Tow Vehicle Rear Differential Care: Ramp Water, Milky Gear Oil and Limited-Slip Additive

BoatUS describes three kinds:

• Open: standard. Power goes mostly to the wheel with less grip, so one tire can spin on a slick ramp.
• Limited-slip: sends power to both wheels, not always equally. The most common option on pickups and SUVs, and the most sensitive to water and heat because most use clutch packs that wear.
• Locking: locks both rear wheels together when engaged.

Your dealer can decode the axle from the VIN if you're not sure. This matters because limited-slip axles usually need a friction modifier in the oil.

Two things break down differential oil and the friction modifier in it:

• Water that gets in through the vent or by condensation, especially when the axle is dunked.
• Heat from towing a heavy load, plus heat from the limited-slip clutches working when tires spin on gravel, mud or a slimy ramp.

BoatUS writes that factory clutch-type limited-slips on trucks used to tow and launch regularly often wear out in less than 40,000 miles, after which they act like an open differential. BoatUS adds that diligent maintenance early on can add another 10,000 to 20,000 miles.

The best fix is not dunking it:

• Back in only as far as the boat needs to float off or drive on.
• If the rear tires go under every time, look at the trailer: bunk height, winch post position or a tongue extension can let the boat load in shallower water (see our trailer bunks guide).
• On a steep or shallow ramp, a helper watching the waterline helps you stop at the right spot.
• Keep the tailpipe out of the water too.

With the vehicle parked and cool, look on top of the axle housing near the differential or along the axle tube. You'll see either a small cap-style vent or a rubber hose running up toward the frame or body.

• The cap should be present and not packed with mud.
• A hose should be intact, not kinked, cracked or pulled off its fitting, and its open end should point down or be capped as the factory designed.

Never plug or seal the vent. Without it, pressure builds as the axle heats and can push oil out past the seals.

Some trucks already route the axle vent up high with a long hose; others have a short cap right on the axle. You'll see aftermarket "vent extension" kits for owners who wade deep or launch often. Only change the vent if your vehicle maker's service information, or your dealer, approves the method for your axle. A poorly routed hose can trap water, kink or pull off, and modifications can complicate warranty claims. If your maker offers a factory extended vent, that's the one to use.

Have the differential checked soon if you notice:

• A whine or howl that changes with speed, or a clunk when you take off
• Oil seeping from the cover, the pinion seal (front of the differential) or the axle ends
• The limited-slip no longer working: on a slick ramp, one rear wheel spins and the other doesn't
• Loud chatter or binding on tight turns

Ram's owner information notes that some noise and mild shudder while turning on dry pavement can be normal for a limited-slip. Rough, loud or new noises aren't.

• Follow the severe-service schedule in your owner's manual if you tow.
• If your rear axle regularly goes in the water, BoatUS recommends draining the rear differential oil at least at the end of every boating season and checking it.
• After the axle has been submerged deeper than usual (a ramp mishap, a flooded road), drain and refill, as Ram's owner information says for driving in water.
• Salt water launches: don't wait; there's no benefit to leaving salt-contaminated oil in the axle.

If you'd rather not do it yourself, a quick-lube or dealer can do it. Ask them to show you the old oil.

Do this after a short drive so the oil flows, but not so long that it's scalding.

1. Park on level ground, set the parking brake and chock the wheels. Use ramps or jack stands if you need room; never just a jack.
2. Loosen the fill plug first (on the face of the housing, about halfway up). If you drain first and then can't open the fill plug, you're stuck with an empty axle.
3. Put a clean pan underneath and remove the drain plug. If there's no drain plug, the cover must come off: loosen its bolts, leave the top ones loose to tilt it, and let the oil run out slowly.
4. Let it drain fully. Check the drain plug's magnet (if it has one) and wipe it clean.

BoatUS gives a simple guide:

• Milky, foamy, or yellow-brown: water got in.
• Burnt smell: the limited-slip clutches are worn out.
• Metal flakes or particles in the oil or on the plug: excessive wear.

A fine gray paste on a magnet is normal wear. Chunks, slivers or chips are not.

If the oil only had water in it and nothing else looked wrong, BoatUS says to refill with the gear oil your owner's manual recommends and add the recommended amount of limited-slip friction modifier. Ram's owner information says limited-slip axles do require the additive, unless the oil is already pre-mixed with it.

• Use exactly the oil type and grade in your manual. BoatUS warns not to use a synthetic gear oil unless it meets your vehicle's limited-slip requirements.
• Add the friction modifier first if your manual says so, then fill with oil.
• Read the gear oil label before you buy a separate bottle of additive. Some gear oils already contain a limited-slip additive. Valvoline's full synthetic 75W-140, for example, lists the GL-5 and vehicle-maker specs it meets and states that a supplemental friction modifier is not required. Match the specs on the label to the ones in your owner's manual, and don't add extra modifier "for good measure."
• Fill until oil reaches the level your manual specifies at the fill hole. Ram's manual, for example, describes using a bent wire as a dipstick through the fill hole.
• Reinstall the drain and fill plugs (or the cover with a new gasket or the specified sealant) and tighten to the manual's spec.

BoatUS adds that a "weak" limited-slip can sometimes be helped just by fresh oil and the correct amount of friction modifier.

Wipe the housing clean, drive a few miles, then look for drips around the plugs, cover and pinion seal. On a gravel lot or wet grass, you can feel whether both rear wheels are pulling. Write the date, mileage and what you found in your maintenance log or on a sticker inside the door.

On four-wheel-drive trucks, the front differential and the transfer case also have vents and can take on water if they go deep. Ram's owner information covers rear and front driving axles together. If the front axle went under, have it checked too.

If the oil smells burnt, has metal in it, or the limited-slip no longer works after fresh oil and additive, the clutches or bearings are probably worn. BoatUS put a limited-slip rebuild at about $500 to $1,200 when it wrote about this in 2013; expect more today depending on the vehicle. That's a job for a certified repair shop or dealer.