4th Generation (1993 - Present)


4.4 Rear End

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Q: Why was the rearend ratio on the 6-speeds changed from 1993 to 1994?

A: The only reason GM went to the 3.42 rear differential for all the 1994 and later 6-speed models (versus 3.23 in 1993) was to accommodate CAGS. A lower rear gear ratio would bog the engine down too much for a 1st to 4th shift. Note the transmission gears between 1st and 4th were also made much closer mostly due to a taller 1st gear for CAGS. If you combine both the transmission and rear gear ratios, the '93 6-speeds equipped with a 3.23 actually have a higher overall ratio from 1st to 3rd.


Q: What GM rear gears are available for my car?

A: The following will fit the 4th generation f-bodies. However, if you're upgraded from a stock 2.73 rear to above a 3.23 or above, you'll need to upgrade your 2 series carrier to a 3 series carrier. If have a 3.23 or 3.42 stock rear, you can upgrade as up to a 4.10 with the 3 series carrier.

         Ratio      Part number
        --------    -----------
        2.56 : 1     26019802
        2.73 : 1     26043705
        2.93 : 1     26016282
        3.08 : 1     14089176
        3.23 : 1     26046642
        3.42 : 1     26029418
        3.73 : 1     14091497
        4.10 : 1     26016309

The ratio's and part numbers refer to GM's ring and pinion gears.


Q: What's that clunking noise from the rear when decelerating?

A: This noise is most common on cars with an automatic transmission. It usually occurs at downshift points where the rear gears disengage. Some people claim that changing the rear differential fluid and adding the rear diff additive (aka "limited slip oil") fixes this. But in most cases it's a problem with the PCM downshift program. A Technical Service Bulletin (#477124) addresses this in the 1993 - 1994 LT1 powered cars. The TSB describes a "3rd to 2nd downshift clunk" and calls for the computer program to be updated on all 1993 cars and all 1994 Pontiacs with a VIN sequence lower than 239227 and Chevy's lower than 194422. All later cars should have the updated program.


Q: What's that whining/grinding noise from the rear on slow turns?

A: The new 4th gens require rearend service (differential oil change) early in life - usually around 7,500 miles (although some have done it even earlier). When you replace the differential oil, make sure you add GM limited slip additive. (Note, the additive should not be necessary if you use synthetic diff oil.) This should alleviate most of the noises coming from rearend. Otherwise, it's probably standard rear gear noise. GM hasn't changed the F-body rear end since 1982 and they all make some form of noise.


Q: How do I change my differential fluid in the rear end of my car?

A: This was taken from the LS1 FAQ which is on www.ls1.com and was original submitted by Mitch Warren. I have made my own slight modifications to the instructions based on what I feel is the best way to do this.

In order to go this job, you will need:

  1. 13mm socket or wrench
  2. A 3/8 inch drive ratchet
  3. At least 1 bottle of GM approved limited slip additive
  4. A little more than 2 quarts of GM approved rear differential lubricant
  5. A gasket
  6. A wide drain pan
  7. Oil soak
  8. Plenty of shop towels.

Let me start by saying you will get lubricant on the floor. It's unavoidable. If you have been driving your car for any length of time, the lube will be hot, so please be careful and have the oil soak and shop towels handy. Also chock the wheels if you aren't using a lift, and leave the transmission in neutral. Remember to always use automotive jack stands rated to withstand the weight of your car.

Begin by orienting the gasket with the bolts, making sure it is the CORRECT gasket. Next, remove the bolts that hold the backing plate. Notice that there are 2 brackets that also use these bolts. They anchor the brake lines. These bolts are slightly longer than the others, so pay attention to this during reassembly. As soon as you get the bolts out of the bottom, you should begin to see some dripping, so be sure to have the drain pan ready. When all the bolts are out, gently pry or tap the backing plate to get it off. Hang on to it, or it'll wind up in your drain pan. The lubricant should flow out of the housing rather quickly and with a big splash. If you use any oil soak, keep the dust to a minimum as it will get inside your differential housing.

Now its time to clean everything. The original document said to use carburator cleaner, however I have heard more against this than for it, so I would say don't use the cleaner. Most people agree that using your hand to try to wipe off as much of the old lubricant as possible it good enough. There is nothing wrong with leaving a residue of old oil on the gears. I then use paper towels to wipe out all the material from the bottom of the housing. Although you don't have to worry about getting all the old oil out, make sure you get all the old metal shavings out.

Notice what looks like a big washer attached to the plate? That's a magnet and is glued to the plate. Don't try and remove it. Make sure the old gasket is COMPLETELY removed from both the housing and backing plate.

The original document also said to use an RTV sealer on the gasket to assure that its on there correctly. As long as you clean the surfaces where the new gasket will go, it is not recommended that you use an RTV sealer.

Put 2 bolts into the backing plate, 1 on each lower side (remember the magnet goes down) and carefully replace the backing plate. Remember the 2 longer bolts go with the brake line brackets and the backing plate is notched to accommodate them. Be careful not to over tighten the bolts.

Use the 3/8 inch drive ratchet to remove the fill/level check plug on the right front of the differential housing. Add the additive first and then fill with the lubricant. This can be tricky, messy and frustrating. I used a small funnel with a clear tube attached so that I could see the lube flowing. You will know its full when you see the oil reaching the top of the fill hole. Once full, replace the plug and you are finished.

I suggest you drive slowly for a couple of miles, allowing the additive and lubricant to mix thoroughly. Stop and check for leaks and after a few miles check the differential level again.

Try using a squeeze bottle with a long nipple to add the fluid. You can pour the additive into the bottle along with the fluid and shake them until mixed. Then use a section of clear motorcycle gas line clamped over the nipple to stick into the rear differential fill hole. This filling method saves a lot of spills and keeps your blood pressure within safe levels.

Please understand that the rear ends between the 1998s and 1999s are different. In 1998 the F-bodies had Auburn rear ends, and in 1999, they had Torsen rear ends. There is some skepticism with whether or not you should use synthetic or dino oil back here. The debate is a little stronger here than it is when people talk about synthetic vs dino for the engine. There are reports of some rear ends not being able to engage the gears because they're too slippery when synthetic oil is added. I would check the user manual for your car and add just what GM recommends. If you hear something different from someone you trust, then go ahead, but I'm sure you won't have any warranty problems if you use what GM recommends and something goes wrong.


Q: Do I really need to change my differential fluid at the first 7,500 miles?

A: Yes. Although some people not familiar with the rear ends in this car will tell you different, the rear ends have a lot of meshing to do when they're new and they will develop a lot of metal shavings in the fluid which much be released. Some people actually change their fluid before 7,500 miles. Once you've changed the fluid the first time, you don't need to keep doing it every 7,500 miles after that. Follow the service schedule in my user manual for your car. Depending on how you drive your car, you may need to do it every 15,000 miles, or ever 50,000 miles.


Q: Should I use synthetic rear end gear fluid and limited slip additive?

A: As for the rear differential fluid itself, using a good 75W-90 synthetic is the best way to go. As for using the limited slip additive, that all depends on your If you primarily use the car for street and strip driving, then use the additive as it will help prolong the life of the rearend and ensure quiet operation of the gears. If you autocross or do any type road racing on a regular basis, then not using the synthetic is will provide better performance in the tight turns, but theoretically, the life of the rearend will be shortened. How much shorter all depends on the type and amount of driving you do.


Q: What does ASR stand for?

A: It's short for "Acceleration Slip Regulator" - more commonly referred to as "Traction Control".