I know this is a very old thread, but I have the identical symptoms on a 1992 400E. I pulled the relay in the ABS unit and it looks like the diode is OK - measured .554 on the fluke diode test. What led you to that relay in the diagnosis? I have not been able to do a real diagnosis at this point. Just things it "could" be, like this relay (may get one as a precaution), ignition switch, alternator....

paul