I don't think ALL the lamps can be out, the cruse unit checks for ground on the brake light line, when that ground goes away it assumes the brake pedal is pressed and the lights are on. Having all the lamps burnt out causes the same thing.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

According to the schematic, pin 8 of N4, the "Cruise Control Amplifier" is connected to S9, the "Stop Lamp Switch," through connector X20. When the brake pedal is depressed, pin 8 goes high and that shuts off the cruise control.

Additionally, the "clutch release solenoid" in M16, the "Cruise Control Actuator" (the thing on the engine) appears to be grounded through the brake lights. If that is so, then at least one brake lamp must be good or the cruise control will refuse to engage. Note the slight semantic difference between "turn off" and "refuse to turn on." But at least one brake light bulb must still be good for the cruise to work.

So you are right and I sit corrected.

Incidentally, some cars have a little green "CRUISE" light in the dash to tell you that the system is working. I wonder why Mercedes didn't do that? Looks like you could wire an LED to pin 5, the "Clutch Control Output," of N4, the "Cruise Control Amplifier." I'll have to try that.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970