Seems nobody responded in this post to "J.HIDALGO's" mention about wrong bulbs causing the cruise control to malfunction!

I have also read where this situation may effect the cruise.

Before ever driving my 87 300DT, I installed red led stops in the vacant spot in the taillight assembly. Since I put it on the road, my cruise has never worked! I read a thread about soldering corroded contacts in the cruise amp and thought I'd pull it out and have a look. In my attempt to pull it out from under the dash, I snapped the circuit board in half! There appeared to be no corrosion and nothing wrong with the amp before I broke it! then I read about having wrong bulb, non MB specified bulbs that is, and is to late for me to find out unless I want to spend $300 for a new amp, only to find out that it was the red led's that may have caused the cruise to not work!

Any input on this possible subject?

The cruise control amplifier uses the brake light "hot lead" as a reference ground. When your foot is off the brake pedal, that connection is seen as ground even though it goes through the brake lamp filament to the actual ground connection. When that circuit is energised by the driver stepping on the brake pedal, the ground goes away (becomes +12v) and the cruise control is disengaged.

The wattage of the bulb is not a factor, internet rumors aside. Putting an LED into the circuit may have changed things just enough for the CC amp to not "see" a ground and therefore not turn on.

Good catch,

__________________
Norm in NJ
Next oil change at 230,000miles