I bought a 1988 Cadillac Brougham a few months ago, and for a month or two the cruise control worked, then it stopped working. I set the car at 40, 50, and even 60mph, well within the operating parameters for the CC, and press the button, it gives me the green light saying it's "engaged", but then the car just coasts and starts losing speed almost immediately. This happens on the highway, backroads, anywhere. It just doesn't work anymore. The cruise control lever sticking outta the steering column, the edge knob is kinda loose (where the button resides), so perhaps the wire got severed up there?
Could be the problem; however it is more likely to be under the hood where the cruise actuator is located - check the electrical connections on the actuator and the linkage between the actuator and the accelerator lever on the engine. Also, if the cruise control uses a diaphragm to control the accelerator, it may be leaking or the vacuum line to it may have fallen off - check the vacuum at the actuator with the engine running.
SOURCE: cruise control
hello , this kind of problems happened due to wrong connecting with the carbrator , or rearly because of wrong wiring of the cruise control , i hope that helps you and , thank you for using fixya.
SOURCE: how to fix the cruise control - won't hold speed
It is vacuum operated. It may currently be disconnected from the carburetor'throttle body linkage.
On the off chance it is connected, it likely has a hole in the diaphragm. So it may be disconnected at the vacuum line to the intake manifold.
There is also a vacuum dump switch which might be stuck open, damaged, or just out of adjustment which is normally pinched off until the brakes are applied and the switch is supposed to dump within the first inch of travel of the pedal. This is a mechanical override of the cruies control, and the brake light energy is supposed to release the control when the brake lights are energized.
There is also a magnet and sensor down either on the transmission speedometer gear housing,where it enters the transmission or on the driveshaft. If the magenr is not there, there is no speed pulse reference for the control to operate from. So it has no reference it can maintain.
It is not a sophisticated system, and it is prone to leaks after 23/24 years.
You can hear the dump valve if it is open, you can hear and see the diaphragm on the cruise control to see or hear if it is damaged when the hood is open. With the right meter, you can ascertain if the magnet is still present in the spot it is supposed to be once you locate the connector for the sensor- it is a hall effect switch usually.
You already know the electrical portion of the system works, so the problem is largely isolated to a vacuum leak or a sensor of unit is still connected. Normally the vacuum unit sits on the inner fender on either side.and the control cable arcs over to the transmission linkage side of the carburator or throttle body, it has one wiring harness connector and one vacuum connection and the cable to the carb, and usually only 2 bolts holding it down.
A wrecking yard might be a source for a good unit used, New old stock may still be in the system although likely aftermarket with the nationalization of GM.
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