This section contains special information on the rear suspension that supplements the Service or the Bentley manuals.
All information contained in this FAQ is provided by BMW enthusiasts who are not typically fully trained in the art of BMW maintenance. As such, all information in this FAQ is provided "as-is". Any use of this information is strictly the responsibility of the using party. The supplier of the information and the Webmeister assume no liability for incorrect information or use of this information.
Worn Diff Mount Causes Negative Camber
Differential Mount Replacement
Rear Suspension -- Self Leveling
Subframe Bushings - Steve D'Gerolamo <firstname.lastname@example.org>
I'm starting to sell a lot of subframe bushings, especially for E30's and E28's which are getting up in age. I've got the tools in stock for shops and car enthusiasts that want to do the replacement without removing the subframe from the car. There are 3 difficulties to this job....first is "removing the knurled bolts" that go down under the seat and through the subframe mount/bushing...an air hammer will pop these bolts up in about 20 seconds per side...with a big enough hammer and a fair amount of patience (& luck), the bolts will free loose without the air tools.
Once the bolt has been removed, onto part 2, "removing the mount from the subframe" The special tools to do this run between $300-400 but the mount can be cut free with an air chisel or reciprocating saw...this will take about 15-20 minutes per side by the time you've removed enough rubber to collapse the mount and removed the pieces from the subframe (if you're using an air chisel, be careful not to damage the subframe or you'll have to spend a fair amount of time cleaning up the edges before the new mount can be installed).
The 3rd difficulty, "installing the new mount", takes 1-2 minutes per side with the factory tool. The new mount is notched...be sure and line the notches with the ridges in the subframe. Put a little kerosene ("tar remover" from your car care kit is essentially the same thing) on the new mount to allow the new mount to slide into the subframe a little easier (the kerosene will quickly evaporate after the installation). The tool is a threaded rod with a top and bottom cap (the top cap is keyed to a notch in the mount to keep it from spinning). As you tighten the bearing/nut on the threaded rod, the new mount will be easily pulled into place in the subframe. Note, that you can also remove the subframe from the car and handle difficulties 2 & 3 above on your shop press...but taking the subframe off the car means removing the differential and adds a few hours to the job.
Remember to check the condition of the differential mount while you're working at the rear of the car...if your car is up in years and miles and you plan on keeping it, I would suggest replacing the diff mount when you do the subframe mounts.
I've begun renting some of the specialty tools (such as the above) to my parts customers that don't want to make the investment for a 1 time job. I imagine that the big mail order parts houses (BMP and Bavarian) do the same when you buy the parts through them.
Steve D'Gerolamo c/o The Ultimate Garage, Emerson,
Worn Diff Mount Causes Negative Camber - Pete Read <email@example.com>
Don Mies doesn't see how a worn differential mount, which lowers the differential height, can cause extra rear wheel negative camber on an E24/E28 trailing arm suspension.
Then Dale Phelps explains that the half-shafts are fixed length and push the hubs out slightly when the differential drops below its design height.
Dale is right about a worn differential mount causing negative camber, but not for the reason he mentions. The half-shafts don't push the wheels out. It's actually caused by movement of the subframe which is rotated slightly by the weight of the unsupported differential.
I've seen this worn differential mount causes negative camber mentioned a number of times, and have always agreed with Don's point of view. I couldn't see any way that differential height affects camber.
The motion of the rear wheels is completely controlled by the relationship between the trailing arms and subframe, to which they attach. Because of the orientation of the trailing arms, as the suspension compresses, negative camber and toe-in increase. That is why sagging springs or shorter aftermarket springs cause more static negative camber.
After this latest mention, I decided to renew my search for a reasonable answer. At the Summit Point Vintage Car races this weekend, I bumped into Tom Baruch, a very experienced BMW mechanic and driving school instructor.
Tom explained that the differential hangs off the back side of the subframe. If the rear diff mount doesn't properly support the differential, the weight of the differential rotates the subframe downwards towards the back of the car.
This rotation of the subframe affects negative camber the same as compressing the suspension. It changes the orientation between the subframe and trailing arms.
