Cruise Control Pulsing
 

Hello,

I recently had my cruise control amp rebuilt and I replaced the turn and CC stalk in my car. My cruise still didn't work after all that. I pulled a CC actuator from a 1984 380SE and swapped it into my car. Now the cruise control works but often times it pulses the accelerator after being set. I have tried it at 40MPH to 65MPH and sometimes it will start pulsing. It can also start when going over small hills or large bumps in the road. It (seems) to work better at higher speeds. Where should I start my diagnosis of this odd problem?

Thanks for any help!

Sounds like you have replaced most of the components in the system. If you feel confident with the replacement components, then look at the connectors.

I'll clean all my connections and report back.

For anyone swapping in a CC actuator, you can use the same unit from many different models. Gas cars to Diesel cars, they are most likely the same or similar enough to work. You may find that the sweep of the actuator differs from your original. Before taking the unit apart and trying to modify it, check the actuator arm. Diesel models have a longer arm and gas models a shorter arm. That is not the only difference. The actuator unit is the same for all models but the arms are keyed differently for different strokes. When swapping one unit for another you want to swap your current actuator arm for the one on the replacement unit. When assembling into the holding bracket, you may need to relocate the spacer. On gas models, it is on the intersecting side while Diesel models are on the shared side. AKA, the side that has two mount points opposed to each other.

The cruise control seems to be working well over 60MPH. I guess that is good enough for me as the only time I really need to set it is on the highway and I cruise at about 62-65MPH. I'll fiddle with the connectors when I have more time. I have read that electrical interference can cause issues because the sensor wires are not shielded. I also need to check my engine to chassis ground strap and clean it up.

Is your speedometer cable/needle bouncy?

No, not at all. The CC only seems to pulse when going down a hill or similar. It is fine on flat roads and going up hills.

Interesting. I e long thought that very minute fluctuations in the speedometer, ones that perhaps are even damped out from the gauge, are the cause of this. I wonder if there are gain settings that. An be adjusted someplace in the circuit. Of course an over damped control will be sluggish, but control with excessive gain will be jumpy and over-excite for minor input. So with a noisy input, too much of the noise will be amplified which will make a pulsation.

Well, that's above my technical understanding of electronics but I agree that the range of acceptable tolerances is not broad enough. I really don't care if the car speeds up 5MPH going down a hill but the CC wants to keep it EXACTLY at the set speed at all times. I believe the CC is overreacting when going down a hill because the car picks up speed but does not sense the accelerator being depressed, so it lets off, drops speed, kicks in, picks up speed, lets off, drops speed, repeat. All the while maintaining 1MPH tolerances. If I slightly depress the accelerator pedal while cruising down hill, the CC does not pulse; assuming it sees an override and enters passing mode.

Have you lubed all the throttle linkage, etc...?

I think pulsing going down hills at certain speeds and grades is normal (though undesirable).

I lubed all the throttle linkages shortly after I bought the car, so maybe 6 months ago. I also cleaned any rust or debris out of them.

You might be right there, small hills are fine but anything where the car would pick up speed, it will do the pulsing. I wish there was a way to widen the tolerances but that probably means changing out a resistor or capacitor in the cruise amp. That is over my head unless someone know exactly which one and what to replace it with.

Is there anyone else out there who drives a W126 with working cruise control that can verify this pulsing issue? Maybe it is normal but I don't have anything else to compare it to.

Your best bet is to ask Peter at GDL. If anyone knows, he would.

I have perfectly working cruise on a 300SDL (with a GDL rebuilt amp). Next time I'm on hills I'll see what happens.

While my memory is a littler fuzzy, I seem to recall that the perfectly-working cruise on my 124's will pulse at times going downhill.