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Cruise Control Jerks on Gradual Downhill

I have a 1982 300 TDT. The cruise control is all electronic, not vacuum.

I have replaced the control unit with a modern rebuilt unit and the actuator with a new VDO actuator (servo). It has about 25-30 thousand miles on it.

The cruise has started jerking when the car is going down a gradual grade. On steep downhill sections the cruise works fine, and on level or uphill stretches the cruise works fine, but on gradual downhill it gives too much boost, then cuts out, then too much, then cuts out, etc.

My speedometer cable is new and runs smoothly.

Does anyone have any ideas about what the problem is? I am pretty sure it is linkage or actuator related. I don't want to spend another $290 for a replacement servo if I can avoid it.

Reply to
heav
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I assume you mean that you replaced those for whatever reason and it then worked OK for 30K miles?

What happens on the gradual downhill when you have the cruise control off and use the accelerator to control it? Any sign of irregular operation then? Wondering if the downhill angle sensitivity could be fuel related, as opposed to cruise control. Last Fall I had the weirdest problem with my 300SD. It was idling rough, but fine when driving. Finally it got so bad I had to keep the RPM's up to keep it running at stop lights. I replaced the 2 fuel filters and it completely fixed the problem. It was exactly the opposite type of behavior I would have expected, as I would have thought a clogged filter would be worse at highway speed, but somehow it was at lower RPM's it just wasn't delivering enough fuel.

Also, not sure how the whole system works on your car, but on my 1980 there is a speed sensor pickup off the odometer, which is the only un- mentioned part of the system. If it was bad I don't see how it could just cause the type of behavior you're seeing though.

Reply to
trader4

Yes.

The engine was remanufactured just 12,000 miles back and runs perfectly. I am sure it is the cruise.

Reply to
heav

I shortened the adjustable link between the pivot on the valve cover and the rest of the throttle linkage by three turns and that seems to have eliminated the jerking in the cruise on gradual down hill runs.

Reply to
heav

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