Cruise Control Switch

[jhtanglewood]

Has anyone ever actually had the cruise control switch on the steering column go bad?

I've replaced the actuator and control unit with no luck, so that was probably money wasted. I've also read about brake lights causing issues and I've recently replaced all of my lights with LEDs - but the crusie control wasn't working before that anyway.

[Alex E]

There is a Mercedes cruise control repair site on the web that has diagnostic tests you can run on the stalk. I don't know if I'm allowed to publish it here so I won't, but you should have no trouble finding it.

[jhtanglewood]

Thanks, I just pulled it out and it was covered in oil - haha I'm sure that's not good for it.

[jhtanglewood]

Well this just doesn't make any sense at all.

Signal from the switch is good in all positions.
Tested the the actuator even though it's new and tested fine.
Amplifier is brand new too.

WTF

[Sugar Bear]

Try replacing the brake light bulbs with the correct wattage bulbs. The problem may have been in the parts you replaced but the cruise still won't work with the incorrect bulbs.

Good luck!!!

[BWhitmore]

James Dean, on this site, repairs cruise control units. Mercedes automobiles are especially sensitive concerning the resistance of the tail light bulbs.

[Marshall Welch]

Originally Posted by BWhitmore

James Dean, on this site, repairs cruise control units. Mercedes automobiles are especially sensitive concerning the resistance of the tail light bulbs.

I’ll second both points. I had bought LED brake lights, which have resistors included to bring the total wattage to a level high enough to work with the cruise control. It stopped working after the LED brake lights were installed. Problem solved when I went back to the original bulbs for the brake lights. The cruise control amplifier had been rebuilt recently, and the actuator had been checked out as well.

[jhtanglewood]

I'm gonna try the original type bulbs today. Fingers crossed.

[jhtanglewood]

Changing the LED brake light bulbs back to the original type worked! - but I really liked my super bright LEDs so I guess I have to buy some with resistors built in like Marshall said.

Thanks guys!

[okyoureabeast]

Cruise Control Repair

James' site. Absolutely great guy and can repair a lot more than just the cruise amps!

[jay_bob]

The reason for this is the cruise control amplifier does the brake pedal depressed detection in kind of a backhanded way.

Instead of looking for +12 V when the brake is pressed, it looks for a low impedance path to ground (i.e. when the brake is not pressed), to give the permissive to engage.

Incandescent bulbs present a near dead short to ground when the filament is cold. As the filament heats up, the resistance increases to the value needed for operation. (Positive Temperature Coefficient). So when the brake is not pressed, the sense lead to the brake bulbs sees a ground through the brake bulbs.

The sensing circuit in the CC amp actually leaks a small voltage onto the brake bulb line. Not enough to light up the incandescent bulb, but enough to prove the brakes are not on.
When the brake is pressed, the voltage on the brake bulb sense line goes to +12 V, which causes the cruise control to inhibit.

LED bulbs do not exhibit similar behavior to incandescent bulbs. LEDs have a forward voltage drop of a few volts (varies per LED type and how many in series). This voltage drop exists whether 12 V is applied or only a few volts.

So when the sensing voltage is applied to the LED, it is insufficient to overcome the forward voltage drop on the LEDs, and thus inhibits the CC amplifier.

[jhtanglewood]

That's one hell of an explanation Jay, now my head hurts.

Hopefully the LEDs w/ load resistors work with the cruise control because I really like the having the super bright lights, the red is deeper too.

[Dubyagee]

Im converting mine to LED. Im going to use the resistors from autozone. Used them on other bulb dependent cruise controlled cars with no issue.

[jay_bob]

The bulbs with bypass resistors allow the CC sensing voltage to pass to ground.

Sorry if the explanation was complex earlier. Simply put, when off, incandescent bulbs act like a low value resistor, while LEDs act like an open circuit. When a small voltage is applied to an LED, and the voltage is less than the forward voltage, the LED behaves as if it is open circuit.

W164 MLs with a trailer hitch connector have a similar problem. The trailer connector sees when you have attached a trailer, when it detects you have connected some incandescent bulbs to it. If you try to connect a trailer with LED bulbs, you get all kinds of errors, as it drives the sensing logic crazy in a similar manner to the cruise amplifier in the 123s. There are aftermarket adapters, that basically have resistors from each signal line to ground, that fool the vehicle trailer control box with the correct readings, as if it were incandescent bulbs connected.

[Marshall Welch]

By the way, I did end up installing the LED bulbs in the tail light running lights and side marker lights, as well as the license plate lights. They are good bright lights, and reduce power consumption.