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Originally Posted by amplifier28
The cruise control sometimes works but most of the time I does not work properly. It slowly disengages gradually once I set the cruise speed. Could low vacuum be the problem? If so, how do I check for vacuum leaks?
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Here's how you can check for leaks if your cruise setup is like mine. If you are at all uncomfortable performing any part of the test (esp. the part requiring electrical voltage to actuate the solenoids in the vacuum servo), you will want to take the car into a mechanic. Mixing up the electrical connections to the servo during the test can be "bad"
1 - disconnect the vacuum line from the servo, and put a vacuum gauge on the vacuum line going to the engine (not on the servo), and start the engine. Make sure the vacuum is in spec for engine vacuum (I think somewhere around 17 is normal)
2 - to test the vacuum integrity of the servo itself, you need to remove the electrical connector, figure out which contacts are for the vacuum solenoid and the vent solenoid, and apply 12v power (to the vacuum servo unit) to open the vacuum solenoid and close the vent solenoid. With a hand-operated vacuum pump hooked up to the vacuum port on the servo, pull a little bit of a vacuum and see if it holds (with engine NOT running). Then release the vacuum and 12v to the solenoids, and repeat the test several times.
2(a) - if vacuum does not hold, remove the large hose (going to the aforementioned brake release switch) and temporarily plug the port on the servo that the hose connected to, and repeat the test in (2) above, then UNPLUG the port on the servo and reconnect the big hose. If the vacuum consistently holds during the test, you may have a leaking cruise brake release switch. It is mounted above the brake pedal. Because of its safety importance, if you have any doubts as to your ability to replace that part, take the car in and tell the mechanic what you've found and have him/her work to fix whatever the problem might be.
2(b) - if the vacuum does hold well, but the diaphragm does not move, make sure you have voltage on the correct contacts on the servo (don't get it mixed up!!!).
2(c) - if the diaphragm sometimes moves and sometimes not, then the problem may be the vacuum solenoid in the cruise servo sticking.
2(d) - if the diaphragm moves, and the vacuum sometimes holds and sometimes not, then the problem may be a sticking vent solenoid in the servo.
2(e) - if the vacuum never holds for long, it could be a leak in the body of the servo or a sticking vent solenoid in the servo.
There could be vacuum leaks contributing to the problem, but I do suspect that one of the solenoid valves in the servo is sticking. When the cruise starts to drop out, will it sometimes kick right back in if you tap on the accelerator pedal briefly? (that was an occasional symptom on my cruise problem)
See my previous post on how you might recondition a servo with a leak or a sticking solenoid.
- GB