I just replaced the rearrotors on my 1990 240 DL, and my Brake Failure lamp is constantly illuminated.The lighthas never come on before.I don't understand what happened, because my brakes work great. Does anyone have any suggestions?

 

Did you open the hydraulic system by disconnecting a brake line from a caliper? Did you replace the pads?

 

no i didnt open the lines or the caliper, and i didnt replace the pads either.

 

Obviously, the brake lights are working ok, or else you wouldn't be posting here. Are you sure all the brake lights are working ok? I would try removing all the brake light bulbs, cleaning the contacts, and seeing if that helps.
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There is a junction block mounted to the bottom front side of the car. Look for all the lines going under in the front. In the middle there is an electrical connection. Sometimes it gets dirty. You should be able to work the sensor out and clean it then reinstall it.

 

Brake lights are fine. I'm more concerned about the "Brake Failure"

 

Problem solved. The light went away. Ok, so that might not mean it's solved, but my guess is since the rotors are slightly thicker (less worn) than the old ones, the caliper's pistons were forced to adjust... which triggered the "Brake Failure" light. Over three days of use, the calipers re-aligned the pistons to a point where it no longer triggered the "Brake Failure" sensor. Basically, time to adjust to the new changes is all it needed.

 

That brake failure light comes on when the sensor gets dirty or if is senses a pressure drop in the system.