BMW DSC failure points revealed, Speed sensor test included

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IF you try this, it's completely at your own risk. Not for the faint hearted!


Here are some of the (most common) failure points in the BMW DSC module.
The wires pointed out, are aluminum "lead in" wires directly from the pins in the connector. These wires seperate at the "kink" where they attach to the DSC block, and substrate. From what I gather, these wires are in a gel material which is causing the seperation problem, with temperature variation. There is another row of these aluminum bonding wires at the top of the box (not shown) which seem to be fine, which are not covered in this gel.



In order to fix these bonding wires, extreme care must be taken when removing the gel that surrounds them. The gel is very soft (kinda like Jello that hasn't fully set.) Right next to these aluminum bonding wires are little gold bonding wires about the size of (or smaller) than a human hair. Pulling on the gel will move these wires, and possibly seperate them. Mind you, the smaller wires seem more robust than the larger aluminum ones.





These bonding wires shown above (Viewed through my stereo microscope) all need to be replaced. You can see that some already have been replaced with red "Kynar" wire. These wires will need direct attachment to the pins in the housing, as the pads on the box side will not solder. The box side pads seem to be anodized aluminum. (aluminum won't solder)



Here you can see all the wheel speed sensor wires have been replaced with "Kynar" wire. All these wires are soldered directly from the board to the base of the pins in the socket. You will need to remove the excess rosin or flux from the pins (only in the socket) after soldering, with acetone or alcohol.



Here you see the "Kynar" wire soldered to the pins directly. The flux has not been cleaned off yet. The case needs to be notched and a small hole made at the base of the socket for wire entry. Direct soldering is the only way to get reliability out of this poorly designed piece of garbage. Conductive epoxy may seem easier at the time, but will eventually seperate again from temperature variation.

When soldering to pads on the substrate, make sure you use solder with "silver" in it, or you will have "pad migration" problems. Ordinary solder will cause the pad on the substrate to "disappear" onto your soldering iron tip (migrate) This is caused by heat and dissimilar metals. The pads (traces too) are silver in make up.

TESTING YOUR WHEEL SPEED SENSORS

Most people go into this blindly, replace sensors, and hope for the best. This will inevitably (almost always) waste your money.

The easiest (non invasive way) to do this is with an milliamp meter and a small Amp clamp. Clip the Amp clamp on either wire going to the sensor (not both at the same time.) This is done (as well as the second and third way) with the vehicle raised and secured properly, and the tires removed. Rotate the rotor by hand, as the magnets pass the hall effect sensor, you will see a rise from 4mA to about 12mA (give or take a small amount). If this is done with an analog meter and Amp clamp, the movement is much easier to see than using a digital meter with numeric read out. (this test is done with the ignition on, but engine not running)

The second way is too directly install a milliamp meter in line (series) with one of the leads going to the sensor. Now heres the catch! If the DSC module is faulty, the sensors will get no power, tricking you into believing the sensor is at fault. This is where sensor swaping may come in to reveal the truth. (this test is also done with the ignition on, but engine not running)


The third way of testing the sensor is with an oscilloscope. I wont elaborate too much on this, cause most people don't own one.

Make sure the scope is isolated from the vehicles electrical system (ground floating) attach the scope across the sensor wires (careful not to short them) and look for a square wave signal (fast rise) when the rotor is rotated by hand. Remember, IF your DSC box has completely failed, one or more sensors will not show action, under any circumstance. (this test is done with the ignition on, but engine not running)


Good luck.