what year model are you referring to?

Cruise Control problems are ususally caused by vacuum leaks.
Especially if it responds weakly to road ups and downs.

In any case, look around engine bay at all vacuum hoses, checking for hissing, dried or hardened rubber, and cracks. You'll often find cracks where hose bends and slides onto nipple. See where hose goes thru firewall, these serve CC and also heater duct flaps. Remember that a leak anywhere reduces vacuum all around, so don't limit your search to hoses directly related to cruise control. Hoses located inside passenger compartment tend to hold up better.

There's another vacuum hose going to brake pedal, you can check that too.

Next likely cause of trouble is the adjustment on brake pedal switch that cuts out the CC when you hit the brake. See the 700/900 FAQ for a discussion of this one. A less common cause of trouble, but it does happen.

There is a brief section, with wiring diagrams, in the Bentley service manual.
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Gary Gilliam Sumerduck VA, '94 940 na Regina 150k, '86 240 180k