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BMW Differentials Part II: Diagnosing, Repairing, Removing, Replacing, Fluid Changes

If your BMW is exhibiting symptoms of differential problems, you can rebuild your existing differential, and use a new ring and pinion gearset from BMW to change your gear ratio. Those with cars that already have limited slip differentials with the rato they prefer will want to go this route. Be warned: BMW gearsets are not cheap. Figure $500 - 900 at the dealer and a bit less used. The course of rebuilding the differential in a E24, E28, E30, E34, or E36 BMW will also require assorted bearings, seals and other bits. On top of this, you will need to figure a good four to five hours labor with the diff out of the car, and another two hours for removal and installation.

How To Tell if Your BMW's Differential is Worn Out

BMW differentials rarely fail as long as the oil level is reasonably well maintained and the oil is changed at least once in a while. Failure is rare even when the oil is not changed, as long as it's in there. The limited slip differentials are fairly bulletproof, but spider gears have been known to break pretty easily on open differentials that are subject to heavy wheelspin-like those that result from autocrossing or becoming stuck in the snow, or being joe cool when accelerating hard from a stop. You will know immediately if the spider gear has failed as the car will not move!

A BMW differential's most common wear item are the output bearings. Output bearing wear is characterized by a hum or whine from the back of the car that will become audible between 3000-4000 rpm, usually more noticealbe in fifth gear at a constant cruising speed. The noise will stop or change when you lift your foot from the accelerator pedal and may also change as the suspension rebounds from dips and valleys in the roadway. The perceptability of the noise is dependent on the load on the bearing The noise may also stop entirely above a given rpm. Also, many cars with differentials showing this wear have gone another 50,000 miles with this noise and no further, so it's not necessarily something that needs to be addressed right away. But the longer you let it go, the more likely it is that other parts, such as the expensive ring and pinion gears, may wear out as well....

It is easy to mistake noise emanating from front wheel bearings for differential whine. But, if you turn the steering wheel, front wheel bearing whine changes or goes away, and also be unaffected by throttle position.

Should You Get a Used Differential or Rebuild Your Existing One?

Buying used parts - often sight unseen over the internet these days- is always a gamble, especially when you are dealing with a BMW! If you buy a used diff from a reputable source and it makes the dread whining noise, take it back. However, they might now have another, a problem that occurs when you have one of the rarer BMW differentials, ring and pinions, or gear sets. A good example is a side loader 3.07 differential on the early E12-based 6 series coupes. When looking at a used diff, cleanliness of the unit is a pretty good indicator of quality or wear or lack thereof.

When looking at used ones, bring a 3/8-in. ratchet, a 17mm socket and a 10mm allen socket along with you. Drain the differential oil, remove the cover and have a look-see. Differential oil is supposed to be changed approximately every 30,000 miles. Is the inside of the housing clean and gray? Do the gears and internals look clean? What about the oil that you just drained out? Is it reasonably clean, or does it look really dark? Another clue to diff maintenance is the drain plug itself. After being removed and replaced a few times, the drain plug will bear some scars. If it looks like it's never been out, it probably hasn't. Run away! On the other hand, don't summarily reject a diff just because it's not pristine inside, especially if the price is right.

Assessing the quality of a rebuilt diff is trickier. Make sure any rebuilt one was done using factory/genuine BMW tools like bearing pullers and drifts, and according to the factory service specs. Here's a good test: Ask the guy who rebuilt it what oil he recommends. If he says something like Redline 75W-90 gear oil, thumbs up.
At rebuild time, check the differential mount bushing. A BMW Motorsport solid bushing made of extremely hard rubber is available for those of you who track your car or otherwise drive it hard.

