Rear axle service simplified

Differentials haven't changed much, but here is what you need to know to stay on task.

undercar rear differentials rear differential service repair shop training technician training A/C training automotive aftermarket Rear differential service, for regular maintenance or for repair work such as replacing leaking seals, often involves removing rear axles and draining and replacing differential fluid.

Frankly, I'm not all that lucky, so I've always had to be extra careful when working on axles and differentials to avoid problems and keep the work profitable.

True, differentials are very simple, straightforward units that haven't changed a whole lot over time. And even though different manufacturers have different ways of doing things, the operating principles involved and the internal components of rear differentials are always pretty much the same from vehicle to vehicle.

However, even though rear differentials all have the same purpose, disassembly and maintenance requirements can vary widely even on seemingly identical vehicles. Complicating things even further, rear axle fluid types might be incompatible with each other and with the seals involved, so guessing at the right fluid can be a very expensive mistake. And certain units need additives in the fluid in order to run properly after service while others absolutely do not.

Even if you know the make, model and year (along with tire size and options) that's often not enough to be sure exactly which differential you're dealing with and how to service it properly — and that can be frustrating and time consuming. But it doesn't have to be.

The best way to remove some of the confusion about servicing differentials and avoid problems is by remembering the basics and by knowing a few tricks. Here's how.

Back to Basics

Most of the differentials that roll into service bays either are full-floating axle or semi-floating axle types, and knowing which is which definitely can save time.

Floating axles are the ones on heavy-duty trucks and are easy to identify by looking at their distinctive hubs — the hubs stick out and the ends of the axles bolt right on to them. This axle design is used on trucks that carry heavy loads because the axles themselves don't actually support any weight. They just float on two bearings riding on a fixed spindle and simply transmit torque. Axles are made tough, but not tough enough to withstand extreme loads. On these systems the hub and bearing are held in place by a nut and the axle is bolted to the hub.

Removing the axles from these units is usually straightforward: You unbolt the axle from the hub and just slide it out. There's usually a blob of oil that shoots out of the housing along with the axle so it's smart — and environmentally responsible — to put a drain pan under the hub when performing this procedure.

One of the nicest things about full-floating differentials is that they usually have a drain plug, which makes it easy to drain and refill the unit.

The important things to be mindful of when removing these axles and servicing these units start with making sure that the bolts holding the axle to the hub aren't stripped and that you don't strip them (which happens when using air tools to tighten them). Also make sure the axle seals aren't damaged during removal (pull the axle straight out, not upward or downward and be ready to support the end), and reseal the hub properly when you're done using the correct RTV sealer or a new gasket (or both) as appropriate.

Other than the fact that they're heavy and awkward, full-floating differentials aren't bad to work on at all, and you can make good money doing it.

Semi-floating axles are the ones used on lighter-duty vehicles and SUVs that don't carry very heavy loads. On these units the vehicle's wheels bolt right on to the axles, which means that the axles are indeed supporting the weight of the vehicle. Depending on where you are in the country, these likely will be the units you'll see most often.

Removing the axles from these units often will take a bit of work and preparation, and the actual removal procedure varies between vehicles so checking the repair manual before starting can definitely save time.

On many GM products, for example, the axles are held in by clips (located inside the differential itself) that must be removed in order to slide the axle out. To access these clips, the rear cover must come off and the center pin and small retaining bolt must come out. When the center pin is removed, gently pushing on the hub so that the end of the axle moves deeper into the differential makes the clip easy to access and remove. Very easy, and the clip actually might fall out into the unit and need to be fished out with a magnet.

After the axles are reinstalled, the differential cover is resealed with a new gasket (often with RTV sealer, too) and bolted in place. The mating surfaces between the differential cover and the differential itself must be clean and free of oil in order to seal properly and this can be tricky. Using a grinder not only is messy, but it can damage the surfaces so badly that a good seal isn't possible. Scraping the old gasket and sealer away by hand is the safest thing to do.

Additionally, when using sealer on the rear differential cover, it helps to make sure the rear end of the vehicle isn't hanging down and allowing any fluid left in the unit to dribble out and get between the surfaces before they can set. Torqueing the cover bolts in a cross-pattern, rather than just tightening them sequentially and moving to the next bolt in line also reduces the chance of leaks.

And if the differential cover is chromed, it's important that the whole procedure is carefully performed using hand tools on the bolts so that the highly visible finish isn't damaged.

Then the unit is refilled with axle fluid and the vehicle is road-tested. If all is well, the vehicle is released to the customer.

I'm describing this procedure in detail, because when I told a Toyota dealer technician how GM does things, he was fascinated. Toyota's differentials have plugs to drain the fluid if that's all that's needed, and have a completely different axle removal procedure.

He described the procedure for removing rear axles (usually to replace rear axle seals, a common job on Toyota trucks) like this:

1. Remove the wheel.

2. Remove the drum (if the vehicle has drum brakes).

3. Block the flexible brake line above the differential that splits fluid to the left and right.

4. Remove the brake line from the wheel cylinder.

5. Remove the parking brake cable from the pivot.

6. Remove the four 14mm nuts from behind the backing plate for the bearing housing.

7. Pull the axle out (shoes and all).

8. Ta-da.

Quite a different procedure indeed! Looking up the procedure for each manufacturer and vehicle can definitely save headaches later on.

