BMW ABS Warning Light – E23, E24, E28, E30
I have a 1989 325i convertible with 95,000 miles. The ABS light stays on. I had it checked by a local German auto repair center. They advised the ABS control unit module needs to be replaced. Is there a separate module, or is it just the rear, front right & left abs sensors that need to be replaced.
On the older BMW models with ABS (E30 3-series, E28 5-series, E24 6-series and E23 7-series), the ABS systems are comparatively simple. There are the four wheel speed sensors, the sensor rings on the wheel bearing hubs or axle ends, the ABS pump, the control module and two power relays (mounted under the plastic cover on the pump). On these models, the most typical problem is either faulty wheel speed sensors or corroded sensor rings.
You can certainly inspect each of the sensors. Check the wiring harness for any visible issues with the installation or the connections to the plug or the sensor body. Remove the sensor and look for damage to the sensor’s head. Check the resistance of the sensors (they should all be within 10% of each other). If there are any suspect inspection points, consider replacing any that are in question. If all looks fine, you may have corroded teeth on one or more of the toothed sensor rings (the wheel speed sensors read/count the teeth on the rings). The rings are difficult to inspect. Typically, the only way you can see them is through the speed sensor mounting holes (when the sensors are removed).
Of course, there is the possibility that the control module or one of the power relays is faulty, or even the pump. However, we would certainly start with the speed sensors and the toothed rings. Most front toothed rings are a part of the wheel bearing hub assembly. The rear toothed rings will either be on the outer axle C/V joint housings (3 series) or will be on the inner wheel drive flanges (5, 6 & 7 series). If the toothed rings are corroded and need to be replaced, the part that the ring is on must be replaced.