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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 11-18-2012, 01:55 AM
Williamou Williamou is offline
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Location: Belgium
 
Join Date: Apr 2010
Posts: 3
Mein Auto: BMW 525dA touring
E39 touring adjust suspension height

My problem is that the free height under the bottom of the car is a bit too small. The car drags over the floor when leaving a garage situated in the basement.
I understand there is a level control system built in the model E39 touring . The height of the rear axles is adjusted by means of pressure control in the shocks (air bellows).

Question: one could imagine, if the height of the car is regulated, the reference must also be able to be adjusted?

Can anyone tell me if this is possible, and how? I suppose there is a sensor to measure the level of the car?
I would be happy if I can augment the height of the rear by inch.

Thanks in advance
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  #2  
Old 11-18-2012, 12:14 PM
vavet5308 vavet5308 is offline
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Location: Mechanicsville, VA
 
Join Date: Dec 2005
Posts: 520
Mein Auto: 2002 525it
I don't know about cars sold for the Belgium market, but in the US, some (not all) cars had self-leveling suspension. This was made possible through the use of airbags. This is important not only for ride and handling, but also to keep the headlights from shining into the eyes of drivers of oncoming cars. Some cars were fitted with two-axle air suspension while others had only single (rear) axle air suspension.
Whatever you do to overcome this, I think it's important for you to be able to disable it such that the system will work properly when you're not leaving your driveway. Otherwise you'll end up with the rear end of the car too high and your headlights will point downwards. Some of this can be overcome with the auto-adjusting headlights, if your car has those. I think that was limited to cars fitted with HID lights.

I see two basic ways of doing this. The first is to trick the air suspension computer into thinking the car is too low so that it adjusts the airbag pressure itself based on the faulty information it's been provided. The second way is more of a brute force method where you provide a power source to the compressor to get it to increase the pressure without input from the module.

So...if you can find the sensor attached to the rear axle support, connect a voltmeter to it with the car setting at a normal position. Then load the cargo area with 100 lbs to create a slight decrease in the ride height. Read the voltmeter again. This will tell you if the voltage increases or decreases when the height is lower. The idea then is to trick the air suspension computer into thinking the height is lower than it is so it increases the airbag pressure. I don't know what the max pressure for these airbags are, so do this at your own risk.

I think the brute force method is the way I would try to try to do this first. Find the relay in the spare tire area that provides power to the compressor. I think there is a black metal dome under your spare tire. Remove the bolts or nuts that hold down this dome. Turn it over (it's still attached with wires and air lines, so you'll be limited as to which way and how far it will move). Find a 12V DC switch to serve as a jumper for that relay (on either the switch side or the switched side), then you wire that into the circuit. If you go to the switched side, you'll need a switch capable of handling that current. The fuse for the circuit is 30 amps and the compressor is the only thing on that circuit, so I'd imagine that's a reasonable capacity.

When you're ready to leave the garage, flip the switch to turn on the compressor to the point where it's an acceptable height, then turn it off and back out of the garage expediently. You might learn that the suspension module adjusts the height too quickly and you have to leave the compressor running while you're backing out. Just don't forget to turn it back off.
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  #3  
Old 11-19-2012, 02:44 AM
Williamou Williamou is offline
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Location: Belgium
 
Join Date: Apr 2010
Posts: 3
Mein Auto: BMW 525dA touring
Thanks a lot for the answer!!

Indeed i have a car with auto-adjusting headlights so there is no problem in changing the position of the rear end of the car.

I try to do the "brute force" method and increase the pressure in the airbags just by starting the compressor manually and lifting the car a bit in this way.
It even has the advantage that the lifting of the car is only temporarely and comes back in "normal" position after i left the garage.
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