Cruise control servo working?

I have a few questions about my cruise control servo. I has two vacuum lines connected to it. A large one and a small one. The small one has vacuum but the large one that leads to the brake pedal does not. I checked the brake switch for a vacuum leak but did not detect any. Should there be vacuum on the large rubber hose on the cruse control servo? Is there a way to clean the cruise control servo? Any help on how to clean the cruise control servo? By the way, I already switched out the turn signal lever which has the cruise control on/off switch on it for a new one. Information greatly appreciated. Thanks in advance!

No, the large line should not have a vacuum on it. That line is there to rapidly release the vacuum in the cruise control servo when you step on the brakes, so that the throttle will shut quickly.:)

Is there a way to clean the cruise control servo?

I had to do this to my '91. Cruise had the trademark problem of running for 10 min and then dropping slowly out of cruise.

I removed the servo and disassembled the solenoid valve section. The problem seemed to be that one of the valves would stick. I thoroughly cleaned the solenoids and lubricated them, as well as applying a tiny bit of seal conditioner (like transmission fix-leak stuff) to the various rubber seals/o-rings. I also put a hose clamp on the aforementioned brake-release hose since it seemed a little bit loose.

That was about 80,000 miles ago, so I don't quite remember which solenoid it was (there are two). I do remember having to very thoroughly clean and lubricate it. I also tested it by applying voltage (just use a 9v battery if you have the servo out of the car) to the solenoids.

The problematic solenoid's armature seemed to be the wrong size - I remember thinking that if I were able to shorten or lengthen it (don't remember which -- but to realign the armature with the coil), it would generate more force and work better.

I think I remember there being a TSB on the problem that I had; if I recall correctly the TSB said to just replace the servo.

The cruise has been working fine since...

- GB

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?

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?

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

How do I know which contacts are for the vacuum solenoid and the vent solenoid? Will the servo be damaged if I connect the wrong contacts? Where could I buy a new servo?

I disassembled the cruise control servo and cleaned the solenoid valves and lubricated them with grease but the cruise control did not work. :( The following day I checked the vacuum line going to the servo with a vacuum gauge and got a 16 psi reading. That checked out fine. Then I disassembled the cruise control servo again and cleaned the solenoid valves again but this time I lubricated them with transmission oil and that did the trick! I never applied any power source to the contacts for the valves because I was afraid that I might mess up the servo. The cruise control works!!!! Yea!!!!!!!!!!! :) Thanks everybody for all your help.

I had Cruise Control problems too.. I racked through all the mechanical too with replacements.. 94' LeSabre.
Symptom was worked for about 3 miles to 10 miles, sometimes wouldnt engage at all.
The cure was a new (replacement junkyard) PCM.