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E34 M5 Discussion 1988-1995 Sedan and Touring

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Old 12th September 2008, 17:31   #1
///Manuel
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EDC Question - Can someone explain why EDC shocks require special expertise for rebuild?

I have read a few posts regarding the subject and heard of a couple of shops in the world which could rebuild shocks for us.
What is it that makes it difficult for a shop to rebuild our shocks at a reasonable cost?

Is it the parts availability? The lack of specs? The unique shock design?
The lack of expertise? The willingness of shops?

For instance, I pulled off this article (QA1 Shocks Revalve And Repair - Stock Car Racing Magazine)
about shock revalve / rebuild out of the numerous ones available online.

Looking forward to discussing this subject.
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Old 12th September 2008, 20:20   #2
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A First Pass

Hi Manuel.

I'll take a stab at answering this.




What makes a shock absorber work the way it does.
  1. The amount of hydraulic oil in the damper
  2. The viscosity of the oil in the damper
  3. The pressure of the gas in the damper
  4. The integrity of the seals on the shock body and piston rod
  5. The valving mechanism at the base of the piston
Additionally for EDC shocks
  1. The valving mechanisms in the shock body
  2. The control electronics for those valves
Lets deal with rears first, because I think they are easier. There is a top cap on the rear EDC dampers just like the top cap for conventional dampers. Remove it and the top seal assembly is easily removed by just bringing the piston rod to the top of it's stroke and giving it a short sharp yank.
  1. The amount of oil in the damper is determined by the hydraulic pump in the engine bay, so this doesn't present much of a problem.
  2. The viscosity of the oil in the damper is set if you use the hydraulic oil recommend by BMW, so this doesn't present much of a problem.
  3. The pressure of the gas in the system is set by the accumulators, so this doesn't present much of a problem.
  4. The top seal assembly of the rears has replaceable seals and is reasonably easy to disassemble, replace the seals and reassemble. The only part I'm yet to identify a replacement for, is a load bearing face that appears to be coated with Teflon (R).
  5. Assuming the seals aren't in really bad shape and haven't let too many foreign abrasive (dirt) or corrosive (salt) substances into the damper, the valves on the piston should present much of a problem.
  6. Wear and tear on the piston rod. The design of the rear suspension doesn't put lots of side load on the rear shocks so asymmetrical piston rod wear (ovalising) on the rears should not be a significant problem.
So far so good.


The fronts are a whole other "ball game". The top cap on the fronts is welded in place.
  1. The amount of oil in the damper is only known by the manufacturer, so this presents a bit of a problem.
  2. The viscosity of the oil in the damper is only known by the manufacturer, so this presents a bit of a problem, although you could test it and ascertain it's viscosity.
  3. The volume and pressure of the gas in the system is only known by the manufacturer, so this presents a bit of a problem. Although the pressure can be calculated but to achieve this would require a fair bit of mucking about.
  4. I haven't opened a front shock yet but I will in the not too distant future and then I'll be able to report on the maintainability of the top seal.
  5. Assuming the seals aren't in really bad shape and haven't let too many foreign abrasive (dirt) or corrosive (salt) substances into the damper, the valves on the piston should present much of a problem.
  6. Because all the securing arms on the front shocks are connected with ball joints, the front shocks have significantly more side loads on the piston rod and wear and tear is much more likely to result in ovalising of the piston rods in the front shocks. Since this part is not available from the manufacturer or BMW, this part would have to manufactured if it needed replacement.
The EDC Controller.
I haven't delved into this, but this device controls valves in the shock body. In a conventional shock absorber, the oil in the shock goes through the valve at the base of the piston, providing damping. In an EDC shock absorber, when the valves in the shock body are open some the oil in the damper can pass through these valves and reduce the damping provided by the shock absorber.

The contoller uses accelerometers on the car body to determine how much damping should be provided by the shock absorbers.



