Cruise control inoperable, sometimes

tlewis said:

99 Cirrus, with cruise control that works 25% of the time. The other 75% of the time, when set , the dash light comes on but does not hold speed and turns off. Replaced with another unit, same result. Would think it is a vacuum problem, except it does work once in a while. It could be a wiring short, but that typically does not happen often. Any suggestions?

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When the cruise control is NOT operative, does the instrument panel lamp CRUISE illuminate? If the lamp does not turn on then I would suspect that there is an open circuit in the clock spring wire in the steering column. Even when the cruise system is operating, the circuit in the clockspring wire must maintain continuity at all times for the control function to operate properly. You turn the steering wheel slightly, the clockspring wire for the cruise circuitry opens, the powertrain control module detects this and disengages the cruise function.

Start the engine and let it idle. Push the on / off toggle switch or button and see if the word CRUISE illuminates on the instrument panel. It should turn on even if the vehicle is not moving. If it does illuminate turn the steering wheel several times full left and full right and see if the cruise lamp stays on. If it turns off with steering wheel movement, you have pinpointed an intermittent circuit in the clock spring wire.

I agree with all that has been said so far. Intermittents can be frustrating to find. Some cruise control malfunctions can set fault codes in the PCM and not light the 'ck eng' light because it is not an emissions violation.
Brake light switch adjustment is critical and if you can drive with your toes under the pedal lifting it toward you and the cruise stays engaged, then the brake switch may need adjustment or replacement.
If the green CRUISE light in the cluster is actually turning off, then the PCM has received a 'cruise off' button signal or something else commanded the cruise off. These steering wheel switches can get sticky or leaky from wear or water. Some active PCM fault codes will keep the cruise from operating for safety reasons.
A good scan tool can show cruise control inputs and states. It can also give the last reason for deactivation (i.e.- what switch last turned it off). A helper watching the scan tool 'cruise control switch states' display during a road test means that you can keep your eyes on the road. If the scan tool has a data recorder, have it watch anything concerning the cruise or brake switches.