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
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....
Should You Get a Used Differential or Rebuild Your Existing One?
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.
Removing and Replacing an E30 BMW Differential - Instructions are similar for E24/E28/E30/E34/E36 BMW Models
More BMW Differential Notes
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?