Since the inception of our Tech Corner column last year, you've been keeping us busy with questions on the maintenance and repair of your modern and vintage Corvettes. We've found that many of your inquiries are similar in subject matter, with queries on cruise-control systems being near the top of the list. The following email is representative of those questions.
I've read your magazine for several years, and your technical articles have bailed me out many times. I like to think of myself as a shade-tree mechanic, so when my 35,000-mile '92 Corvette came out of winter storage this year with a dead battery, I thought it was no big deal. Well, since I installed the battery, the cruise control has stopped working. Do you think I caused a voltage spike and blew one of the modules? Is there anything I can do, other than take the car to the dealer?
There's a common scenario that occurs when installing a new battery in a C4 Corvette. If you look at your cruise-control servo, which is located just above the battery, you'll see two vacuum lines coming off of it. The act of sliding the battery into the tray can break these lines or dislodge them from the servo. Remember: Even though your Corvette only has 35,000 miles on the odometer, it's still almost 20 years old. At that age, the vacuum hoses have probably become brittle and are prone to cracking. Examine them and replace as necessary. Assuming the hoses appear OK, it's time to dig a little deeper.
A Closer Look at Cruise-Control Systems
The cruise-control system in most GM cars operates a mechanical linkage to the throttle body by using a vacuum motor called the cruise servo. This servo has a diaphragm, which is moved when vacuum is applied to one side. There are two solenoids located inside the servo. One connects the vacuum motor to the vacuum tank and allows for the application of more throttle. The other solenoid vents the vacuum to reduce throttle position.
On the '84-'89 Corvettes, these solenoids are controlled by the instrument panel (IP), which controls vehicle speed (throttle position) by pulsing the two solenoids on and off. Out of all the cruise-control problems I've repaired through the years, very few have had to do with a faulty IP. In the '90 and newer C4 models, the cruise-control module is a stand-alone unit mounted either on the right side of the instrument panel or below the passenger-side dash.
There are a few common problems to look for on the C4 cruise-control systems. Let's go through how to test a system with minimal tools. All you'll need is a test light, a multimeter, a vacuum pump with gauge, and a little knowledge.