BMW Air Supension............

[fatdaddy]

you probably think that is a strange title for a post here !

Hello, first post.

I know nothing of electronics, not bad with auto electrics being a qualified LPG installer and having worked on Harvesting machinery for years.

My car is a BMW 540 Touring. These have rear air suspension as standard, being a system to provide self levelling regardless of load. Both sides of the axle are fitted with a "level control" and these feed to a control unit. I know the suspension height can be altered via dealer level diagnostic interfaces and that its normal operation is "overseen" by the car ECU systems.

[more details here
WDS BMW Wiring Diagram System - 5 E39 from 09/98
under Chassis>Springing and damping>Single axle air suspension EHC>EHC supply>Rear axle air suspension level control ]

I can see from that site that the level control devices at the axle are Hall Effect units and I am wondering (in my simplistic way of thinking!) if the signal from those to the control unit could be altered into fooling the control unit to set the car at an alternative higher ride height.

My previous 5 series was a saloon with wheels that allowed the fittment of snow chains. My present Touring is a Sport model and BMW quote that this cannot be fitted with snow chains. Having looked at mine and spoken to several other owners on Forums the "clearance issues" that BMW cite as the reason are actually that it is physically very difficult to fit chains due to clearance for your hands rather than any issues in driving with chains on.
So what struck me was could I have a two position switch which would send the level control signals via "something" that would give a raised height when required so that chains could be easier to fit.
I may be way of the mark but I'm thinking that the Hall Effect units send a differing voltage to the control unit and the control unit, either directly or via the ECU, allows pressure in or out of the air springs as required. If the "something" altered that voltage then the car would settle at a different height and the rest of the BMW systems would be none the wiser...............................


Sorry this post is long and I hope I'm not wasting your time by talking rubbish.


Thanks

Iain

 

[Diver300]

I don't think that it is a stupid idea.

The circuit looks as though the sensor is a simple potentiometer with 3 wires. If so, two of the wires will be at approximately constant voltage whenever the ignition is on. It could be they use 0 and 5 V. The third wire, the wiper, will move from one end voltage to the other (0 - 5 V) depending on the suspension height. It probably won't cover the whole range, so if you find that it does 1 - 4 V that would be correct.

To fool it, you need a resistor from one end of the potentiometer to the wiper. Don't go below 1 k ohm and you won't do any damage.

If you do switch in a resistor, it is likely to upset the headlight level. Be aware of funny handling if you have different grip on the back from the front.

Alternatively, it looks as though you have a valve to connect up the left suspension, one to connect up the right suspension and one to let the suspension down. There is a compressor that runs to raise the suspension.

You could just turn on the left and right valves and the compressor to raise the car. Leave everything disconnected and it should stay up. Put it back to normal and it will go back to the normal level.

 

[fatdaddy]

thanks for the reply.
My first thought was that it would be a variable resistance thing of some sort but then I read that the system works on the "Hall Effect"...................... [maybe that it the correct description of what a potentiometer does ?]

What you have detailed is what was going through my mind. I envisage just using the raised position to enable easy access to fit the chains, then dropping to the normal position for driving.
Could I use a variable resistor in the switched side of the circuit just 'cos I don't know what value I want to start with?

I didn't want to mess with the air side as I think this is what the dealer diag does and the manual says you need to reset the height after using it.

Thanks again for your help

 

[RF_Burns]

I have one of those GMC motorhomes they built back in the 70's with rear tandem wheels and air suspension.

At some point I'm going to change the way the suspension works from air valve sensor to an all electronic system. I want the ability to press a button and set a higher rider when navigating uneven campground roads etc.

I agree with Diver300, the sensors appear to be potentiometers. The resistors can be added right where the sensor wiring plugs into the control module to keep the connections out of the weather.