While I can't take credit for the explanation, at least I was able to recognize a good answer <grin>.
Differential Mount Replacement - "Wise.Avery"<wise.avery@Orbital.COM>
Per some of your requests, here's my procedure for changing a Euro 6-Series diff mount:
1. Keep all four wheels of car on ground. Do not jack car up. If you have problems crawling underneath the car without jackstands, you may want to use ramps on the rear wheels. This advice was based on a diff. mount procedure for another German car with a similar rear suspension.
2. Disconnect the two rear muffler hangars and the hangar adjacent to the differential.
3. The muffler will now hang down slightly, but will not touch the ground since there are two hangars near the cat/center resonator holding it up.
4. Place a jack underneath the diff and pump the jack a few times to take the weight off of the diff. Do not jack the rear wheels off the ground.
5. Loosen the (19 mm head) bolt holding the diff to the mount. You'll need some type of long ratchet or breaker bar. Lower the diff down slightly with hydraulic jack.
6. Remove the four (17 mm head) bolts holding the mount to the chassis. Note the location of the notch in diff mount (it's on the passenger side).
7. Lower the diff a sufficient amount to pull the old mount from the car. (You won't have to lower the diff that much).
8. Inspect the diff mount nutplates and mounting surface for cracks, etc.
9. Install new mount (~$67) with blue Loctite and new wave washers on the four (17 mm head) bolts. Note notch location. The loctite is just for "piece of mind."
10. Jack the diff back up so that it butts against the new mount. Install the large (19 mm head) bolt with blue loctite.
11. Torque the four (17 mm) bolts to ~43 N-M. Torque the large (19 mm) bolt to ~85 N-M.
12. Lower jack slowly and re-install exhaust hangars.
Rear Suspension -- Self Leveling - Don Schmidek <Dis@Metricom.com>
The '88 & '89 sixes have self leveling suspensions. The suspension consist of special shocks, hydraulic accumulators in the wheel arches, a pump and control system under the spare tire. There are also linkages connected to the rear torsion bars, etc. which sense the height of the car, plus piping, hoses, etc.
If your car rides very harshly or the height level keeps on changing, i.e. you hear the pump cycling frequently, the chances are that your accumulators are shot.
The good book states that the life of these may be as short a s 50,000 Km. Typically it is probably about 100,000 miles.
You can check out this problem by rocking the rear of the car -- push down on the rear bumper hard. If you get very little motion, you may the problem.
You can also check by bleeding the accumulators --- see below -- by disconnecting the big hose connection and seeing how much fluid runs out. If you get about 200-400cc from each accumulator, that suggests that they are defective. If both old accumulators were completely full of fluid, it indicates that the Nitrogen pressure charge had been lost. This may be due to the bladder braking or the Ni just leaking out.
To replace these, jack up the rear by the differential (use supports on the two sides via jack stands). Turn on the ignition (do not start the engine) to allow the system to normalize and depressurize -- for about 1 min. After that, just remove the two pipe connections to each accumulator (be sure to cover them, not to let dirt into the pipes) and let the fluid bleed off into a pan. Remove the old accumulators (3 nuts) and install new ones. Be careful not to miss-thread the fittings. Add fluid to the tank -- if you remove all the fluid before you start the process you will in fact rejuvenate the fluid -- all it takes is about 1 liter -- and you are ready to roll. Do not have to bleed the system -- but after you let the car off the stands, turn on the ignition and see if your car adjusts. Sit on the trunk edge, and see if it self adjusts. You can also measure the height from the ground to the bottom of the rear wheel arch -- it should measure about 27 inches. Finally, you should now get a little bouncing motion if you push down on the rear bumper.
Once done, you will find that your 6 rides comfortably over road irregularities as it was meant to, corners 200% better and no longer skitters in turns.
Now for the costs: Accumulator run about $130 each --- plus $10 shipping -- total $270. You may get quotes up to $270 each, but keep on shopping. Note that the retailers pay the wholesaler just $85 for each -- so there is plenty room to bargain.
You should use Pentosin 7.1 CHF as called out for hydraulic fluid -- costs is about $16 per liter. It is available in foreign car shops.