Originally Posted by motomaster99

Does anyone know about the process of rebuilding a G80 Posi? I have the factory one in my truck and I'm pretty sure the clutches are worn out because it only works half the time and not that well.

I would be willing to bet that is has greater issues than worn clutches. Those things are "marginal" on their best day. It has probably just "given up". I have never seen one actually wear-out the clutches. The gear "works" is the failure point in most cases.

I wouldn't waste a nickel on rebuilding it. Replace it with a proper spring-loaded posi unit.