S-EDC Accelerometer Troubleshooting

[a777fan]

The S-EDC button in my 2001 750iL will come on for about 10 seconds when I start the car, and then it will shut off. The button then becomes un-responsive and the suspension also 'feels' quite stiff, which is the default setting when the system has issues. I had pulled codes via INPA, and there was the following stored:




The first code pointed to a defective accelerometer, so I thought checking those out would be a good first step. There isn't a whole lot of information pertaining to the specific sensor locations, but after rooting around on the internet a bit (mostly posts from QSilver7 on this very forum ), I figured out that there are two sensors, one fore and one aft located on the right side of the vehicle within the wheel wells. You'll need to remove the wheels as well as the fender liners in order to gain access to them. The front fender liner comes out fairly easily once you have all of the eleventy billon screws out. The rear liner is another story all together however. The hydraulic lines for the self-leveling suspension and the electrical connections to the Brake pad wear sensors and the wheel speed sensors (?) complicate things. The rear accelerometer is mounted at the rear of the wheel arch in this location, so focus your efforts on the fender liner in this area. I still found it easiest to take out all the nuts, bolts and plastic clips, as well as disconnecting both electrical connectors to give me as much room as possible to manipulate the fender liner itself to gain access to the accelerometer itself.

Some shots of the sensors in their 'natural' habitat:

Front Sensor location; A bit aft of the very top of the wheel arch, outboard of the shock tower, under the plastic wiring loom which runs around the wheel arch. You'll have to remove the plastic speed nuts which hold in the loom in place, as well as an 8mm bolt down by the washer fluid tank in order to be able to pull it away from the body and access the sensor.





Rear Sensor location; Rear of the right rear wheel arch.





Now the important part: The BMW documentation indicates you can either use INPA to check the function of the sensors, or you can bench test them. I did have INPA available, but I wanted to bench test them instead to ensure I could see how each was performing. I found the system description that BMW provides (and posted by QSilver7 here) useful, but it's definitely inaccurate. The car does NOT provide 12V to each sensor. It provides 5V.

Front



Rear



If you attempt to bench test the sensors with a 12V supply, you'll get incorrect results and it will appear as if the accelerometer is broken when it is not. (It will read 5.8V at all times....) Apply 5V to the sensor at the appropriate pin, orient the sensor the way its oriented in the car (flat mounting surface pointed UP), and you should see 2.5 V on the sensor output when you hold it steady. This is its 'resting' measurement. You'll see the voltage move up or down relative to that 2.5V as you move the sensor around. Its quite sensitive!

I checked both sensors in my car, and they both appear to be working normally, IE - the voltage responds to moving the sensor around, and the voltage returns to ~2.5V when stationary. If you reference the wiring diagram, the sensors are wired directly back to the EDC computer in the glovebox (no connectors between the sensor and the control module). This plus other posts on S-EDC trouble shooting has led me to buy a replacement EDC module off eBay. I'll report back when it arrives. I'm hopeful the 'new' unit fixes it.


Here is the pin out for each sensor for reference:

Pin 1 - Sensor Output
Pin 2 - Sensor Ground
Pin 3- Sensor Power

Both the sensor and the connector have very clear Pin numbers on them, which makes trouble shooting easy. The pins themselves are the 'micro' type, and I was able to use some breadboard breakout pins, which made it WAY easier to hook my power supply up to the sensor.

[psjr]

Thanks for the info!