The cruise control has two switches, one electrical/one vacuum, mounted
on the brake pedal support. The electrical switch turns off the power
to the controller when the brake pedal is depressed. The vacuum release
valve is also mounted on the brake pedal support. It vents cruise
control servo vacuum when the pedal is depressed. The venting action
serves as a second disengagement system.
The ECM should show trouble codes associated with a cruise control
problem. The ECM also has the capability of testing the various
switches in the cruise control system.
Hope this helps
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