Q: Hi James. I’m so happy to have the tech column in Vette. It has helped me so many times. I’m in need of some advice on my cruise control. It was working fine until I replaced the battery terminal ends due to corrosion. I love driving my Corvette on the open road when the weather is nice and I use the cruise control to keep out of trouble if you know what I mean. Is there any way I can diagnose the problem myself?
Thanks for your help,
A: Hi Chris, and yes, you can diagnose this problem yourself. I have had a lot of questions on how to diagnose and repair cruise control for all different generations of the Corvette. Since you did not include the year of your car we will discuss how to diagnose and check the components that will cover most of the cruise controls on the earlier generations of the Corvette.
The easiest way to diagnose would be by using a scanner. If you don’t happen to have one of those there is an “old-school way” to figure out your problem. It is very common to have cruise control issues after changing a battery because the movement of sliding the battery in and out of the tray can cause the vacuum lines of the cruise control servo to dislodge or break. Since you only replaced the terminal ends, and assuming there wasn’t much movement of the battery, we will have to go old-school and start from what I call the “jumping off point.” Basically, we have to begin somewhere so let’s start at the most common element, the cruise control servo.
Examine the vacuum lines to ensure that they are intact and not leaking. The photo at the beginning of the article shows the most common failure of cruise control systems, vacuum hoses that are leaking or deteriorated to the point they will not stay attached to the cruise control servo. If you don’t find any problem there we will need to search a little deeper. Since we jumped off at the most common failure—vacuum lines—let’s back up and test the fuses with a test light. I always recommend testing all of the fuses at this time, not just the ones related to cruise control. Sometimes you will find fuses feed multiple components that may not have been listed in the owner’s manual.
First, turn the key to the On position. Probe the back side of each fuse. There are two places on the back side of each fuse to probe; if the test light does not light on both sides of the fuse you will find that the fuse is most likely blown. If the test light does not light on either side of the fuse; you will need to look in the owner’s manual to see what that circuit feeds. Certain circuits will require that you activate something to illuminate the test light, such as turning the headlamp switch on to energize that particular fuse. If all of your fuses check good we will continue down our pathway of the cruise control system.
Our next step would be to make sure the servo operates freely. The cruise control system operates a mechanical linkage to the throttle body by using a vacuum motor called the cruise servo. This servo has a diaphragm that is moved when vacuum is applied to one side. There are two solenoids located inside of the servo. One solenoid connects the vacuum motor to the vacuum tank and allows for more throttle position. The other solenoid vents the vacuum to reduce throttle position.
While squeezing the diaphragm with your hand, you can watch the throttle open and close, and the cable should move freely with no binding or tight spots. If you see any restrictions or binding, we know the cable or its attachment point is the culprit. If all is working properly our quest continues.
There are two hoses going into the cruise servo, the smaller hose is a vacuum supply and the larger hose is the dump hose. First, check the vacuum supply hose using a vacuum gauge. It should read between 15-20 inches of vacuum at idle. If not, check the supply hose for a leak. This hose originates from the intake manifold.
Feeding just off of the intake manifold you will find a solid black or a black and white vacuum check valve with two or three ports. This is a one-way check valve that prevents loss of vacuum to the air-conditioning controls and cruise servo when the manifold vacuum drops under acceleration.
To test the check valve, remove the valve from the hose and apply vacuum on the port closest to the intake. It should not hold the vacuum applied. Next, apply vacuum to one of the smaller ports and cover the other small port with your finger, it should hold vacuum. Remember, vacuum hoses and check valves can deteriorate and become brittle with heat and age. This is a very common problem.
If the vacuum supplied is good then we need to check the operation of the dump hose. Use a vacuum pump to apply vacuum to the dump hose to approximately 15 inches, and then press on the brake pedal. The vacuum should be dumped and the gauge should read 0. If the vacuum is not dumped we will need to check for a pinched vacuum hose or a faulty or out of adjustment brake vacuum-control switch.
If vacuum is getting to the servo and can be dumped, check to see if you are getting power to the servo. While placing your hand on the servo, have someone turn on the ignition and turn the cruise control switch to the On position. You should hear and feel the solenoids in the servo click. This lets you know the switch is working and that you have power to the servo. If you did not hear or feel a click in the servo, you will need to check to see if you are getting voltage to the servo itself. Disconnect the connector at the servo and connect a multimeter—on most systems—between terminals B and D. It should read approximately 8 volts.
