Can an occasionally bouncy speedo needle cause the infamous surging cruise control in a W123?

I have checked, lubed, and tightened up the throttle linkage on the car, so I'm pretty confident that is not the problem.

The actuator has been tested and is good.

I installed a used cruise amp - thought I'd gamble and see if it was any good. With the original cruise amp, the cruise did not work at all. With this amp, the cruise will set but it will give you whiplash with all the surging.

Before I buy another cruise amp, wanted to see if this condition can occur due to the sometimes-wobbly speedo needle.

It's the bouncy speedo needle. The amp gets it speed info from the speedometer head. A worn or binding speedo cable will cause the speedo to oscillate up and down 2-3 MPH and that will make the cruise surge as well.

Apparently, the cruise amp is a bit Teutonic about measuring speed...inside the amp can, I'll bet there is some sort of adjustment to damp that input signal.

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Never a dull moment at Berry Hill Farm.

The speedo needle is probably jumping because the cluster was taken out and the cable now has more of a bend than it should. The wire inside can hang up on the casing and make it jump at low speeds.

But, I would think that it should smooth out at about 20-30MPH.

Are you sure your last linkage to the actuator is proper length?

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1982 300CD Petrol/Black Leather

The speedo needle bounces wildly at <20 MPH. Sometimes it is steady at <60 MPH, sometimes is oscillates a bit. Regardless, I'm going to lube the cable and check for excessive bends first. If that doesn't solve the surging cruise, then I may need another amp.

What is the "proper length" for the last linkage to the actuator??

I've been down this road, believe me...you can put ten different amplifiers in there and in if the needle isn't steady, the thing will still surge. Replace the cable if lubing it doesn't help..

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Never a dull moment at Berry Hill Farm.

If the cluster was pulled out and then the cable removed, it now has too sharp a bend in the cable housing and it will cause the needle to bounce. I've had good luck straightening the housing just as I reinsert the speedo cluster. To verify the cruise cable is the proper length to the pivot linkage, you are to adjust the slack by depressing the manual shut off red lever and hold it forcefully depressed and with the other hand adjust the nylon nut in or out to have the ball stud on the cruise cable just a couple of mm. away from the grommet it passes thru. Do this with engine off. Mine would loose approx 2 mph once set then regain that 2 mph within a half mile and remain steady. For the system to work correctly the speedo needle must not vibrate or bounce and the cable length has to have the proper slack.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

Thanks all. I'm 99% sure that it's the damn speedo needle causing the surge, not the amp or the linkage.

Like the other 99% of Atlanta drivers I don't pay much attention to the speedo anyhow. ;-)

Will look for kinks or bends and lube cable. If that doesn't work, I'll replace the cable and report back. It will be NICE to have working cruise.

I have an 81 300sd and the cruise works off and on. And when it works, it surges up and down. Occasionally it will just take off and speed up continually and sometimes it works perfectly. Tach is bouncy but not speedo.

I have a similar problem to jap. When I got the car the cruise would work, but would oscillate 5 or so mph. The needle is steady. I was told the actuator can ware at a point (say where the throttle is at 60 mph) and that can cause the problem. When I went to test this theory and set the cruise at 40 MPH it would not turn on and I have not seen any signs of life from it since. Is my amp bad?

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

The SD is a different beast because is has an electric speedo that uses a sensor on the tranny tailpiece....no mechanical speedo cable. Your problem is a hosed amp.

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Never a dull moment at Berry Hill Farm.

Seems to be a simple task, but here we go...

How do I get the blasted cable out?

I pulled the cluster (easy) and disconnected the cable from the speedo. Every speedo cable (on other cars) would then come out with a good tug on the core with a pair of pliers or Chan-el-locks. Ideally I want to pull out the core, clean and lube it, then slide it back down through the sleeve.

Is it attached to the trans or what? I pulled on it pretty good but did not yank with all of my God-given strength. I've learned the hard way over time that these cars do not respond well to the "brute force and ignorance" method.

I'm pretty sure there's a flange on the drive end of the inner cable that will prevent it from doing what you want to do.

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Never a dull moment at Berry Hill Farm.

Bouncy tach is probably an amp issue. Take apart, wedge something in, worked for a while in my '82 D, same set up.

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83 SD

84 CD