Friend drives a 1990 300SL-24. The ABS light comes on with the ignition, but doesn't go out - it stays on all the time, and we're not able to find out why.. ABS is of course non-op.

Wheel sensor resistance OK, both front and rear. Haven't tried the signals from the sensors on a scope, but a bad or weak signal anyway shouldn't trigger the light directly after startup before the car has even moved - then it would go out and come back on when getting faulty readings.

Checked out the troubleshooting procedures in the factory shop manual, but it refers to some weird, old ABS tester that we of course don't have.

Any ideas on where to look next? Need to find out what's causing this, as the car won't pass inspection with a lit ABS light and it's overdue.. (and no, pulling the bulb temporarily won't make it pass - they check that it comes on and goes out)

__________________
'95 E36T AMG (124.092)
'98 E200T (210.235)
'00 E430 4Matic (210.283)