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C4 Cruise-Control Repair - No-Cruise Blues
Vacuum Valve Solenoid
C4 Cruise-Control Repair - No-Cruise Blues
How To Diagnose And Repair C4 Cruise-Control Problems
By James Berry, Photography by James Berry
May 17, 2010
This is a one-way check valve that prevents a loss of vacuum to the A/C controls and cruise servo when the manifold vacuum drops under acceleration. To test the valve, remove the valve from the hose and apply vacuum to the port closest to the intake. It should not hold the vacuum. Next, apply vacuum to one of the smaller ports and cover the other small port with your finger; it should hold vacuum. (Remember that the vacuum hoses and check valve can deteriorate and become brittle with heat and age. This is a very common problem.)
This is a one-way check valve that prevents a loss of vacuum to the A/C controls and cruis
If the vacuum supply is good, you need to check the operation of the dump hose. Use a vacuum pump to apply vacuum to the dump hose to approximately 15 inches, then press on the brake pedal. The vacuum should be dumped, and the gauge should read 0 inches of vacuum. (If the vacuum is not dumped, you'll need to check for a pinched vacuum hose or a faulty or out-of-adjustment brake vacuum-control switch.)
If the vacuum supply is good, you need to check the operation of the dump hose. Use a vacu
If vacuum is getting to the servo and can be dumped, check to see if you're getting power to the servo. While holding 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 vacuum is getting to the servo and can be dumped, check to see if you're getting power
If you didn't hear or feel a click in the servo, you'll need to check to see if you're getting voltage to the servo itself. Disconnect the connector at the servo and connect a multimeter between terminals B and D. It should reed approximately 8 volts.
If you didn't hear or feel a click in the servo, you'll need to check to see if you're get
If you aren't reading 8 volts between terminals B and D, have someone release the steering-tilt arm and, with the key and cruise control in the "on" positions, move the steering column up and down slowly. If you are reading an intermittent voltage at the multimeter, the wiring from the cruise-control engagement switch could be broken inside the column. This problem can result when the wires become brittle with age. As you use the tilt-wheel feature, the wires bend and eventually break.
If you aren't reading 8 volts between terminals B and D, have someone release the steering
If you think you have a solenoid problem inside the cruise-control servo, there's a "no tools required" test you can perform. Simply pull the servo vacuum pod and cover the two vacuum ports with one finger, then push another finger against the dump servo plunger. The diaphragm should hold as long as your finger is on the plunger. If it doesn't, there's an internal leak in the servo or diaphragm, and the servo will need to be replaced.
If you think you have a solenoid problem inside the cruise-control servo, there's a "no to
There's a plunger and a pin inside the dump servo. Make sure they move freely and aren't stuck.
There's a plunger and a pin inside the dump servo. Make sure they move freely and aren't s
If you have a multimeter, you can use it to check the resistance of the solenoids inside the servo to make sure they're operating properly. The following procedure will work on most C4 servos. The connector will be labeled A, B, C, D, E. Unplug the connector; remember, you'll need to refer to the correct alphabetical locations at the servo.
1. Check the resistance between pins A and C. It should be somewhere between 30 and 50 ohms.
2. Check the resistance between pins C and E. It should be somewhere between 30 and 50 ohms.
3. Check the resistance between pins B and D. It should be somewhere between 10 and 30 ohms.
If any of the resistance readings falls out of range, a defective solenoid is likely to blame and the unit should be replaced.
If you have a multimeter, you can use it to check the resistance of the solenoids inside t
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 hold the brake pedal up with the top of your left foot while pressing the cruise-control button on a testdrive. If the cruise control tries to engage, the brake switch will most likely need to be adjusted. This is very common. Manual-trans Corvettes have a clutch switch that has the ability to disengage the cruise. This switch can be tested in the same way as the brake switch.
If all of the above test out OK, check to see if the brake switch is out of adjustment. A
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 insert it. After inserting the switch fully, pull the brake pedal back against its stop; you'll hear the switch click into its correct position. Reconnect the brake switch's electrical connector and vacuum hose (if equipped). Have someone check your brake lamps for correct operation before you go on a testdrive.
To adjust the brake-pedal switch, simply depress the brake pedal fully. Insert the brake-p
If you suspect that the brake switch may be faulty, you can use a test light to probe both sides of the switch. The test light should illuminate on one or both sides of the two wires, depending on the year of your car. If the light only illuminates on one side or the other, or both sides light up, the switch is OK. Next, press the brake pedal down 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, both sides should illuminate with the pedal applied. If both sides illuminated on the first test, only one side should illuminate when the pedal is applied. (If there was no change at all, then the brake switch should be replaced. If you have no illumination at all when you perform the first test, check for a blown fuse or an open circuit between the fuse box and the brake-light switch.)
If you suspect that the brake switch may be faulty, you can use a test light to probe both
If your cruise control and speedometer aren't working, the Vehicle Speed Sensor (VSS) could be the problem.
The VSS, which is mounted on the output shaft of the transmission, sends electrical pulses to the computer. A magnet spinning past a sensor coil generates these pulses. 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.
If your cruise control and speedometer aren't working, the Vehicle Speed Sensor (VSS) coul
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By James Berry
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