After the cruise has been engaged and criusing down the road it will disengage by itself. What is the probable cause? The older cars had a vacuum switch and hoses. Were they still using vacuum in '89?
Thanks a lot, Wayne

 

There are a few things things that can cause the CC to disengage: the two main things are the brake switch, and the VSS. Under normal circumstances, it will disengage when you depress the brake pedal, or you drop under 35mph. If something fools the CC into thinking one of these two things have happened it could disengage the CC prematurely. The 89's CC still has a vacuum servo, but doesn't use the seperate CC module like older vehicles (85-earlier I guess) did. Since it's just a vac. servo, an internal vac leak in the servo would cause bad performance under CC, but normal performance without. A vac leak on the lines before the CC servo would act like a vac leak anywhere else on the car.

Have you scanned and monitered the vehicle speed information via a scan tool? If not, then I suggest doing that. Watch it for a while for strange readings (and they're possible, I've dealt with odd speed readings due to a dirty connector behind the gauge cluster). Or a faulty wire somewhere along the way that would open intermittantly could give the car a 0 mph reading momentarily, enough to drop the car out of cruise control.

Then you'd have to check to see if your brake switch is adjusted right (not "hair trigger" or a bare/crossed wire, etc.), and rule it out as a cause of your CC problem.

Another thing that would obviously cause the CC to disengage is turning it off. If something is wrong with the switch, say the detents are worn and it wiggles around near the OFF side enough to actually "touch" the off select, that would disengage the CC and it wouldn't re-engage if it happened to bounce back to ON, simply because it "loses" it's memory of the desired speed. Or any number of other things on or near the switch could cause a problem.

 

don't overlook the multifunction switch itself. wires for cruise control rub against the plastic steering column housing everytime the turn signals are used.