is there a quick and easy mityvac-way to check this?

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Jennifer
90 350sdl

Not that I know of. But it's much more likely that your amp is the problem, not the actuator.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

There is always exception to the rule.
When I bought the SD, cruise was bad. Tried measure the actuator for motor open circuit and variable resistance etc. The variable resistance is not functioning right. Opened it up, and found the shaft was worn. A used actuator cures it. I was lucky that the cruise controller worked.

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Shane83SD

If I could make my speedometer work, I would know if my cruise eworks.

Any way to bypass the speedo or make a potentiometer to take the place of the amp and speedo?

I would imagine a rheostat and toggle switch could be rigged to power the actuator direcly. Could this work?

Hi 82-300TD

Spending time and effort to "re-invent" the cruise seems like rather involved task. It has to be able to over-power the pedal linkage and design a feedback control loop for desired cruising speed....

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Shane83SD

Not reinvent, but bypass the cruise control amp and control the stock actuator directly by manualy activating the lock and changing the current/voltage sent to it like the amp does. It's not automatic, but it will save your foot alot of strain.

Can you make your Benz accelerate with the cruise lever? If so, then your actuator is working properly. If not, then your problem may be with one or more components of the system, including the actuator. On a 240D I once owned, I could accelerate and decelerate with the lever, but the amp had no short term memory whatsoever.

Phil
'84 Euro 300D, peasant class, no cruise
'85 300D, drives itself

Ya'll keep in mind that the one Jennifer is asking about has the vacuum actuator instead of the electric. Over the next month I am going to try to get the one in my '81 working and will post whatever I find.

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Jim

Speaking of '81 cruise control....

Mine will accelerate and decellerate per the control lever. It will also accept a set speed. However, a few seconds after the speed is set, it will accelerate to 3 mph or so over the set speed, and then return to the set speed, it cycles back and forth like this until deactivated.

There is both a vacuum component and an electric component to my system. I wondered if some cable or linkage was just loose - so I tightened the little plastic nut that tensions the control cable to the vacuum unit. This helped a little, but not much.

I now suspect that the electric control unit is the problem. Its an expensive unit to replace, so I'm holding off.

My speedo occasionally bounces around by 2 or 3 mph around my actual speed. This might be related, yet its strange that it only happens occasionally, and not very often. Perhaps a grounding problem (?).

Engatwork - I'm very interested to hear how your cruise repair job turns out.

Anyone with cruise control problems - do you also have a problem with occasional speedo fluctuation?

-Zach
1981 300D (220 k Miles)

Zach - I have a minor fluctuation in my cruise control. I also have a slight speedometer needle wobble (intermittantly). I'm changing the speedo cable this weekend and will post the results.

82-300TD - I don't think you can "fool" the cruise amp with a rheostat. The signal that it reads is a series of pulses, not changing resistance. Maybe you could mount a magnet on the driveshaft and a reed switch nearby that would pulse in time with driveshaft speed. Going to all that trouble, though, you might just as well fix the speedo problem....

my $.02,
Wes

I don't think I'm giving the right idea. I will control the actuator *directly*, by controling the electromagnet and electric motor inside the actuator via switches.

I'm getting the components today and should have everything in by the end of the week.

82-300td - My bad. Skimmed your posts too quickly. Let us know how it works.

Wes

I tried to use my CC in my '85 300D last week but after I tried setting the speed it just started accelerating as if it was floored and I was alreay doing 70 at that point. It used to sort of work before that it would keep speed for a short time but then it would start surging by accelerating and decelerating wildly. It would work better when I would be driving on a slight incline but if I was driving on a slight downgrade, forget it. I've tried doing everything suggested in this forum short of taking the amp apart and checking it with a meter. if these things are so unreliable is it worth even going out and buying a new amp if its just going to go bad again in a short time?
Why are these things so quirky and unreliable?

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012

You know, after reading all the cruise control threads (and experiencing a few of the problems myself), it strikes me that someone could put together a reasonably priced aftermarket kit to retrofit these cars and eliminate the expensive fixes and to add some reliability. There ought to be some money to be made there somewhere.

Methinks the next time I have a problem with one of the cruise controls in either diesel I'm going to investigate aftermarket.

Funny, the cruise in the Tacoma works perfectly. Course, its only 9 years old compared to 22 on the diesels.

Just my $.02,
Wes

My CC in my 11 y.o. Silverado works flawlessly as well as everything else.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012