1998 K2500 limit slip or locking?

[dmg92001]

Hi all,

I've been wanting a truck for a minute and didn't realize until after I got one there was a whole community dedicated to them.
So if anyone knows the answer to my question, I guess it would be you guys.

I'm only now really learning how to do more advanced maintenance work and still trying to learn all the terminology.
Trying to take it slow so I don't mess up and hurt someone or the truck.

I just bought a 1998 K2500 "Cheyenne" 5.7 Vortec Single Cab 8ft Bed that I'm planning on making my daily driver.
and I want to make sure everything is good before I get rid of my current vehicle.

The brakes go pretty far down before it starts to slow and the front brake pads look basically new.
So I am told it might be either the rear drum shoes are low or the adjuster is shot and I need to take off the drum to check. (the drum are also flaking off a lot of rust)

But anyway - my main question is does this thing have a limited slip an automatic locking? I am told I need a different fluid or an additive depending on this.
I am getting conflicting information from the posts I've been reading.
The RPO code is G80, and when I run that through a decoder it says:

• Differential, heavy-duty locking rear
• Differential, automatic locking rear
• Differential, limited slip
• Auto-locking rear differential
• AXLE,REAR,POSITRACTION,LIMITED SLIP

Which doesn't narrow it down. I saw one post mention that some G80s were locking and some were limited slip.
Is there a way to identify this from the outside? It is 8 lug if that makes a difference.

I just need to make sure I put the right oil in it after I take the axel out.
I'm also having trouble determining which would be the right fluid to put in. The previous owner says he just put in whatever GM recommended but he doesn't know if it had a posi.

Any help is much appreciated.

 

[RichLo]

Its a mechanical locker, you dont need limited slip additive.

The G80 is a weird design but you have a 9.5" SF axle so its not as bad as the 8.5s that came in the 1/2 tons. It lets the loose tire spin around 3 revolutions then slams shut and locks the other side in until you either let off or it spins up to around 35mph. Its fine for winter driving but its not ideal for anything else. Lots of people upgrade to a real Detroit style locker.

 

[GoToGuy]

A limited slip is a generic term for either a locking or positrac, clutch type differential. Additive lube in clutch type diff only. No lube additive in mechanical or gear locking.
A generic decoder is ok. If you download the FSM in the first chapter it breaks all the RPO codes for that year, and more specific info on options.

 

[evilunclegrimace]

https://www.bing.com/ck/a?!&&p=39dd3b1e937082caJmltdHM9MTcyNjM1ODQwMCZpZ3VpZD0yZDE3OGI4Mi1lYjc1LTZjMzItM2I1YS05ZjM2ZWE2NzZkYmMmaW5zaWQ9NTYxNA&ptn=3&ver=2&hsh=3&fclid=2d178b82-eb75-6c32-3b5a-9f36ea676dbc&u=a1L3ZpZGVvcy9yaXZlcnZpZXcvcmVsYXRlZHZpZGVvP3E9Ry04MCtsb2NraW5nK2RpZmZlcmV0aWFsK29wZXJhdGlvbit2aWRlbyZtaWQ9NjE4MEREMUU2MzgwREEwQ0U3RUQ2MTgwREQxRTYzODBEQTBDRTdFRCZGT1JNPVZJUkU&ntb=1

 

[Cadillacmak]

Straight out of the 1998 manual - "G80: Rear Axle: Positraction". BUT, I think its just limited slip
Manual also lists this for all Diff's this year - "Axle Lubricant (GM P/N 1052271) or SAE 80W-90 GL-5 Gear Lubricant."

 

[evilunclegrimace]

Straight out of the 1998 manual - "G80: Rear Axle: Positraction". BUT, I think its just limited slip
Manual also lists this for all Diff's this year - "Axle Lubricant (GM P/N 1052271) or SAE 80W-90 GL-5 Gear Lubricant."

All of the G80 differentials that I have seen in GMT 400's for the last 36 years have been Gov-loc's

 

[Sean Buick 76]

The Gov lock is a crappy mechanical locker. Avoid burnouts, HP and off road or replace it with a good quality limited slip or locker.

 

[Schurkey]

1998 K2500 "Cheyenne" 5.7 Vortec Single Cab 8ft Bed

I'm planning on making my daily driver.
and I want to make sure everything is good before I get rid of my current vehicle.

There are other threads devoted to this, I'm too lazy to look them up.

Short story: Safety first. Verify suspension, steering, brakes. Replace all fluids and filters (except for A/C refrigerant and desiccant, assuming the A/C works.) Flushing the brake fluid will require a scan tool to get all the old fluid out of the ABS system.

Everything rubber is suspect: Hoses, belt, tires, mounts, suspension bushings.

Then a proper and complete engine tune-up. Connect a scan tool, verify EVERY sensor and computer output. Look at fuel trims, misfire counts, and so forth.

Some guys shampoo the carpet and upholstery, wax the paint, and so forth.

The brakes go pretty far down before it starts to slow and the front brake pads look basically new.
So I am told it might be either the rear drum shoes are low or the adjuster is shot and I need to take off the drum to check. (the drum are also flaking off a lot of rust)

There's multiple reasons for low brake pedal. You're right about the adjuster(s), and you'd want to remove the drums to verify. But that's just scratching the surface. Air anywhere in the system--including inside the ABS--is another possibility. Oversized (worn) drums. Weak brake hoses. Weak master cylinder--in particular the third chamber or it's check-valve.

Your truck likely has JB6 brakes, therefore low-drag calipers in front. This requires a Quick Take-Up master cylinder, and replacing that with a standard, but larger-piston master is popular but crazy.

does this thing have a limited slip an automatic locking? I am told I need a different fluid or an additive depending on this.
I am getting conflicting information from the posts I've been reading.
The RPO code is G80...
...Is there a way to identify this from the outside? It is 8 lug if that makes a difference...

...I'm also having trouble determining which would be the right fluid to put in.

I'm expecting that axle to be a 9.5" semi-float; G80 represents a Gov-Loc differential.

But the easy way to put this to bed, is to remove the differential cover, drain the fluid and take a look. Look at the gear ratio and gear condition while you're in there. Inspect the magnet for debris. The fluid you put back into a Gov-Loc needs to be the correct stuff; in this case the GM recommendation is the really-expensive "Grape Juice" stuff: ACDelco 10-4016.

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But some of us have been using less-expensive Valvoline gear lube that contains NO limited-slip additive.

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"Limited slip differentials top-off only"

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[454cid]

A limited slip is a generic term for either a locking or positrac, clutch type differential.

I disagree. There may be people that use the term universally, but it's incorrect. A limited slip, and a locker are not the same thing.

 

[454cid]

...Its fine for winter driving but its not ideal for anything else.

My dads old 96 had one. I never realized it had the locker until right before he sold it. I borrowed it in the winter and wondered why his 2wd shortbox with wide-ish tires seemed to do better in the snow than my 1-ton 4x4 with AT tires, but with an open diff.