87 300D cruise control jitters

[SHARK1007]

Friends:

My 87 300D has come along nicely with one nagging issue left...
The cruise has the tiniest of surge almost feels like a recurrent 25rpm drop on and off rather quickly, does it intermittantly but most of the time..

I can't actually see it on the tach but can feel it easily, more pronounced going down a hill.

Thoughts? Suggestions?

Thanks

[R Leo]

Replace the speedo cable.

SWMBOs 300D has a slight drag/bind in the speedo cable which causes the speedo to pulse up and down about 1-2 mph. This pulsing causes the speed control to react to the increasing and decreasing speed indication which serves to exacerbate the movement of the speedo needle ultimately developing into an increasing phugoid oscillation.

When the car is cold evidently there's enough drag in the lubricant between the inner and outer cable to attenuate the wind-up and unwind of the inner cable that causes the needle pulse. But, after the car is hot, (ie: an hour or so on the highway) the speed control is basically unuseable.

A new cable will fix it.

[Marcb]

My wife's 300D has a rebuilt amp in it and it also surges on her. I've tried lubing the connecting rods from above the engine and wonder if removing them and lubing them with ATF would help. From looking down, I can see some small cracks in the plastic pieces which house the little snap area.

Any thoughts? The cruise always seems to work and never cancels itself.. it just drives her nuts with the surging.

Thanks so much!
Marcb

[R Leo]

Originally posted by R Leo
Replace the speedo cable.

SWMBOs 300D has a slight drag/bind in the speedo cable which causes the speedo to pulse up and down about 1-2 mph. This pulsing causes the speed control to react to the increasing and decreasing speed indication which serves to exacerbate the movement of the speedo needle ultimately developing into an increasing phugoid oscillation.

When the car is cold evidently there's enough drag in the lubricant between the inner and outer cable to attenuate the wind-up and unwind of the inner cable that causes the needle pulse. But, after the car is hot, (ie: an hour or so on the highway) the speed control is basically unuseable.

A new cable will fix it.

FWIW, the new cable did fix it (BTW, the cable was one of SWMBO's x-Mas gifts). Strangely enough, the new cable has enough friction in it to dampen the torsional flex and allow the speedo to display and maintain a constant speed.