Cruise control only works with headlights off
 

92 400e - 66K

Recently my cruise control stopped working when I'd have my headlights on. I did a search and found that I might check my tail light bulbs. Sure enough my right rear brake, running lights and turn signals were out. But the bulbs were still good but tried new bulbs anyhow. They still did not work. Any thoughts on where to go from here?

Check the wiring, you have a ground problem at the right rear taillight, and are getting 12V on the stop lamp wire when the headlights are on (this is called feedback, by the way, something automotive systems are VERY prone to!).

I'd bet the ground connection on that side is a bit corroded or the wire is broken off.

Peter

Thanks for the input. I checked the ground and it seems to be firmly in place on both ends. Upon my further inspection done at night, my rear running light is on but was quite dim (when I originally inspected it was daylight, thus bulb did not appear to be on) and my turn signal does flash, but again, a bit on the dim side.

I did however notice an audible buzz from the rear passenger quarter panel (right around the rear tire). Sounds like a faint version of my interior lights on buzzer. Could this be related to my lighting/cruise problem?

Hi tdk,

I don't know your car, but I assume you had the engine running when checking for the lights. On my 87 420, the fuel pump is under the car near the passenger rear tire and if yours is there too, you might have heard the fuel pump.

I still would scratch the grounds and use some dielectric grease. My motorcycle trailer had very dim lights and it was a bad ground on the receptacle into which the plug went on the motorhome.

Now whether/how it affects the cruise control, I have no idea. I would look at the fuse box and see if they are on the same fuse.

Regards

Reinhard Kreutzer

If the lights are dim, the ground is bad. Either the wire or the connection (don't know where it is on that car) have high resistance.

The cruise control amplifier grounds through the brake light filaments, so that when you step on the brake and the brake lights come on, you apply 12V to the ground and the amplifer switches off. If you have a bad ground on the tailamp assembly, you get significant voltage on the brake light filament from the marker lamp being on, with a bad ground, so the CC amp is switched off.

Check the ground with an ohmeter, I'll bet you find significant resistance on the ground side of the bulbs.

Peter