93 Seville Cruise Control Problem

[Mike Shawgo]

Hi guys,

I know most of you don't have the newer cars, but maybe one of you can help me with this. I have a 1993 Seville, 4.9 engine, 59,000 miles. The cruise control is driving me crazy.  It started out where it wouldn't set, until about the 5th or 6th try, then it would set and work fine. The message "Cruise Engaged" would appear on the IP, but it would't be. Since the car thought it was engaged, I would have to turn it off and on on the stalk in order to try it again.  I have replaced the brake switch, the speed sensor, and swapped out the servo with another one. I seem to have made some progress, but now it is even more strange--I will push the "set" button, IP says "Cruise Engaged," I take my foot off the accelerator, and it starts slowing down.  However, if I just briefly tap the accelerator, it will suddenly speed up to the speed where I set it, and work OK. If I touch the brake, then press the "Resume," it behaves the same way--won't re-engage until I tap the accelerator. In addition, the "Accelerate" feature does not work (even though the "Resume" does--sort of.  Any ideas?

Mike Shawgo
#20545

[TJ Hopland]

I'm trying to remember is this still vacuum actuated or is it all electric?

The cruise engaged light kinda makes me think much of the electric is fine and its under the hood that has the issue.  Do you know for sure the servo you installed was good? 

If its vacuum chances are good the valves are not sealing well if they are working at all.  If its electric my guess is chances are good that there are plastic parts and or lube that hasn't aged well.

[Mike Shawgo]

Yes it's vacuum activated, but my mechanic checked vacuum. Maybe he only checked vacuum going to the servo though. The other servo we tested was was one from a parts yard.

[TJ Hopland]

Since its vacuum that probably means its got the failsafe vacuum line running down to the brake pedal.  If you look at the servo it should have 2 vac lines on it.  One attaches to the solenoid looking thing and doesn't likely go too far before it attaches to other vacuum stuff or a port on the engine. 

The other line should be closer to the main body of the servo and should ultimately lead to the brake pedal inside the car with no other junctions.  At the brake pedal there is just a on off valve. Pedal not pressed the valve is off so it and the servo can hold vacuum.  If the pedal is being pressed that valve opens and will release the vacuum (thus the throttle) no matter what the electronics are telling it what to do. 

Historically that brake valve has been reliable but it seems like in the last couple years a few people have had issue with them so it may be one of those things that just has a 30ish year life?  If that's the problem in your case that would explain why the junkyard one acted the same since this was part of the car but we also don't know that there isn't some plastic or rubber part in the servo that also has a 30 year lifespan that has failed on both units. 

The safest way to test that circuit is with a hand vacuum gauge pump.  Disconnect it at the servo and give it a few pumps and see if it leaks.  You could also pinch it off and go for a test ride but that could be bad if it did turn out that the servos were sticking and without that vacuum dump it would stay engaged. 

Those hand pumps are pretty cheap now days and really handy for lots of car and some other around the garage stuff. 

This one is about $25 nowdays from Harbor Frieght but many of the parts stores sell something similar in price and quality.


 https://www.harborfreight.com/brake-bleeder-and-vacuum-pump-kit-63391.html

[TJ Hopland]

Another quick test you could try while you are driving to buy a vacuum tester is lift up on the brake pedal with you foot while trying to engage the cruise.  Maybe that valve is darn close to sealing and just needs the pedal to be a MM higher to seal properly.  If that helps you will know where to be looking.

[Mike Shawgo]

I think it was the hoses under the hood.  I hadn't noticed one hose was a little bit split at one end.  But I also replaced the servo just for good measure, and it is working perfectly now.  I have a feeling it was the hose though. Thank you everyone for suggestions.