If you’ve spent any amount of time around cars or trucks, you’ve probably heard the saying “heat is the enemy of performance.” While this statement is universally true, it’s even truer under the stress of towing, which will cause an unprepared or over-stressed vehicle to buckle under the weight of its burden. Our ’04 Avalanche 1500 Z71 came equipped with the 3.73 axle ratio, rather than the 4.10 that came with the towing package. The 4.10 ratio would help in towing, but would take its toll on fuel economy, which in the nearly 6,000-pound Avalanche, isn’t great to begin with.
As equipped, the Avalanche is rated for 7,000 pounds, which may seem weak by contemporary standards, but was about par for 1/2-ton trucks and SUVs of its day. It’s more than enough to handle a lightweight travel trailer, a pair of watercrafts, a ski boat, or a lightweight car trailer and track car.
The Avalanche was bought used with 163,000 miles, and only a partial service history. We knew the motor oil had been changed regularly, but some other items had not been touched, such as the brake rotors and spark plugs, among others. Since transmission and differential fluid aren’t frequent maintenance items, we went under the assumption that both were factory fill. The original owner claimed the truck had lived a life of leisure consisting of highway miles and light utility use. The condition of the transmission and differential fluid would tell the tale. Thankfully, both were in remarkably good condition considering the mileage, with minimal metallic debris, no strange odors, and still-decent color.
| While any underbody work is made easier with a lift, with some jack stands, it’s possible to have enough clearance to work underneath the vehicle. Make sure you have a chock behind the wheel on the ground, and that the jackstands are rated to handle the weight of the vehicle you’re working on.
However, we took the opportunity to change out the stock fluid for Amsoil Signature ATF synthetic automatic transmission fluid and Severe Gear gear oil. While we were at it, we swapped out the stock stamped steel transmission pan and rear differential cover for some finned aluminum units from PML. The coil spring rear suspension and rear sway bar suspension—the same design on the GMT800 SUVs like the Tahoe, Suburban, and Yukon— limit the options for differential covers on the Avalanche. The PML cover is one of the few finned aluminum covers that fits behind the stock rear sway bar.
The aluminum covers give both the transmission and rear differential a higher fluid capacity, as well as better heat dissipation than the steel covers. The Amsoil fluids have a much higher thermal capacity than the factory fill as well, ensuring that even if things get hot, we’ll have a greater safety margin. Amsoil offers specialty fluids for nearly every application, and PML offers a wide variety of transmission pans and differential covers for trucks, SUVs, sports cars, and classic cars.
Because the vehicles on which the differential cover fits came with different thread pitches and bolts, PML recommends removing one of the bolts from the top edge of the differential and sending it in to make sure the replacement bolts are the correct specification. Follow along as we take the first step in making our Avalanche into a reliable daily driver.
| The Avalanche has the same coil spring rear suspension design as the Tahoe and Suburban, meaning it also has a sway bar around the rear differential housing. If you’re installing the diff cover on a Silverado or Sierra, you won’t have to worry about this.
| The first step to give yourself some more working space around the rear differential is to disconnect and remove the rear sway bar. First unbolt the sway bar top endlinks.
| Then you can unbolt the lateral bushings, and remove the sway bar.
| Now we will have much easier access to the rear diff cover.
| As expected for a vehicle of this mileage, there was some weeping along the bottom edge of the cover. However, it appeared the leakage was over a long period of time.
| We loosened the stock diff cover bolts. Be sure to have a drain pan beneath the differential when you do this to catch the gear oil.
| Thankfully, there was minimal metal debris on the magnet of the stock cover, and the fluid was still translucent and didn’t smell burnt.
| The ring-and-pinion and G80 limited-slip mechanism looked in good shape for the mileage.
| We took a razor blade to scrape off any gasket or silicone from the diff housing.
| With the differential housing surface clean, we apply a bead of RTV silicone to the surface. It’s important that you apply the bead to the inboard side of the boltholes, and not get any in the boltholes themselves.
| We applied some silicone to the differential cover to hold the gasket in place while we installed it.
| We centered the supplied gasket on the diff cover, making sure the holes are lined up. Because the aluminum cover is thicker than the steel, PML provides new hex bolts that will secure the cover to the differential.
| Put a small amount of Loctite 242 or equivalent medium-strength thread locker on the end of the bolts. Go around the perimeter and tighten bolts to 20 ft-lb (240 in-lb). The drain plug is the same torque spec.
| To fill the differential, it helps to have a fluid transfer pump. These are generally available online or in stores for less than $20. We used Amsoil’s 75w-90 Severe Gear oil.
| PML says its covers increase the fluid capacity of the rear differential by 1/2 to 1 quart. Our differential took more than three full quarts until it started coming out of the fill hole—the usual indicator of max fill level.
| PML recommends tightening the fill plug to 12 ft-lb (144 in-lb).
Reinstall the rear sway bar.
| Even with its relatively slim size, the cover gives a higher fluid capacity, and the external finning is designed to dissipate heat more effectively.
| The transmission pan proved to be a little trickier to install than anticipated, because of some of the tight clearances around it. If you have a service lift, it could make this job much easier. We did it in a driveway with some jackstands, and were able to complete it, but not without some bloody knuckles and a few non-PG words. The first order of business is to drain as much of the old transmission fluid before removing the pan as possible. Thankfully, the transmission fluid was still in relatively good condition.
| As noted above, clearances around the transmission were a little tighter than anticipated. We ended up having to remove the shifter cable bracket to get the pan out.
| While we had the pan off, we replaced the transmission fluid filter. While there are a lot of similarities between the 4L60E and the earlier 700R4 transmissions, there are some differences. We got an ACDelco 24208576 “deep pan” filter. Note the notched corner.
| Like the differential cover, the transmission pan comes with new bolts to work with the thicker flanges of the aluminum pan. PML recommends tightening them in a crisscross pattern to 10 to 12 ft-lb (120-144 in-lb).
| We had to loosen the transmission crossmember and use a pry bar to get the thicker, larger PML pan in. We then reattached the shifter cable bracket, and tested the shifter to make sure the mechanism was working correctly.
| Once we had the new PML pan fully installed, we added Amsoil’s Signature Series automatic transmission fluid. With the engine running, we continued to check the fluid level, and added fluid as needed.
After we came back from a testdrive, we checked the level again, and were satisfied that we had sufficient fluid.
We plan to later install an auxiliary transmission cooler, but the larger capacity of the PML pan, along with the higher heat resistance of the Amsoil fluid makes us more confident the old Avalanche will be ready for towing.