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E39 steering box into 540i - servotronic questions

26124 Views 27 Replies 8 Participants Last post by  Retset
I've had a good old forum and internet search on fitting an M5 steering box into my 540i including some useful information from Steve Wright (built an E39 M5 Touring). He has the M5 box on the 540i steering components. However he also has the full M5 loom so has been able to connect up the servotronic.

I have sourced an M5 box at reasonable cost from a BMW breakers.

My 540i does not have servotronic. If I simply fit the M5 box in with no servotronics connected can anybody tell me what would happen apart from a welcome reduction in turns lock to lock? ;)

I assume it would probably run at the maximum heaviness (equivalent to your M button being pushed). Now my 540i is fairly heavy at all times on the steering so I can live with that happily as long as it's no heavier. If it ran full light all the time I wouldn't be as happy.

So, the specific questions are:

1) Does anybody know if it will run light or heavy?

2) If you leave your M button on all day in your M5 is it still a reasonable weighting to park and manoeuvre?

3) Is there anything else I should have considered?
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Correct!

The E39 M5 box with servotronic unplugged was noticeably heavier than the E39 540i non-servotronic box.

It was a bit heavy (but OK) for parking but made it really nice once on the move.

If I had an E39 M5, I would certainly run it for a few weeks unplugged, to see which I preferred.
Gonna try that, and maybe rig a circuit to control resistance from inside the car :)
 

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Gonna try that, and maybe rig a circuit to control resistance from inside the car :)
Just FYI, I had a friend hang a scope on the servotronic control module of another Servotronic-equipped vehicle (in that case a '96 Ford Taurus SHO, which uses a ZF steering rack, and the Servotronic control circuitry is in the same black box that controls the electric shock valving) and we found that the output from the module (the cube is a pulse-width-modulated solenoid) maintained high assist until approx 30mph at which point it backed down to minimum assist by 38mph. I've got that setup in my '91 SHO now, but I find it's actually too light with the variable assist connected so I pull the fuse on it.

I've got a chart of the Servotronic assist somewhere for the Opel Omega B (Cadillac Catera) box - which is a recirculating-ball box very similar to the M5 box, but with a much longer output shaft housing with the mounting lugs down the shaft housing (it fits beautifully in a '64 Ford Galaxie...) and the Opel assist tapers off much more gradually with speed, closer to what I perceive the M5's setup to do. I would not care regularly to drive the M5 with the Servotronic unplugged.

Side note - the BMW setup is a sucky design, most Servotronic solenoids have a Bosch fuel-injector-style plug on them but since the E39 puts the thing hard up against the subframe there isn't room for a plug, instead you've got a wire that comes out of the cube and seems to want to shed its sleeving at that point. I didn't like the potential for water getting in there, it took no little bit of work to finagle a couple layers of glue-filled heatshrink onto the cable to act as a seal and strain-relief there.

Servotronic-equipped Audis at least of a few years back use a standalone relay-package Servotronic control module, though I can't tell you what the assist curve looks like or what the needed input vehicle-speed pulse rate is. I keep meaning to try to find a base A6 4.2/A8 module for our C5 S6 to make it a little more spousally compliant (the steering's pretty heavy in that car.)
 

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Took my wheel off today to do this...here's a photo. What do I unplug here? I see a bunch of wires.

 

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Just did this "mod" today and drove for about 30 minutes. Definitely much heavier at low speeds, but not as noticeable after 20 mph. It's like having sport mode on at all times sans the super sensitive throttle. There is a small black box attached to the side of the car in the photo that I posted. You need to open it, and unplug the cable with the white connector (the one on the right side of the box). I left the longer part of it back in the box, and sealed off the other plug and zip tied it to the other cables. No codes showing on the dash of any kind.

Will leave it like this for a while and see how I like it. If anything changes, I will update you guys.
 

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The idea of hooking up a switch to the servotronic is, I think, an excellent one. Turn it on when running around town, getting groceries, etc., but turn it off when on the highway, the twisties or the track. This I may have to do.

--Peter
 

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Oh yes, oh yes, OH YES :cheers: :7: :applause:

It's all worked superbly. I can't type out all my hints and tips a second time so please view my thread on BMW5.co.uk. Most of what you want is on page 4 of the thread which is here: forum.bmw5.co.uk • View topic - E39 M5 steering box into 540i - DONE IT!

So worth the time spent (lots) and money spent (not much ;)).

Done 200 miles since the conversion and am delighted. Not wired up the servotronic yet but I'm happy as it is. Future project will be to wire in a volt or two to just lighten it a little whilst maintaining a fixed rate of assistance throughout the range.

If I can be of any help just PM me.
I may be about to do this upgrade myself. Did you ever hook up voltage and figure out what the proper assist level would be?
 

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Discussion Starter · #28 ·
I sold the car about 18 months ago without ever doing it! The new buyer liked the meaty feel and said he would probably never try to sort the voltage - he had a Carlton 3000GSi before and said that the M5 non-servotronic in mine felt very much like his old Carlton!

Sorry I couldn't help :(
 
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