Modified Chevy Small Block - 1227730 with $8D - Cruise Control Issue

[zaut]

This may not be the spot for this thread, but it is related to my engine tuning and modifications. My vacuum control cruise worked fine before I built the new engine. Now that I have the new engine in, the cruise does not work properly. When I engage it, the control overshoots, then it lets the car slow too much, then gives it too much gas and overshoots again. If you can imagine it acts like a lead foot driver, flooring then coasting below the target speed then flooring it again and again, etc. Has anyone run into this and if so, were you able to get it to work correctly? I have a much bigger throttle body then the stock one. Small opening of the throttle gives much more air to the engine. It there anyway to dampen the cruise servo module or change the feedback so it doesn't overshoot so badly?

[RobertISaar]

the size of the TB is exactly why it's happening..... now identical actions cause wildly different changes.

the trick here would be to lengthen the arm that the cruise control module controls to give it a finer resolution. or swap in a module with less gain, assuming you could find out that information.

[zaut]

From my classroom training 30+ years ago, you are correct, this is defiantly too much gain, hence the overshoot. I have thought about the bigger radius on the throttle linkage but haven't wanted to anything that looks homemade. The BBK throttle linkage/bracket looks like the stock GM one I replaced. I know I need to weigh aesthetics versus function here. I think the gain is controlled in the module that is under dash. My guess is there is no way to modify it. The servo has a variable inductor that gives that module position information. I doubt adding an inductor in series of that circuit or a cap in parallel would affect gain. But if I could slow the servo down, I think that may work. Maybe a flow restrictor in the vacuum line to the servo could do that? Any ideas??

[gregs78cam]

If you can't get a longer lever then you could try to get the cable to attach to the arm at a 90* angle to the shaft, so that when engaged the cable's movement makes the shaft rotate as little as possible. Also you can try lowering with the DE numbers so it doesn't cut AS MUCH fuel when cable lets throttle close a little, that helped tame my cruise control.

[FSJ Guy]

You may also be able to adjust the gain and threshold settings on the cruise control itself. I don't know what system AMC used, but it has a little "brain box" that has potentiometers that control gain and threshold. Others may be similar.

[1project2many]

I might have some information about these systems at home. I'll try to remember to look tonight.

[zaut]

The controler is up under the dash. I will see it it is easy to get out and take a look.