This weekend, it failed completely.
The electronics appeared to be operating, both the 'On' light and the 'Cruise' lights would come on at the appropriate times, but when I removed my foot from the accelerator pedal, the car just dropped in speed until it hit the low speed warning point (10 MPH < set) and the cruise was automatically cancelled.
The shop manual troubleshooting guide was not much help. Their decision tree is based on reading the diagnostic code from the computer. In my case, the computer indicated 'Normal'.
An initial visual inspection of the system did not show any obvious problems. So I started pulling things apart to do a little hands on diagnostics. The first thing I wanted to check was the vacuum pump and solenoid assembly. Unfortunately, this unit is buried under the throttle cable junction box by the firewall. (Non-cruise cars don't have this. 1990 models use a mechanical system). It looked like it was going to be one tough job to get it out.
Fortunately, early in the removal process, I noticed that the vacuum line to the throttle actuator had fallen off of the vacuum pump. Hmmm, that's not supposed to happen.
Turns out that the inner diameter of the vacuum line is considerably smaller than the nipple on the vacuum pump, stretching the hose beyond its limits, causing it to split. Not what I would call a particularly good design.
I cut the split end off the hose and struggled to press it back onto the vacuum pump. I expect that it will split again in a year or two. Maybe next time I'll replace the whole hose with a better one.
Has your cruise control been acting funny? Check out that hose. The bigger the leak, the lower your top set speed will be. A leaking hose will also cause your vacuum pump to run all the time prompting an early failure.
Happy Cruising,
Terry Wells
twells@cp10.es.xerox.com