Is my G80 diff broken?

I'm kicking myself for not investigating this while I had the diff open and drained, but wanted to throw it out there.

My truck has the G80 locking diff with 3.73 gears. When I first changed the diff oil, I confirmed it is a G80 by seeing the cam gear on the side of the outer pinion cage. So, I thought cool.

One day I had to pull over to let a emergency vehicle go by. It was a 2-lane road with no shoulder and had been raining. So I had one tire on the pavement and the other in the mud. I thought, no problem and slowly added gas to what I thought would be enough to make it engage. However, the rear didn't lock up and I sat there spinning my right rear tire while cars waited behind me. The left one didn't do squat.

So, the other day I had finished changing the rear axle bearings and belatedly thought I'd do the test. Blocked the front wheels, put it in neutral, jacked up the rear and spun one tire hard by hand. The opposite tire did not lock and spin in the same direction. Instead it spun slowly in the opposite direction. The driveshaft spun too but can't remember which direction.

I wish I had thought of doing this with the cover still off so I could check the cam and weights. My question is what is wrong? Any thoughts or help would be great. Thanks!

I'm kicking myself for not investigating this while I had the diff open and drained, but wanted to throw it out there.

My truck has the G80 locking diff with 3.73 gears. When I first changed the diff oil, I confirmed it is a G80 by seeing the cam gear on the side of the outer pinion cage. So, I thought cool.

One day I had to pull over to let a emergency vehicle go by. It was a 2-lane road with no shoulder and had been raining. So I had one tire on the pavement and the other in the mud. I thought, no problem and slowly added gas to what I thought would be enough to make it engage. However, the rear didn't lock up and I sat there spinning my right rear tire while cars waited behind me. The left one didn't do squat.

So, the other day I had finished changing the rear axle bearings and belatedly thought I'd do the test. Blocked the front wheels, put it in neutral, jacked up the rear and spun one tire hard by hand. The opposite tire did not lock and spin in the same direction. Instead it spun slowly in the opposite direction. The driveshaft spun too but can't remember which direction.

I wish I had thought of doing this with the cover still off so I could check the cam and weights. My question is what is wrong? Any thoughts or help would be great. Thanks!

did you check the vehicle codes ...your sure you have a locking diff...? the reason I ask this is because you do not have to open the diff to see if it is the locking diff...10yrs ago I found out just by giving it some extra throttle on wet pavement and you can feel very easily the lock locking....

if you bought this used it is possible it had the locking diff ,,,but because of problems with it , it was removed ....some have done this rather than replace the diff..

The truck does have the RPO code in the sticker and I confirmed it is a G80 when I changed the oil the first time (noted in original post). When I had it open last week, I saw the locking cam assembly. What I can't recall is if the pawl that catches the cam when it spins was there or not...knew I should've grabbed a camera and snapped a shot! I know the bottom of the case was clean with no odd bits and the magnet was clean. It is possible it was disabled by removing that pawl, but I'm hoping I was just not paying enough attention and didn't see it.

I have gunned it before on a dirt road and observed both tires making marks, but I don't recall a clunking noise when it locked in. Guess I'll have to crack the pumpkin open again someday...until then, thanks.

The truck does have the RPO code in the sticker and I confirmed it is a G80 when I changed the oil the first time (noted in original post). When I had it open last week, I saw the locking cam assembly. What I can't recall is if the pawl that catches the cam when it spins was there or not...knew I should've grabbed a camera and snapped a shot! I know the bottom of the case was clean with no odd bits and the magnet was clean. It is possible it was disabled by removing that pawl, but I'm hoping I was just not paying enough attention and didn't see it.

I have gunned it before on a dirt road and observed both tires making marks, but I don't recall a clunking noise when it locked in. Guess I'll have to crack the pumpkin open again someday...until then, thanks.

you can feel the lock cut in ..this time of the year in snow country the shoulders of the road are litered with sand..if the rt rear tire is in the sand then the lft rear tire in the middle of the road where it is clean you will feel the locker lock up when power is applied to the lft tire on that solid clean pavement....it don't ease in ...

now what I found was if the locker fluid is good the locking is faster so you get less shocking when it locks..if the fluid is bad the delay in locking can be a quite harsh engagement...

my driveway has a slight upward grade so when the snow flies I can feel that lockup with a slight rear end bounce....


I have the defective parking brakes , so years ago when I was trying to mod this so the park brake would continue to work, I rotated the rear tires both off the ground and with just my rotating I got those rear wheels to lock ...now this messed up my park brake adjustment !

It was worth it because I did fix the failing park brake ,,,the brake that is never used ,,that wears out ...

It is a locker that has clutches. If the clutches are worn, the thing will lock but nothing will engage. Your locker may be ok, the clutches may be out. You have to hold the driveshaft to get it to lock properly with the wheels off of the ground. Leave it in park and try again.

If the wheel is spinning too fast the locking unit will not engage. You will notice a spring loaded centrifugal weight by the cam gear you referred to. If the differential is turning too fast the weight is thrown outward preventing the locking feature. Saves axles and differentials. You can prove this by jacking one side up and putting the truck in gear and carefully allowing the wheel to turn slowly until it locks.