In a message dated 98-10-05 18:23:14 EDT, jbaker@APBOARD.COM writes:

> Could I get one of you to give me a brief conceptual explanation of how
>  cruise controls work.
>  I have always wondered plus I need to fix mine.

Basically there are 4 parts.

Speed Sensor - Can measure rotational speed of an axle, a driveshaft, or
engine.  Most systems use the pulses of the ignition coil to measure the speed
of the engine.  Some aftermarket systems use magnets attached to the axle or
driveshaft and a sensor that counts the rate at which the magnets pass.  I
installed one of these on a Diesel Rabbit Pickup - no coil pulses!

Driver Switches - Used for on/off, accelerate, decelerate or coast, etc.

Controller - Takes inputs from speed sensor, driver switches, etc and
determines proper actuator position.

Acutator - Typically a vacuum powered device that pulls open the throttle when
more power is needed, allows throttle to move toward closed when less power is
needed.

Here's how the pieces function together.  The driver sets a "setpoint" of the
desired speed.  The controller determines the actual speed, as measured by the
speed sensor and compares it to the setpoint.  If the two match, no action is
taken.  If there is a difference between the actual speed and the setpoint,
the controler causes the actuator to move to either open or close the throttle
a little, depending on whether actual speed was above or below setpoint.  By
now about 1/4 second has gone by and the controller goes through the routine
all over again.

Other inputs can come from the brake pedal position, and clutch pedal
position.  Sometimes the controller and actuator are assembled together into a
single unit.  Most units also have a function whereby if the actual speed if
too far from the setpoint the unit automatically turns off.

Hope this helps.

Regards,
Jim Davis