85 300D (W123)...cruise control works at times. when it does work, it doesn't seem to work correctly. once i set the cruising speed, instead of holding steady, it starts to slow down. when it goes down about 1 or 2 mph, it speeds back up to the set speed, and repeats this process until i turn it off. anyone else had this problem, and if so, what did you do to fix it?

Mine is way more wild than that When it does decide to turn on, which is usually now since I cleaned the contacts for the acutator under the hood, it sometimes will hold the speed, but most of the time it drops about 3-5mph then it just takes off, literally, the pedal goes all the way to the floor and it doesn't let off unless I tap the brake or click it off with the stalk.....eventually once I figure out how to get my amplifier out easily, I will resolder all the connections, this seems to often solve the problems like you (and I) are experiencing. The cruise is just a tempermental electronic system, if all parts of it aren't perfect it won't work properly.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Numerous Posts are available - via Search......

Here's a couple:

DIY page. Introduction to Resoldering Climate Control Unit

http://www.mbca.org/pages/tech/MBCA_Troubleshooting_cruise_cont.htm

I fixed mine by re-soldering the Circuit Board. Now all works fine. Somewhere I created a thread - Oh, here it is Fixed Cruise Control

If your Cruise Control needs to be re-soldered - then also plan on pulling your Climate Control. Same Fix for both.

Best of luck.........

Stephen

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98 K2500 Suburban 6.5 Luxo Barge

"We cannot change the wind - But we can adjust our sails"

I've resoldered three cc amps and none ever came back to life - at least not for long. You may get lucky. If not, I've bought two amps from this guy:

http://www.benz-store.com/cruise_control_amplifiers.html

They're $125-155. So far, he's taken my resoldered ones in for core credit, though some places won't take them if you've screwed around with them.

In one case, the new amp didn't work, and I had to replace the actuator, which he also sells (used) and which I have a spare I'd be willing to sell.

I use my cc during about two thirds of my 130 mile commute and it's worked perfectly since I replaced the amp (actually replaced twice since the first rebuilt one was defective). When they work, they're awesome.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer