W123 Cruise Control Problems

I installed cruise control on my 230CE. It has the wiring harness and amplifier from a 1982 300D and the actuator on the engine is from a 84 190E. The engine is from the 190E.
It doesn't work as well as I expected. First, it will not engage until about 45 MPH. The manual says it should be about 25 MPH.
Secondly, it over shoots the set point. If I use the accel mode it will continue to accelerate for 3-4 MPH after I release the lever. Same thing when I use decel, it will slow some more after I release the lever.
At about 60 MPH it uses wide throttle positions to maintain speed. In other words it gives hard throttle and gets +1 to 2 mph then no throttle to give -1 to 2 MPH. It is so bad that I have to turn it off. This is on level road. At higher and lower speeds, it holds speed smoothly.
I tried an amplifier from a 190E and the accel works but it will not hold set point.
So, I guess my question is, is this the way cruise control works on these cars, or is there something wrong?
Thanks, Scott
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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.
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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.

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speedometer cable and squirted teflon lube into the cable from the end. It worked. I had a quiet sort of intermittent squeak at very low speeds with corresponding jerks in the speedo needle. After I gave the whole deal a good soak down with the teflon lube the squeak was gone and the speedo got smooth at about 25 mph and holds nice and steady at any speed where the cruise will hold, which is above 25 mph.
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Contact this guy:
Automotive Technical Services (ATS) - MB electronic units remanufacturer
Englewood, Colorado 8010
(800) 865-6211
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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
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wrote:

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????
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This doesn't use a cable. Just a link. I am pretty sure that I got it adjusted right. Thanks.

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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.
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