Preliminary checks

Road testing any vehicle before bringing it into the service bays is always a great idea. For rear differential service, it actually can save time and generate money.

Indications that the differential needs more than just a fluid change and some new gaskets or seals include growling noise, chattering, pulling, inoperative 4x4 system or in a worse-case-scenario, the vehicle just won't move at all.

Noises that come from the differential, as opposed to the tires or transmission, usually will occur when decelerating and accelerating, but not when cruising at a steady speed. Also, noises from the differential usually start to initially happen at higher speeds but as the wear gets worse the speed the noise initially starts at gets lower.

After the preliminary road test, if you suspect internal damage, see if the differential unit has a drain plug. Most drain plugs on differential units have a built-in magnet, and debris will stick to the magnet indicating trouble inside.

Note though that it's also good practice to never drain a differential unless you're sure the filler plug can be loosened, because they can strip quite easily due to corrosion and wear. Warning the customer about the possibility of additional costs before you realize that the drain plug isn't budging will save embarrassment later on. It's also good practice to torque the drain plug when you reinstall it so you're not the one causing problems for someone else.

Selecting Fluid

When looking up the correct fill level for the differential unit, you'll probably find that the correct fill level is at, or just below, the bottom lip of the refill hole for most vehicles, but it's still a good idea to check.

In general, the two things to consider about differential fluid are the type of fluid needed and whether or not additives are required (usually a consideration for limited slip differentials (LSDs), but not all limited slip differentials need additives).

Selecting the right fluid to refill the differential can be a challenge, so it's good practice to look this up before draining the unit. It's so challenging, in fact, that one vehicle manufacturer tried to create a quick and easy reference guide for its dealers to use when replacing differential fluids on vehicles. That was three years ago and the reference guide is still incomplete. What they didn't count on is that the older vehicles had a variety of differentials and the manufacturer had no way to link each vehicle to each of the differentials they had used. To this day, even with the VIN number and the dealer's parts catalog, the older vehicles have multiple results listed for each differential. They might have just given up on the guide.

It's easy to find out if a GM vehicle has a limited slip differential because the clutch packs are obvious when the rear cover is removed. Also, it's easy to look this up if you're not sure by checking the option codes that are usually in the owner's manual, glove box or in the rear compartment near the spare tire (the repair manual will show where the white sticker is mounted). If the differential needs the special LSD additive, that's easy to look up, too. One important note though: If the vehicle does indeed need the additive, hold your nose! It's possibly the worst smelling chemical in all of automotive.

But unlike GM vehicles, Toyota vehicles with LSDs can be difficult to identify and may not even need any additives, LSD or not. Additionally, using the wrong fluid can cause chattering on take-off and turns, LSD or not.

The best way to check a Toyota vehicle to see if it has an LSD is by looking for a sticker that says, "LSD," mounted to the differential near the fill plug. The sticker warns technicians that it's a LSD differential but doesn't imply any reference to the correct fluid or viscosity.

In fact, Toyota uses several differential fluids and they can't be used interchangeably. For example, the synthetic oil used in 2007 vehicles can't be used in certain 2002 vehicles because the fluid isn't compatible with the seals.

For Toyota vehicles, as with most other manufacturer's vehicles, even though the owner's manual usually lists a fluid type and quantity it's really important to be aware of updated fluid specifications.

Often newer fluids that weren't available when the vehicle was built are the ones recommended for the vehicle to prevent noise and wear. Online repair manuals and TSBs have the most current information and should always be consulted for the final answer, regardless of the manufacturer.

Conclusion

There's no one-size-fits-all approach to rear differential service, but with a bit of research it absolutely can be both profitable and trouble-free. And because the related differential seals do indeed leak and the units need regular maintenance, it makes sense to get familiar with some of the more common rear differential systems on the road, and with several different manufacturers' vehicles.

Trucks and SUVs are still incredibly popular vehicles and need to be serviced regularly and properly so that they work well for years to come. With a bit of preparation working on those vehicles will be no problem at all and your customers will thank you for it.

Vanessa Attwell is a Master Technician for two major manufacturers and has also worked on the bench of an independent shop. She has developed and delivered training for both vehicle manufacturers and independents, and helped develop government training and regulations standards. She drinks too much coffee and spends her spare time sitting in traffic.

CHECK THE Checklist

Five things you might not know about rear differentials

1. If you really can't find the specifications in any document for the gear ratio when ordering parts, just take the time and count the pinion and ring gear teeth and there's your gear ratio.

2. If the differential seals are leaking, check the breather. It's usually small, mounted inconspicuously and can stick easily (due to contamination), which allows pressure to build up inside the differential housing, stressing the seals.

3. Setting pinion bearing preload and ring gear backlash is an art more than a science. The repair manual gives you a range, but ultimately it's the gear contact pattern that tells you if you have it right.

4. Incorrect tire inflation pressure, multiple tire sizes being used, and uneven tire wear can all affect differential operation. (More than 1/4 inch or 2/32 of tread difference between tires on the same axle can affect operation.)

5. Electronic controllers and actuators usually fail before the mechanical components of the differential do — don't be quick to assume there's a mechanical problem if electronic parts are involved.