I have an '81 240 B21F Calif. in which the criuse control recently stoppped responding. I installed the system myself from a volvo dealers kit.

Instructions don't give much info other than where to install stuff and how to adjust. This system is I believe the "older" style. The vacuum servo is mounted under the wiper motor and has the brand name "Daton" on it. I think the newer styles had a smaller servo.

Anyway if this sounds like yours, the compontents are: servo, decel sensor ( a mercury switch mounted on a little angle bracket near servo and is normlly closed), and the cable. All under the hood. The control box is under the dash on the right side just behind the clove box. Then ofcourse there are the clutch and brake peddle(electrical and an air valve) switches and the control lever. There is an inline fuse on the control lever connector.

Typically one checks fuses, all connectors for tightness, and all switches for continuity. The vacuum servo has two internal electrical air valves. One opens the vacuum line to the vacuum source and the other vents the vacuum in the servo to the atmosphere thus moving the cable in or out. You can check these for continuity at the servo's connectors. If you think the contol box is bad, you may find another cheap in a pull-it-your self savage yard.