Changing the differential fluid in an 840

[wedge31]

Hello all,

I'm planning on changing the auto transmission fluid and filter in the next few weeks, and am considering changing the differential fluid at the same time. The car has 90,000 miles on it and on the whole drives very well - smooth gear changes and no noise / whining from the diff, so you could call this preventative maintenance.

My questions are:

Considering that BMW states that the diff fluid is lifetime fluid - see: (http://www.realoem.com/bmw/enUS/show...diagId=33_0362) does anybody know if it is a straight forward job to change the fluid or if it is overly complex and hence expensive (I will not be doing the work myself)?

If it is worth changing, anyone have any info on the correct fluid type and quantity required? I would like to know this so I can order it.

Any advice is much appreciated as always.

[wedge31]

Hello all,

Does anyone know the correct amount of fluid (Pentosin CHF 11 S) required for changing the hydraulic fluid in an 840 with ASC+T?

I'm considering also having the hydraulic filter changed (pn:32411137788) any thoughts on whether this is worth doing or best left alone? The steering is good at the moment, nowhere near as light a that of some US cars like a Cadillac Seville, but certainly not the heaviest steering I've experienced. Is it possible that changing the filter would make the steering a bit lighter?

The filter in question is item no 11 shown on the following diagram from RealOEM:
http://www.realoem.com/bmw/enUS/show...diagId=32_0654

Cheers.

[e31gixxxer]

* Mobile VS500, 75W/140. 1.9 litres. Rear diff. (As if there is a front one). Always worth changing. There is no drain bung, so remove the rear cover plate to drain. The gasket for this has been discontinued and BMW supply sealer instead.

* Pentosin fluid change is generally done by sucking it out of the ressy with a "turkey baster" or syringe, 1 litre per go should do. Same again, this only dilutes the fluid, so give it several goes once its been run through the system.
There is an error on the P.N. for the filter on some of the 840s, you can clean your old one out with brake cleaner if you have issues getting a new one.

[Timm]

Both are easy to do - here's the differential (which does have a drain plug!):

http://www.meeknet.co.uk/e31/BMW_E31...al_Service.htm

....and here's the power steering/brake booster including the correct part number for the filter (which is wrong on realoem):

http://www.meeknet.co.uk/e31/BMW_E31...ter_Change.htm

[wedge31]

Guys many thanks for the feedback, very much appreciated.

-Timm, I noticed that you have a LSD, are you able to confirm if the non-limited slip diffs have the drain plugs as well?

A couple of questions if you don’t mind regarding the hydraulic fluid change you did:

1) Did you notice any difference in the weight of the steering after you changed the fluid and filter?

2) How much did you pay for the replacement filter?

Cheers.

[Timm]

I can't confirm or deny the fitting of a drain plug on the original diff! I have put up links for a syringe extractor and a fluid transfer pump (which I have used to good effect)

1) The braking action got a lot better - and the steering was lighter
2) Not much, a tenner or so from BMW

[wedge31]

I can't confirm or deny the fitting of a drain plug on the original diff

Thanks for clarifying that.

Can anyone confirm if the original (non-limited slip) diff has drain and fill plugs?

[Timm]

They definitely have fill plugs as that is on the rear plate (which is the original one on my car)....

- - - Updated - - -

Update: Yes, they all have drain plugs - here is a shot underneath my first 840ci - and that had the standard diff:

[wedge31]

Update: Yes, they all have drain plugs - here is a shot underneath my first 840ci - and that had the standard diff:

Good man 👍🏻👍🏻

Since we’re on the subject of fluid changes, can I be a bit cheeky and ask the quantity of DOT 4 required for a brake fluid change?

[Hyper]

My opinion on these wedge issues...

1) Pentosin goes out at the system low point, thru the pump port or banjo connection.

Bottom side of typical single vane, 840.


2) If there was a change in steering effort, it was not by much. Probably the same difference in whether your tires are at 25psi or 35.
[professor-speak]The assisted steering is a high gain feedback loop. The nature of "feedback" systems, with sufficient gain, is that they tend to minimize the affects of components and means (and fluid viscosity, etc). The exceptions to this are the gain, which in this case is set by the rigidity of the torsion bar internal to the input shaft {ed: and the piston pressure modulated by the servotronic valve}. A less rigid bar increases the gain, reduces the steering effort.[/professor-speak].

3) Here is the big brother to what you see at meeknet.
Known as a suction gun, about 1-liter, it's my go-to tool for filling.
For instance, tranny fluid via the check-level tube at the back of a bay, on other cars of course.
Might (?) be able to extract some diff fluid via the filler port. Haven't tried that.

[Timm]

Goodness knows, my man that does the 'rusty bits' changed it for me a couple of years ago. Here's the blurb from BMW:

http://www.meeknet.co.uk/gen_bmw/BMW...rake_Fluid.pdf

[wedge31]

All good info guys, many thanks.

[e31gixxxer]

Update: Yes, they all have drain plugs - here is a shot underneath my first 840ci - and that had the standard diff:

I stand corrected T, it's been a while since I was under there.

DOT 4 is the spec of the fluid, it meets the required Dept of Transport standards, so if it says DOT 4, then DOT 4 is ok.

[rob1210]

E31s all use large case diffs, also known as 210mm. The internals (LSD or open) are interchangeable. All large case diffs have both drain and fill plugs. The large case diffs only differ in the number of front mounting holes: 2 or 3. The E31 only needs the forward most 2, therefore, any large case diff will function in an E31 but, not necessarily, in for example, an E34.