Obviously a bad amplifier.

No, it's not how they should work.   Not sure of the
interchangeability of an 82 diesel and 84 gas engine cruise amp and
servo, but presumably you checked that out already and made sure they
are the same ones that are supposed to be in your car.   Next question
is where did you get the parts?   The MB amps, as much of their
electronics of that era, is very failure prone.   I would not buy an
amp from some random source.  I see them listed on Ebay all the time,
stating something like "Pulled from a running car"   Which could mean
it was pulled from a wreck hauled to the junk yard and the engine
still would run.  But the cruise control?  Who knows.   I'd only buy
one that was new or rebuilt and guaranteed.

Thanks for the responses.

The needle for the speedometer needle is very steady. By the way, how should
the cable be lubricated?

The amplifier was with the parts car that I pulled the wiring harness, an 82
300D. I don't know how I'd find what one Mercedes specified for the euro
230CE. I tried different amplifiers that I pulled from wrecking yards
including 190E's and they had different problems. One would accelerate but
not hold. The other used way too much throttle to try to hold speed. None of
them would engage below 40 MPH or so. But the one that I have in it now will
decelerate to about 25 MPH if I step down the speed until it will not hold.

I hope that the actuator is ok. I think that I'd have to remove the intake
manifold to get to it.

Contact this guy:

Automotive Technical Services (ATS) - MB electronic units remanufacturer

Englewood, Colorado  8010

(800) 865-6211

I would contact Peter Sanford at General Development Laboratories.  I
have purchased actuators and a rebuilt amplifier from Peter and he
provides excellent service after the sale and is most helpful in
figuring out what is wrong.

http://www.gdl-online.com/begin1.html

His spam filtering is pretty strong, so you probably want to call him
the first time and make sure he gets your e-mail on his accepted list.

I have a 1982 300 TDT that I have been driving for 16 years.  It has
over 400,000 miles on it and I have had a few problems with the cruise
control over that extensive period, but it usually works perfectly.
It's rock steady and smooth at any speed and can hold speed up a steep
mountain pass when it's working right.

There are a few possible causes of surging.  Your speedometer has a
sensor on it that tells the amplifier how fast you are going.  If your
speedometer is jerky, the cruise won't run smoothly.

But your issue sounds like a worn circuit board in the actuator.  Some
contacts run along an etched strip of conductor on the board.  When
the strips of conductor become worn and pitted, the cruise will get
jerky at the speed which that part of the conductor corresponds to.
This explains why an actuator would be jerky at only a given speed
range and yet be smooth at other speeds.

Here is a link to a web page Peter of GDL was working on.  I don't
think this is part of his public web site, so maybe you shouldn't
directly mention it to him because I am not sure he wants this out
generally.  He sent me the link in an e-mail when we were trying to
figure out why my cruise was surging.  In my case it turned out to be
a jerky speedometer cable this time.  I have a pretty new actuator, so
it was unlikely that it would be a worn board.

Peter makes those boards, and I think he will sell them as parts, but
I am not sure, you will have to talk to him about that.

http://www.gdl-online.com/act_boards.html

Scott - the description you gave is exactly the behaviour of my 1990
Jaguar Sovereign cruise control if I tighten the throttle cable (from
the accelerator pedal) up too tight and do not allow a LOT of play in
the linkage/cable.  

I get a bit more zoom by having greater throttle movement, but I lose
the cruise control function because of the constant over-correction.
If I put lots of play back into the cable, and the cruise control is
fine again.

Maybe yours works the same way????

Post the VIN numbers for your 230ce with the numbers from all donor
vehicles and I'll run them through the MB EPC and tell you why these
mixNmatch components are incompatible.

Mercedes is constantly improving and enhansing their technologies. The
part numbers for any given component changes with almost every new
model year and from model to model their
is little interchangeability. The idea that one can scavinge parts
from several different models and bring them together into a properly
functioning group is unlikely at best.