If you are not reading 8 volts between terminals B and D—with the key and cruise control in the On position—have someone release the steering tilt arm and move the steering column up and down slowly. If you are reading an intermittent voltage on the multimeter, the wiring from the cruise control engage switch could be broken in the steering column. This is a problem that can be caused from the wires becoming brittle with age. Then, as you use the tilt steering wheel feature the wires naturally bend as the column is tilted, causing the aged wires to break. So, as the tilt column is moved up and down, the frayed wires can make contact at times.
If you think you are having a solenoid problem inside the cruise control servo there is a “no tools required” test you can perform at this point. It is to simply pull the servo vacuum pod and cover the two vacuum ports with one finger, then push another finger against the dump servo plunger. As long as your finger is on the dump servo plunger the diaphragm should hold until you remove your finger. If the diaphragm will not hold, there is an internal leak in the servo or the diaphragm and the servo will need to be replaced, unless the plunger and a pin inside of the dump servo are stuck.
Make sure they move freely and are not stuck. No problem found? Our trek continues.
Using a multimeter, check the resistance of the solenoids inside of the servo to make sure they are operating properly. The following will work on most C3 and C4 servos. The connector will be labeled A, B, C, D, E. Unplug the connector. Remember, you will need to refer to it for the correct alpha locations at the servo.
1. Check the resistance between pin A and C. It should be somewhere between 30 and 50 ohms.
2. Check the resistance between pin C and E. It should be somewhere between 30 and 50 ohms.
3. Check the resistance between pin B and D. It should be somewhere between 10 and 30 ohms.
If any of the resistance readings fall out of range, a defective solenoid should be suspected and the servo will have to be replaced.
If all of the above test out OK, check to see if the brake switch is out of adjustment. A quick way to diagnose this is to try holding the brake pedal up with the top of your left foot at the same time as you are engaging the cruise control while on a test drive. If the cruise control tries to set, then the brake switch will most likely need to be adjusted. Adjustment of the brake switch is very common. If your Corvette has a manual transmission you have a clutch switch that has the ability to disengage the Cruise Control. This switch can be tested in the same way as the brake switch. To adjust the brake pedal switch, simply depress the brake pedal fully. Insert the brake pedal switch until it seats on its bracket. You will hear it click as you are inserting it into its bracket. After inserting the switch fully, pull the brake pedal back against its stop, you will hear the switch click into its correct position. Reconnect the brake switch electrical connector and vacuum hose (if equipped). Have someone check your brake lamps for correct operation before you go on a test drive.
If you suspect that the brake switch may be faulty you can use a test light to probe both sides of the switch. If the light only illuminates on one side or the other, or both sides light up, this is OK. (This switch can be a commonly open or commonly closed switch.) Now, depress the brake pedal and perform the same test. You should have the opposite reaction. If one side lit up and the other didn’t on the first test, with the pedal depressed both sides should illuminate. If both sides illuminated on the first test but when the pedal is depressed only one side should illuminate. If there was no change at all, then the brake switch should be replaced. If when you began the first test and have no illumination at all, check for a blown fuse or an open circuit between the fuse box and the brake light switch.
OK, we have made it this far, and if your cruise control and speedometer are still not working; the Vehicle Speed Sensor (VSS) could be the problem. The vehicle’s speed sensor, which is mounted on the output shaft of the transmission, sends electrical pulses to the computer. These pulses are generated by a magnet spinning past a sensor coil. As you increase the speed of the vehicle, the frequency of the pulses increases. Usually, if this sensor has failed it will leave a fault code 24 in the Engine Control Module (ECM). These codes can be retrieved using a scanner or a simple paper clip.
Good luck, Chris. This completes our journey of the cruise control system. I like to use the phrase “jumping off point” to get to the root of the problem. You have to start somewhere so you pick a place to start and then follow that part from beginning to end or until you find the cause of the problem. It involves multiple steps but it gives you a thorough inspection of all elements of the system you are checking. I hope you find the problem with your cruise control.
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