W140 BAS/ASR light, not switch or bulb
 

Okay, I have searched my face off, here and elsewhere, but I'm not sure what is the next step in diagnosing this issue.

1998 S320 - When the car is started, all trouble lights are off. As soon as the brake pedal is depressed, the BAS/ASR light comes on. ABS light does NOT come on. I confirmed all brake light bulbs are good. Replaced the brake light switch today.

After you shut the car off and restart, the light is off again until you press the brake pedal. No OBDII trouble codes. I have not yet attempted to pull codes using the 38 pin connector.

Cruise control and ABS work even when the light is illuminated. The only thing worth noting is that brake pedal travel is long most of the time. The brakes work fine, but they don't take up until the pedal is pretty close to the floor. If it's like most other makes, that indicates a bad master cylinder. I don't know what trips the BAS light, so I don't know if the two issues are related. The light comes on even if the brakes are "pumped up" and pedal travel is in what I consider a "normal" range.

Any diagnostic tips or other input are welcome.

I'd check the wheel sensors first.I hope you bought a German brake switch,the cheapo's won't last a year.

I believe any issue with the brakes trips the BAS. You REALLY need to pull the codes with a STAR system.

Sounds to me like my low pedal/master cylinder issue may be the root of this. I'm planning to order the interface so I can pull all the codes before I start throwing money at it. I doubt the wheel sensors are involved, since the light comes on as soon as you touch the pedal after starting the car (without moving or even shifting out of park.) But I suppose anything is possible.