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The cruise control electronic module is grounded via a connection to the brake lights. When the bulbs are off, that circuit looks like a ground to the module. Braking (lighting the bulbs) kills that ground and the cruise drops out. The ground connection is also broken by the cruise switch on the brake pedal opening that circuit, and dumping vacuum from the cruise dashpot that opens the throttle. Double safe.
If you remove the underdash panel and can access the brake switch (buried a little further above and behind the cruise sw.) you can see if all wiring is properly connected. A disconnect here will affect both cruise and shiftlock.
In my 1994-940 wiring diagrams, Yellow/Brown wire(s) comes from the brake switch and goes to the Cruise unit, the bulb malfunction relay and to the shiftlock relay (also buried up in there - I think it's coloured Yellow). The shiftlock relay operates when the ignition is turned on, and then passes current to the shiftlock solenoid when the brakelight circuit gets 12V.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD and 89 745 (LT-1 V8); hobbycar 77 MGB, and a few old motorcycles)
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