Thanks in advance to all who try to help.

I've had my '86 DeVille for 6 years. Since I bought it from the elderly original owner, the cruise control never worked. When slide the dash switch from "off" to "on", the "on" light glows. Then, when I press the engage button on the end of the turn signal, another light on the dash glows to show the cruise control is now engaged. I then remove my foot from the gas and pray that my freeway speed will hold steady. It never does.

In an effort to solve this, I have made the following four attempts to solve the problem:

1) checked the play on the linkage between the vacuum servo and the throttle. It has a slight amount of slack on it. When I unclip the servo end of this linkage and try to position the linkage in the next of the staggered holes of the server mounting, goes from slightly slack to a bit tight...to the degree it puts a light tension on the flexible servo body. Thinking this is worse than having slight slack, I restored the linkage to the original position. I have not tested the cruise control in the state where there is tension on the server body.

2) checked the 3 AMP fuse. To confirm that I had identified the correct fuse, I removed it. When removed, it causes the cruise control "on" light to go out. Reinserting it, immediately causes the "on" light to re-illuminate.

3) replaced a 6" length of dry and cracked vacuum hose that connects in a "U" shape from the back of the servo to adevice that sits just below and behind the server. It appears to be a switch with two leads running from it. Once removed, old hose still appeared to be vacuum tight, but I replaced it with slightly heavier gauge new hose to be on the safe side.

4) replaced a portion of longer vacuum host that also connects to the the rear of the servo and extends down toward the electric vacuum pump under the battery. I have no ideal where this particular hose terminates, as it is bundled with tow other hoses on a protective plastic casing. I just trimmed off a couple of inches, added a coupler and a short length of fresh vacuum hose then reconnected to the original fitting.

None of the actions I've performed have changed the behavior of the cruise control. I am encouraged that I see the "on" and "engaged" lights glow in response to the activation of their associated switches.

I am hoping to tap into the experience of others who know the most probable cause(s) for the behavior exhibited by my cruise control system. I'd also like to know good sources for replacement parts, if needed.

Many thanks,

Dave

 

Does electric vacuum pump work?(in left front fender) How much vacuum is a available at the 1st inline vacuum solenoid? Do you have FSM?

 

Many thanks for your prompt reply to my post. Below are my answers to your excellent questions...

1) Does electric vacuum pump work?(in left front fender)
Answer: It runs, but the amount of vacuum is unknown. As soon as the ignition is turned to the 'on' position, this pump makes a noticeable pump sound, much like that found on an aquarium...just more rhbust. Opon visual inspection, there is no doubt about the source of this sound. This pump is below the battery. It can only be seen from below the left front fender.
2) How much vacuum is a available at the 1st inline vacuum solenoid?
Answer: Unknown. I didn't know to check this, but knowing how much vacuum is being supplied to this system makes perfect sense. I must admit that I'm unsure where the 1st inline solenoid is located. I wager it's just below and behind the servo. Please set me straight if my guess is flawed.

Question: Once I isolate the correct testing point, I wonder if getting a reading whilst the engine is idling will be sufficient, or if I'll need to rev it to some specific RPM level.

3) Do you have FSM?
Answer: Inspired by this question, I ordered both 1986 Cadillac Deville/Fleetwood Service Manual (black cover), and the product service publications manual (gold cover). I expect to have them by the week's end.

Again, many thanks for your contribution to my troubleshooting efforts.
Dave

 

Engage light comes on which normally indicates vacuum issue. You need vacuum gauge or pump (better) Vacuum release switch (at brake pedal) can cause this, if leaking or mis-adjusted. Follow vacuum hose from servo, soleniod on servo - controls speed, there so be another one in front of it to enable(power). Need to know how much vacuum how much vacuum is available to make cruise work.

 

Thanks again, Slowroll, for your generous help on this. Reading your post describing that the symptoms I'm observing are consistent with a vacuum leak goes a long way towars narrowing my search for the root cause. Once I recieve the '86 DeVille factor service manual which I ordered the other day, I should have the informatoin trace and test all relevant vacuum lines. Stay tuned...

 

Nice question and answer thread.

 

Thanks for the comment, joshtimber81.

Slowroll, I just received the black service manual and the gold product service publications books today. They're a treasure chest of diagnostic informaiton and flowcharts. Following the instructions in these bibles, you can isolate the cause(s) of problems with the many systems in the complicated sled, that the 1986 DeVille is. The diagrams showing all of the elements of the cruise control and vacuum systems alone are worth what I paid for these books.

I'm just about to head out of the country to India for a few weeks, where having a 125cc, two-wheeler is a luxury. (...and if you have a 3-wheeler, and auto-rickshow, you can make money hauling people around for a living) When I return in early May, I'll get back on this cruise control project. In addition to testing the vacuum, there are some functional and eletrical tests I want to perform on some of the switches and the servo solenoid. Stay tuned.