When you're having problems with an EDC system,
  1. Have one or both of those valves failed ?
  2. Have they failed electrically or mechanically ?
  3. Have they failed opened or closed ?
  4. Has the controller failed ?
  5. Has one or more of the accelerometers failed ?
  6. Has the control electronics failed ?
  7. The performance of the system changes based on road speed. Has the input from the speedometer been lost ?
  8. The performance of the system changes based on the "S" or "P" setting on the dash.
Only the manufacturer and BMW know what inputs from the accelerometers will drive what opening and closing of the valves in the shock body and similarly how that valve control will change based on road speed and "comfort" setting.

In summary, repairing the rear legs doesn't appear that difficult, assuming of course the problem is a malfunction in the leg. So far no-one has mentioned a source of the valves, so problems with the valves could be an issue.

Repairing front legs is much more of a challenge. Removing the top cap and being able to repeatedly replace it accurately could present a challenge, depending on how it's located. I haven't opened one yet. Getting the right volume and viscosity of oil will take a fair amount of homework as will getting the right volume and pressure of gas. Refilling a refurbished unit and comparing it's performance against a new unit is one possible approach to discovering the unknowns in a front EDC shock. Can a local shock absorber repair place ascertain that info in less time than the cost of a new unit.

Mapping the EDC controller will take a considerable amount of patience. Better to just buy another one. Checking whether the unit truly performs properly, probably requires checking using the BM diagnostic tools.

I'm sure there's much more that others can add to this and I'm sure others will correct any mistakes, but it's start.

Reg.
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Last edited by E34_nut; 12th September 2008 at 21:02.
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Old 12th September 2008, 20:55   #3
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Additional only & not corrective.
The EDC controller is driven from the vehicle speed sensor & the overall EDC cal sensor, the steering angle sensor.
This dual wiper system measures angle & rate of turn.
The 3 acceleration sensors combined feed the EDC control unit with the vehicle movement, that combined
with steering rate & angle will give a crude yaw calculation.

Compared to current EDC refinement, it is a little slow & can be caught out
in some weather conditions.

The dampers are just the recipient for signals.
As Reg said, if the solenoid valves circuits for one or both pairs on each leg fail, limited operation will result
with default to harder damping control planes.

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Old 12th September 2008, 21:12   #4
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I know the attached isn't M5 specific, seems to cover E31 and E32 EDC operation, but may help explain a few things to those interested......

The M5 will be similar with perhaps a different response graph........(?)

Mark

Attached Files
File Type: pdf EDC111.pdf (196.3 KB, 635 views)
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Old 12th September 2008, 21:20   #5
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As the lowly owner of a 3.6 devoid of these issues I cannot offer much, but I seem to remember some people investigating rebuilds having great difficulty in that re-welding the unit closed again after the top was cut off was killing seals/internal stuff with the heat.

I do not recall how they got around this.

Regards

Ivan..



P.S. Did anyone ever put to bed the idea that one of the Masarati Quattroporte dampers is the same unit as the BMW unit? At £550 from BMW the Maser unit might have been more, but at the £££ BMW now want a definitive answer may be worth investigating.
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Old 12th September 2008, 21:41   #6
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Excellent and very detailed response thank you, I wasn't expecting that much, I am impressed with the level of knowledge on this board.

It does sound like the rear units could be (and have been) rebuilt fairly easily and that the front ones may need to be purchased from BMW given the unknown stock specs combined with the required welding (how nice of them).

Anything that helps reduce the final bill is always welcome!

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Old 13th September 2008, 15:11   #7
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Quote:
Originally Posted by IvanDias View Post
P.S. Did anyone ever put to bed the idea that one of the Masarati Quattroporte dampers is the same unit as the BMW unit? At £550 from BMW the Maser unit might have been more, but at the £££ BMW now want a definitive answer may be worth investigating.
Or was it the Alfa 164 Cloverleaf / 4WD model? I don't ever recall reading a positive write up of the Italian electronic damper systems of the early nineties though.
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Old 13th September 2008, 16:30   #8
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Quote:
Originally Posted by Baddie View Post
Or was it the Alfa 164 Cloverleaf / 4WD model? I don't ever recall reading a positive write up of the Italian electronic damper systems of the early nineties though.
It was indeed the alfa 164. I doubt that the dimensions/attachment points match though.
If they do, this could be a source for seals etc, if alfa has a little bit different view on supplying parts...

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