[This mod applies to all diesel and gasoline 124 models that DO NOT have an electronic accelerator system (400E/500E) and may also apply to models 123, 126, 201, etc. but the wiring may not be the same. Caveat emptor.]

I had always wondered why Mercedes didn't have any indicator to show that the Cruise Control was active. Maybe they figured it wasn't necessary but I always thought it was a nice touch and many other manufacturers have put such things in their cars. (I have since learned that the current Mercedes vehicles do have something to indicate speed control in the speedometer.)

Looking at my collection of diagrams, I realized that the wiring from the cruise control "amplifier" (under the dash, driver's side) to the "actuator" (somewhere under the hood) could also be used to turn on a light. Pins 5 and 8 of the amplifier control an electric clutch in the actuator; when the clutch is engaged the cruise control maintains a steady speed. If a lamp is connected between these pins, it will illuminate whenever the cruise control is active.



You can see in the picture that I used an LED as the indicator lamp. This LED is not designed to run directly from 12 Volts so I used a resistor in series, 240 Ohms, but anything close ("a few hundred") will do. The LED and the resistor are both common Radio Shack parts. Try #276-022 for a green LED and #271-011 for ta 220 Ohm resistor. [My picture shows a 240 Ohm resistor because that's what I had in my junk-box. The exact value doesn't matter -- a few hundred Ohms is fine.]

If you use an LED, remember that the positive (longer) lead must connect to pin 5 of the cruise control amplifier. The resistor goes in series with either lead of the LED. If you use an incandescent lamp, it doesn't matter, of course. Run the leads and put the lamp or LED somewhere out of the way but where you can see it while driving.

Rather that open up the amplifier's 14-pin connector plug (and risk an explosion of pins) I simply wrapped the wires around the pins of the amplifier. When the plug is put back on, the wires are trapped and can't move. This is simple, crude, unprofessional -- but it works! Use fine-gauge stranded wire, 24 - 28 gauge is plenty big enough. Make sure the wires don't short on the metal body of the amplifier. Secure with duck tape.

I went on to do something fancier as the following picture shows. It automatically dims at night and looks "factory" (actually it IS factory). This took a lot more work for the same result. If I get some positive responses I'll post the details. For most folks the KISS method is recommended.

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