Removing and Replacing an E30 BMW Differential - Instructions are similar for E24/E28/E30/E34/E36 BMW Models

  • If there's a lot of dirt and grease under the back of your car, do yourself a favor and pressure wash under there before taking on the job. You'll cut the grief and aggravation factor by at least 75 percent.
  • Jack up the rear of the car as high as possible. Don't jack on the differential; use the rear suspension carrier. Place jack stands under the rear suspension carrier mounts.
  • Drain the differential oil. Reinstallation torque on the drain and fill plugs is 50 Nm (36 ft-lb). If you've removed the cover, reinstallation torque is 50 Nm (36 ft-lb), and you must use new wave washers.
  • Cut the safety wire with a pair of side cutters and gently remove the speed sensor cable connector.
  • Using a long-handled 3/8-in. ratchet and an 8mm allen socket (preferably a long one), remove the inner CV joint retaining bolts on both sides. You can use the parking brake to lock the drivetrain. Reinstallation torque is 58 Nm (42 ft-lb).
  • Using a 17mm short open end wrench and the park brake, remove the four driveshaft to differential flange nuts and slip out the special bolts. It's impossible to get a torque wrench on these nuts. Just make 'em real tight. They're lock nuts, so if they've been off more than once or twice buy new ones-part no. 07 12 9 964 672.
  • Using a 22mm box wrench and a 1/2-in. drive ratchet with a 22mm socket, remove the 12mm bolt that toes through the rubber differential mount. The diff will drop about 2 or 3 inches now. Reinstallation torque is 87 Nm (63 ft-lb).
  • Place a floor jack with a piece of plywood under the differential. Using a 19mm short box wrench and a the 1/2-in. ratchet with a 19mm socket and a 1-in. extension, remove the differential housing to axle carrier retaining bolts. Reinstallation torque is 123 Nm (89 ft-lb), if you can get a torque wrench on the fasteners.
  • Carefully lower the floor jack and say hello to your diff. Follow these instructions in reverse to re-assemble. Voila!

    More BMW Differential Notes
    BMW differentials need a fluid change around every 30,000 miles. If filling a limited slip unit be sure to use the correct fluid with friction modifiers. Undo the filler plug first, this will prevent the embarrassment of having a drained diff and no way to fill it if the plug is stuck!

    A limited slip differential is desirable as it provides improved traction, if one wheel slips the other will continue to turn and get the car moving on ice, mud or snow. More modern BMWs have ASC+T electronic traction control and don't need LSDs so much although some cars did have both.

    The best lubricants (& change intervals) for BMW manual gearboxes and differentials: It's generally accepted wisdom that BMW's have special needs for the oil used in the manual gearboxes. Most of us know that you can't just go down to the local autoparts store and buy gear-oil off the shelf that's compatible with your BMW. There are a few gear-oils that are OK for your BMW, but to simplify this issue as, let's go directly to the best solution.

    Since you should be using a premium synthetic gear-oil, Redline makes one that is appropriate for your BMW. Virtually all of us who race BMW's use Redline gear-oil in our gearboxes and differentials. Why? A premium synthetic gear-oil will make the parts inside your gearbox and differential last longer. Reduced "hydraulic drag" will allow more of your engine's horsepower to get from your engine to the rear wheels. Synthetic gear-oils themselves last much longer than conventional oils, so you can reduce maintenance.

    Which type of Redline or other synthetic oil should you use in your bimmer? In the gearbox (manual transmission), some BMW's made between 1986 and 1992 had "labeled" gearboxes with a red or green label on the passenger side of the bellhousing. These had 17mm EXTERNAL wrenching oil plugs. If the label is green, use Redline MT90. If the label is red, use Redline D4 ATF. If there was no label, the gearbox will have 17mm INTERNAL wrench oil plugs, and should use Redline MTL GL-4 70W-80. Virtually all BMW manual gearboxes used before 1986 should use the same Redline MTL GL-4 70W-80 as you would use in the no-label / internal plug 1986-1992 gearboxes. For BMW's 1993 and newer, Redline recommends D4 ATF for all gearboxes, manual OR automatic.

    Now, for differential oil, it is easier. ALL BMW differentials use Redline 75W-90 Gearoil. This is ideal for open or factory limited-slip units, as well as the torque-sensing type if you are lucky enough to have one of those.

    How often should you change the tranny or diff oil?

    In racing, back in the days when people used non-synthetic gear-oils, the useful life of gear-oil in the gearbox and differential under summer racing conditions was just a few hundred miles, or just a few hours. Synthetics now can go an entire racing season, so racer's just make changing them part of the off-season winter maintenance, just once a year. For street cars, you can get away with never changing a synthetic, although I certainly would recommend doing it at least every two or three years for optimal gearbox and diff life.

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