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Curing Your C4’s Cruise Control

Auto Pilot Adversity

Andy Bolig Jun 1, 2002

Step By Step

There are two hoses going into the vacuum motor. One is a vacuum supply and the other is the dump hose. Check the smaller (supply) hose vacuum. If you have no A/C or cruise control, this is most likely the problem. Check that the hose is connected properly and that there are no holes in it.

Check the engine vacuum at the intake. You can pull one of the vacuum ports at the passenger-side rear of the intake to find the vacuum. The small hose going to the vacuum motor should be within 2-3 inches of the engine vacuum. If it is more than 6 inches more, start checking for a problem between the vacuum motor and supply.

The next step would be to make sure the vacuum motor operates freely. Squeezing on the diaphragm, you’ll be able to watch the throttle blades open and feel if there are any tight spots as the cable moves in the shielding.

Next, check the operation of the dump hose. Use a vacuum pump to pump up vacuum, and then press on the brake pedal. The vacuum should dump immediately.

Check the solenoids in the vacuum motor. Turn on the key; when you turn on the cruise-control unit you should hear the solenoids in the vacuum motor click. This shows that you have power to the solenoids. If not, check the fuse.

Next, remove the underdash panel and check the operation of the brake vacuum-control switch. If your Corvette is a standard, there will be another switch on the clutch pedal. Use a test light or a multi-meter to check the operation of the clutch and brake switches. We removed the switches for this photo, but you can check them in place to make sure they’re adjusted properly. We discovered that our car’s brake switch needed adjustment. That’s why the system wasn’t working properly.

Also check the cruise-control-switch connector. The pink/black wire and the two double blue wires should have power.

Check the servos inside the vacuum motor to ensure they’re operating properly. You’ll need a multi-meter to check the resistance at the connector. The connector is labeled “A, B, C, D, E” and you have to check between two of the connections to get the right reading. (See chart.)

Another way to check the servo is to push in the diaphragm and put your finger against the dump servo. It should hold the diaphragm until you remove your finger.
At this point, Chris says you should have found the problem, and that he’s never replaced the instrument cluster due to a cruise control problem. If you feel your cruise control may be the problem, Chris suggests you purchase a shop manual for your Corvette before progressing any further. We got our ’85 shop manual through Mid America Designs. You’ll be dealing with electrical systems directly, and the potential for doing damage increases greatly once you remove the instrument cluster. If you don’t have a shop manual and aren’t comfortable working on sensitive electronics, seek out professional help.

The cruise-control system operates via a mechanical linkage that actuates the throttle according to input from a vacuum motor. Solenoid-operated valves regulate the linkage which, in turn, open or close the throttle according to the dictates of the speed sensor and module. One valve opens to increase vacuum in the vacuum motor while the other valve opens to dump the vacuum to close the vacuum motor. The speed module (included in the instrument cluster on ’84-’89 cars) controls the vacuum motor and throttle by pulsing the solenoid valves on and off.

The C4’s cruise-control system can be intimidating if considered as only a part of the myriad of hoses, wires, and electronics that comprise the car. When viewed as a smaller system of components with an understanding of what they do, the fear factor is greatly reduced. We’ll be following Chris Petris as he troubleshoots an ’86 Corvette’s cruise-control system. The major difference between this car and the ’90-and-up Corvettes is that the module for the cruise control will no longer be part of the instrument cluster on the later Corvettes. Obviously, if your ’84-’89 Corvette’s instrument-cluster speed indicator doesn’t work, don’t expect the cruise to work. The good news for owners of ’90-and-newer Corvettes is that if your module goes bad, you don’t have to replace the entire instrument cluster. Chris says that the modules are rarely the problem, though.

Follow along as we go looking for trouble in a C4’s cruise-control system.


Corvette Clinic
Sanford, FL

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