ABS Lamp Symptom
 

1991 300D. ABS lamp illuminates when I start the car and then goes off. After 5-10 minutes the lamp comes on and stays on. If I turn the car off and restart, the same thing happens: off for a few minutes then on.

To the best of my knowledge, ABS sensors have never been cleaned and ABS relays are original.

Does this sound like dirty sensors, a bad relay, or something else?

Going on old memory, I would suggest finding the plans for the simple code reader, build one and read the codes on the brake system. It is a blinking light type. Then go ahead and replace the bad sensor. You don't need ABS till you do.

Mismatched tyres ?

Dirty ABS sensors can lead to intermittent ABS light issues, as the system may not be able to read the wheel bearings speed accurately. Over time, dirt, rust, or debris can accumulate on the sensors or their reluctors. Try to clean it.

I think I'm going to eliminate control module and relays as variables. Then dive into cleaning sensors.

If none of that works, I will change to antique registration so I won't have to worry about inspection.

Replacing the control module appears to have solved the problem.