 

[Rbeckett]

Iain, As a former BMW tech I would caution you against atempting to make any modifications to the systems in that automobile. I have been perplexed on several occasions due to line losses of as little as 500 Millivolts that caused other unrelated systems to not funtion correctly. Specifically the Sound system was modified in a very minor way and the theft and door locking system failed. When I replaced the stereo head with an identical BMW head unit it worked again perfectly. Euro cars such as Mercedes and Bmm'ers are tuned and adjusted to a very fine line and seemingly minor mods have very strange effects often in very different sub-systems. Better to leave that alone since it is handling related and get a set of good Blizzak type tyres for the application in the correct size. Sorry,, I am a gear head and tinkerer at heart, but those cars just do not lend themselves very well to that type of tinkering. Wish I could be of more positive help, but just cant in good concious make that recomendation.
Bob

 

[fatdaddy]

I appreciate that advice Rb and know exactly where you are coming from......................... it just seems that it could be such a clean simple answer to getting chains on.
I know I could do it by making the tie link remotely adjustable which would have no effect on anything else but that would be a clumsy way to do it.


hmmmmm

 

[fatdaddy]

just a tesst to see if me posts still need approval

 

[RF_Burns]

Just curious, But I never would have thought southern UK would get enough snow to warrant the use of chains...

This coming from a Canadian who lives where the average winter snow is 12feet (3.7m) and studs and chains are illegal.

 

[fatdaddy]

Just curious, But I never would have thought southern UK would get enough snow to warrant the use of chains...

This coming from a Canadian who lives where the average winter snow is 12feet (3.7m) and studs and chains are illegal.

As a nation we are hopeless on roads in the snow
Majority of people are completely unprepared and don't know how to drive in it. The roads get blocked quickly with a few inches of snow 'cos cars get abandoned all over so the plows/gritters can't do their job.
My case is unusual in that I live in the sticks on a lane where we don't get gritted and I have a 30% gradient down and back up again from the highway. The local authority come out when we have more than a few inches and "plow" the lane but all they actually do is smooth out the snow to leave a perfectly smooth shiny surface for the nights frost [sometimes as low as -10 just to give you another laugh ] to harden. So my problem is just grip for that first mile then I'm fine. It only happens a few days each year normally [last year it lasted a couple of weeks and this year looks as if it may be the same].

 

[fatdaddy]

.........so back to my "circuit"

**broken link removed**

I need to put a resistor
1. in the wire from 4 to 1 ?
or
2. between terms 20 and 1 ?

As I don't know what value I need [and I need two as each side of the car is separate] can I use something like this
**broken link removed**

It struck me that I don't know if adding resistance will actually lower or raise the car at this point but I can cope with that by altering the linkage if needed.

 

[RF_Burns]

You would put your resistor between either 1 and 20 or 1 and 18. In one position it will go up and the other it should go down. Trouble is we don't know what the total resistance is across the sensor pins 1 & 5, so its hard to determine what value of resistor you will need. Using a pot is a good idea since it will let you adjust the height once you find the correct pins to connect it across.

Also, be aware that the module will likely also detect if there is any problems with the sensor or its wiring. I suspect that might be the purpose of the resistors shown in the control unit. If so your added resistance could trigger a malfunction alarm.

Automotive sensors tend to be low resistance units to improve noise immunity, so start at a high resistance, maybe 10K, then 4k7, 1k, 470ohm, 220, 100, until it starts to react.

Just what I would do... no warranty implied.

 

[fatdaddy]

just to understand that more..................

the resistor I am adding will let a controlled "leak" of voltage from one connection to the other ?
So starting with a high resistance unit will have the least effect ?

thanks again

 

[Diver300]

Yes, the larger resistor will have the least effect.

I would describe the resistor as letting current leak from one connection to the other. That current will move the voltage of the wiper closer to the voltage at the other end of the leak resistor.

As we don't know the value of the potentiometer, we don't know how much current to leak, which is why the 100 times range of resistor values.

 

[fatdaddy]

Hi.
I finally got round to trying this


and it works
I used a 220 on each side to get a 20mm lift, made up a short jump lead with a plug and socket so i could remove things easily. The system is VERY sensitive to the value and it got a bit confused about self levelling again after trying wrong values but disconnecting the battery, letting all the air out then reconnecting sorted things each time. Now just need to wire it properly with a two way switch in the luggage area.

thanks for your help............

 

[InventionUnlimited]

thats great news, glad it worked for you - now off to see it it works on my '7'

 

[fatdaddy]

Invested in some Alpine tyres this year so I only used the chains once in fact, still nice to know that I can get them on when I want without a jack

SO that would be a 2003+ 7 ?
or maybe you had different in the States ?