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1982 300D Turbo
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67 Posts
Discussion Starter #1
just replaced the cruise control amplifier on my 84 300 D turbo. now when the cruise is engaged and I drive down hill, the car seems to surge. it's as though it can't decide whether it's going downhill or needs to continue to accelerate. when it's going up hill it does fine, and pretty good on flat land (which we don't have much of here in East Texas. Is there some mechanism in the cruise control system that can tell if the car is going downhill and makes the cruise react accordingly? or is the amplifier bad?
 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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1,418 Posts
What is the story on the replacement?

Is the speedo needle stable?
 

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1977 300D 4 speed
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6 Posts
I had the same issue with my 240d, surging with cruise control on. I solved the issue by loosening the speedo cable at the speedo head by two turns reducing the tension of the cable inside the sheathing. The cable was to tight and was 'binding', the needle was steady. The cable could also just be worn out or in need of lubrication.
 

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1982 300D Turbo
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67 Posts
Discussion Starter #6
I had the same issue with my 240d, surging with cruise control on. I solved the issue by loosening the speedo cable at the speedo head by two turns reducing the tension of the cable inside the sheathing. The cable was to tight and was 'binding', the needle was steady. The cable could also just be worn out or in need of lubrication.
I"m wondering if it's the speed sensor...is that in the amplifier?
 

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1977 300D 4 speed
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6 Posts
the speed sensor is the 'little black box' located on the back-side of the speedo head, next to the speedo cable. My 240d was surging even though the needle was NOT fluttering at ANY speeds - again I just loosened the cable two turns and the issue was solved... from my experience when the amps start going bad they hold the speed set and as you drive they loose the 'set speed' slowly over distance, eventually you have to reset your desired speed. My 300d has never had surge issues
 

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1982 300D Turbo
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67 Posts
Discussion Starter #8
well, through the magic of the self healing w123, the problem wasn't as bad today driving to and from Dallas. I will loosen the speedo cable though to see if it gets rid of the problem...
 

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1982 300D Turbo
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67 Posts
Discussion Starter #9
crazy car!

had to put a new cruise control amplifier because the old one kept not working...this one seems to be doing the same thing going down hill...but now it seems to be kicking the AC off...when I was driving home from the shop the car actually DIED at a red light...first time that has ever happened...then the AC stopped working (compressor was on but blowing hot air) stopped to get diesel, aC began to work for about 10 minutes, then quit. turned aC off, 15 minutes later turned it back on and it was working again...
 

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Brown 83 300SD I have a few other projects before I get to her!
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1,845 Posts
I had the same issue with my 240d, surging with cruise control on. I solved the issue by loosening the speedo cable at the speedo head by two turns reducing the tension of the cable inside the sheathing. The cable was to tight and was 'binding', the needle was steady. The cable could also just be worn out or in need of lubrication.
What is the procedure for lubricating the speedo cable? Mine on hot days jumps from 0-10 while going slow. So I'm sure I should at least add some grease or whatever is needed.

Thanks,
Dave
 

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1977 300D 4 speed
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6 Posts
Stephen, It seems to me you have other issues with your car too. From what you said about the A/C intermittently working, your compressor may be failing. I have never had a diesel die unless there is fuel starvation or excessive loads applied. If the compressor locked up at idle it could well 'kill' the engine but the belt usually makes a horrendous noise and the smell of burning rubber (belt) is quite evident. Do you notice anything unusual under the bonnet? Look at the compressor clutch, they usually turn a yellow colour when they start going bad.
 

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1977 300D 4 speed
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6 Posts
Hi Dave, I usually replace the entire cable assy when they need replacing but you can lube them they make a special grease fitting to force grease in the cable. I have seen very few available. The easiest way is to remove the cable from inside the sheathing and lube it with either white grease or a moly based grease and then slide it back into the sheathing. Another option is to use a liquid graphite impregnated oil and put put some oil in the speedometre side of the cable (disconnected from the speedo) and let gravity work it down the cable without removing the cable from the sheathing. I hope this helps you.
 
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