Hello! I've got a 1996 Checy Blazer 4WD LT with about 248,000 on the OD. I've taken the leap and am replacing every fluid in the car. So far I've replaced the transmission fluid and filter, power steering fluid, brake fluid, engine oil, transfer case, windshield wiper fluids and finally switched the coolant to 50/50 after flushing it! I'm looking to change my rear differential oil and have absolutely no idea how to tell what gasket it takes as well as the oil I should use in it?

(If it's on Advance or Autozone links would be greatly appreciated in your replies!)

 

It's either going to be a 7.5 or an 8.5. Most likely the 7.5.

The easiest way to tell them apart at a glance is the protrusions on the bottom of the housing.








It should take regular 75w90 GL5 gear oil, weather or not it has the G80 locker or not.


I was in the 7.5 of my 04 4x4 just the other week for other reasons aside from the fluid change. Pretty sure it's the first time the fluid had EVER been changed in it's 216k miles...

 

Thanks, I'll check it out in the morning and see what's up. Out of curiosity, my brother has told me my blazer has the locking one where both wheels spin but I tried doing a mini donut with some super bald tires a long time ago and it peg wheeled, one wheel spinning while the other did not. However, in the woods I gassed it on a dirt hill in 2WD and it spun one tire again. Then I tried it once more and both tires spun. Have any ideas?

 

Unless either the rear end has been changed, the glove box door has been changed, or the sticker is just missing, you can verify if it has a locking diff in it. In the list of codes on the sticker, look for "G80". If it's present, then your diff has the locker in it. If the code is not there, then you have a standard non locking diff.

The G80 diff relies on the speed difference between the two tires to engage. Once one wheel breaks loose and gets up to some speed and the other one remains stationary, the mechanism inside will trip and lock the diff. If you simply tried for a second and didn't get much wheel speed, it may not have been enough to engage the locking mechanism. It's purposely designed to not be excessively easy to engage, so that it doesn't kick in while normal every day driving.

The easiest way I've found to "bench test" the engagement of the G80 locker is to get the rear end off the ground (both tires), put the truck in neutral, then grab one rear tire and try to spin it in either direction as quickly as possible. When doing this, it surprisingly doesn't take much effort, and the diff should engage and "click" and lock together.

Even with an open diff, on lose or slippery ground, it's not uncommon to break both tires loose, but it's not predictable or controllable, hence the reason for lockers