I decided to try lubricating the speedometer cable to see if the continued small surginess of the cruise control is due to that. The needle _is_ still a bit jumpy, especially at low speeds.
I used M1 5W-20, since my tube of speedometer cable lube was empty. As a light-weight synthetic, it should be pretty good in the cold. I jacked up the rear of the car so I could put it in gear, and dripped it into the open end of the cable as it ran. Then I took it for a test drive. The speedometer jumpiness is much reduced, as is the surging in the cruise control. I will need to do this in the 450 SL too, my only other example of a vacuum cruise control, as it also exhibits both symptoms. I had theorized that since there was a large capacitor smoothing the output of the road-speed signal that a bit of jumpiness in the sensor would be smoothed out, but I now believe that to be wrong. (You also have to account for the gain of the error amplifier, versus the output impedance of the circuit driving that capacitor.) I then decided, since I had the instrument cluster out, to see why the lamp-test feature didn't work on the low-fuel light. After much fooling around, I figured out that in 1979 there _was_ no lamp-test feature on that light! (The only one missing that feature.) But since I was there I added it. Just takes a diode and a resistor soldered to the back side of the big connector inside the cluster. The car's still for sale! http://cathey.dogear.com/mb240dsale.html, as is the 450 SL: http://cathey.dogear.com/JSLsale.html -- Jim
