Buick's cruise control is none too steady

(MCT) -- QUESTION: The cruise control is intermittent on my 1998 Buick Regal LS, which has 130,000 miles. Outside temperature seems to determine if it operates. It works OK if the temperature is below about 75 degrees. In the mid-80s, there is no control. This started shortly after the coolant change at 100,000 miles.

My son is an airline technician. He would like to check for heat expansion causing a short or break in the circuit but needs a "road map." Where is the speed sensor located? Maybe he can follow the wiring to the trouble spot.

Your help would be greatly appreciated. I am 90 and want to make this Buick my last car.

-Ray Forrester

ANSWER: Ray, I'm going to try to help you sort this out, but I need to keep things fairly general so that the information will be of possible interest to others. Your Buick's cruise control system consists mainly of the cruise control module/actuator unit located on the left strut tower, the control switches, the brake pedal release switch, and cooperation with the powertrain control module, or PCM. The cruise control module is the place to go for testing, and thankfully it's easily accessed. I tell my students a control module is like a train station - all the trains come and go from it. Almost everything you'd want to know can be observed there.

The Buick's PCM shares vehicle speed input with the cruise control module and can order the cruise control to stand down if diagnostic trouble codes are present or transmission gear, vehicle speed, battery voltage or engine speed are out of allowable range. If you have not seen an illuminated "service engine soon" light, it's fairly safe to assume there are no current diagnostic trouble codes or vehicle speed sensor issues. Troubles with the vehicle speed sensor, or VSS, would also show up in the speedometer/odometer functions.

The first thing I always check with an inoperative or intermittent cruise system is the brake pedal release (cruise inhibit) switch. Next time the cruise control acts up, try sneaking a toe beneath the brake pedal and pull back slightly, then attempt to re-engage the cruise. If this restores operation, the pedal switch is out of adjustment or faulty. Another thing to try: With the cruise working normally and engaged, wiggle and twist the multifunction lever, or turn signal arm, checking to see if you can make it cancel. If so, frayed wires or a flaky switch may be the cause of the intermittent operation.

Since your cruise system operates correctly at times, we'll assume there are no faults in the mechanical linkage connected to the throttle. Have your son obtain the very clear one-page cruise control system wiring diagram via Alldata or On-Demand-5, hopefully online at a public library. A shopping list of module terminals and expected voltages can be determined to check switch and PCM inputs. Observing good test values would be very helpful before checking the same circuits when it's acting up. It's anyone's guess where the intermittent connection may lie, but testing at the module will narrow this down quickly.