Here's are the steps to how to remove and inspect the ABS pulse sensors on your 7-series E23.
I did this during a brake job, so the rotor and caliper have already been removed, but you don't need to do that. The black, rectangular object with a beveled edge and a wire on the opposite end is the ABS pulse sensor.
Once the 5mm hex bolt is removed, all that holds the sensor in place is friction. You could grab the plastic head with channel locks, but be careful not to break the sensor at it's an expensive part.
If your sensor refuses to budge, it might be that the hole has rusted to the sides of the sensor's metal canister. In that case, you might have to douse it thoroughly with PB Blaster or WD40. With a little patience, you will be able to remove it as shown above. The end of the sensor is a strong magnet. I sprayed the sensor and the hole thoroughly with brake cleaner.