The problem:
The problem is well known within the e24 community, and probably other models but I don’t keep up with everything. Above a certain speed the orange ABS warning light comes on and although the brakes work OK the ABS assistance is turned off. The simple (and I mean simple as in not-so-intelligent) solution that I have seen on occasions suggested in fora is to remove the warning lamp. I had the problem for a number of years but only by fairly high speed levels, but the last couple of years it had started to happen under 100 kph so something had to be done.
A little theory:
An ABS system includes a central electronic control unit (ECU), four wheel speed sensors, and valves within the brake hydraulics. The ECU constantly monitors the rotational speed of each wheel; if it detects a wheel rotating significantly slower than the others, a condition indicative of impending wheel lock, it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel; the wheel then turns faster. Conversely, if the ECU detects a wheel turning significantly faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied, slowing down the wheel. This process is repeated continuously and can be detected by the driver via brake pedal pulsation. The ECU is programmed to disregard differences in wheel rotative speed below a critical threshold, because when the car is turning, the two wheels towards the centre of the curve turn slower than the outer two. If a fault develops in any part of the ABS, a warning light will usually be illuminated on the vehicle instrument panel, and the ABS will be disabled until the fault is rectified.
The most common ABS problems are related to the sensors and the tone wheels in the hubs which are used to measure the rotational speed of the wheel. The sensors on most vehicles are magnetic and generate an alternating current (AC) signal that increases in frequency and amplitude with wheel speed. These are sometimes called “variable reluctance” (VR) or “passive” WSS sensors because they generate their own voltage signal when the vehicle is in motion. They have two wires: signal and ground.
Inside a passive WSS sensor is a permanent magnet core surrounded by copper wire windings. When the teeth on the tone ring rotate past the sensor tip, it changes the magnetic field and induces a current in the sensor windings. The result is a classic sine wave current pattern that changes with wheel speed. Since the voltage induced in the sensor is a result of the rotating wheel, this sensor can become inaccurate at slow speeds. The slower rotation of the wheel can cause inaccurate fluctuations in the magnetic field and thus cause inaccurate readings to the controller.
Traceable faults:
The sensors can malfunction but the basic functionality can be tested by connecting an ohmmeter across its terminals. The resistance specs will vary depending on the application, but most sensors should read between 450 and 2200 ohms (always look up the exact specifications because they can vary a great deal from one vehicle application to another). If a sensor reads open, shorted or is out of spec, it can’t generate an accurate signal and must be replaced. You can also test the sensor’s output by spinning the tyre by hand at a rate of about one revolution per second. With a voltmeter attached to the sensor’s terminals, a good sensor should generate between 50 and 700 millivolts AC. Other situations can occur due to metallic debris and dirt sticking to the magnetic tip of the sensor or degradation of the tone wheel affecting the air gap or the quality of the generated magnetic field.
My solution:
Although never having had an electrical problem with the sensors I have had to replace them as they had become rusted solid in the hub carrier when I refurbished the rear axle and front suspension assemblies. The tone wheels are another problem area and this where I used a little ingenuity and my workshop facility to solve the problem. The front tone wheels are easily accessible as they are relatively exposed on the hub and are as such easy to keep clean. The rears are very difficult to access as they sit deep inside the trailing arm mounted on the hub installed in the wheel bearings. As mine where untouched for 25 years they had deteriorated to the extent that there was very little left of the teeth. You can see this in the slide show at the bottom. To actually get at them means removing the drive (half) shafts so you can get at the drive flange which is fastened to the stub axle with a large nut. To remove this (and to replace it) you need a very large (torque) wrench. Once removed the state of the teeth was obviously the cause of the problem so I made enquiries at the dealer for replacements. The drive flanges are only sold complete, it is not possible to just buy the toothed tone wheel and so it starts to get expensive (€175,- each side).
In the slide show you can see that I turned the tone wheels off the flange and so found that they are separate parts. I made my own replacements by buying a short length of 70mm dia round steel bar for €20 and hollowing it out on the lathe deep enough to give me the two rings I needed. Next stage was to transfer it to the milling machine to cut the teeth. This meant setting it up between a rotary table and mini tailstock so that I could cut the teeth at the correct places on the circumference of the steel bar. I had counted the old teeth and needed 95, resulting in a spacing of 3.789 degrees between each tooth, so I created a spreadsheet to work out the degrees for each increment between 0 and 360. I have to confess that I needed 2 attempts to get it right as the first attempt was based on the assumption that the teeth were pointed like a normal gear wheel. Looking at the old ones it did seem like that as the teeth were so badly eaten away. So anyway I cut them with an angled cutter as you can see from the photo and re-assembled everything to find that it didn’t work. For the second attempt the teeth were cut with a 1mm slitting saw and this gave the correct profile for the pulse to be generated by the sensors. After re-assembling again I tested it and everything was OK, that was even the case when driving on the autobahn in Germany in 2013 at 200+ kph.
Now although this seems like a significant cost saving it has to be remembered that a) you need the workshop facilities and b) you don’t need to cost the time spent doing it. It gives you a tremendous amount of satisfaction to create something like this but it isn’t something you would do as a commercial undertaking, unless BMW stop producing the spares of course.
