1986 BMW E30 325e Locating ABS Control Unit & ABS Pulsing Pedal(E23, E24, E28, E30)
This answer is applicable for many BMWs in addition to the one listed below.
I am having trouble locating the abs control unit. where is it located and what do I have to remove to get at it? Note: I have abs trouble and replacing the abs relay does not help. I have considered checking the wheel sensors but it seems that the abs comes on the minute I use my brakes then shuts off and the abs light comes on.
The ABS controller is mounted under the dash, to the left of the steering column. It is a silver box that is about 3″ x 6″. I doubt that your issues are due to a faulty ABS controller. The symptom that you describe are not uncommon and would typically be due to an issue at one of the wheels (this would apply to all 3, 5, 6 & 7 series models, through the ’80s, that have ABS). I would suspect that you may have a faulty wheel speed sensor or a corroded/dirty toothed hub ring (that the sensor reads), at one (or more) of the wheels. You can start by unplugging the sensors and measuring the resistance through each sensor. If one is out of line with the other ones, you may have found the issue. As for inspecting the toothed rings, this is not so easy since you can only see the edges of the rings through the sensor mounting holes.
The front ABS rings are typically integral with the front wheel bearing assemblies. The rear rings are typically integral with the outer CV joints on the rear axles, on later models, earlier models may have the ABS ring mounted to the drive flange that the outer CV joint bolts to.
See the toothed ring on the inboard end of the wheel bearing hub, below: