Cruise control works for a while then starts to lose speed

[mibars]

My Cruise control is bit erratic. I've searched the forums and I couldn't find a thread that would describe similar issues.

My cruise control works, but starts to misbehave after couple of minutes of driving. First it works correctly then it starts to slowly lose speed, applying too little throttle. If I manually accelerate and bring it back to speed then it sometimes manages to take over and keep the speed for a minute or two. I can also feel that pedal is stiffer when cruise control is off.

If I keep driving with cruise control engaged but not doing it's work (manually pressing the gas pedal) and I start to lose speed uphill, I can feel that actuator takes over as the pedal gets softer and sinks under my foot a bit. Usually not enough to be able to bring the Burb back to target speed.

Couple of times the cruise control was keeping the speed correctly and then lost all the support, like it shut off, despite being on. Turning it off and on again makes it work again, however usually it won't be able to hold speed.

In any case when the cruise control starts to lose speed it does not react to "set" or "resume". It needs to be shut off and turned back on again.

Any idea how to troubleshoot that? On a 1st glance I can't see any specific issues under the hood.

 

[PrairieDrifter]

Sounds like vacuum loss. Hoses good? Vac pod is probably tired. Get a vacuum tester and see what it's doing.

 

[gmbellew]

agree, sounds like it is having an issue holding vacuum. check the hoses and the vac reservoir on the driver side fender under the hood in addition to the pod that attaches to the throttle.

 

[Snoots]

Does your truck have the vacuum or digital Cruise?

 

[oldretiredafguy]

Roger,
What year did they start putting digital cruise modules in the regular production vehicles?

 

[PrairieDrifter]

Roger,
What year did they start putting digital cruise modules in the regular production vehicles?

It woulda have to have been after 85. That's as far as my knowledge would go on that subject. Looks like he's got a tbi unit.

 

[gmbellew]

mines a 90 and is still is a vacuum system, although there is an electrical control box that is between the switch and the vacuum servo pod that tells the servo what to do.

not holding speed is most likely a vacuum issue in my limited experience.

chapter 9 of the GM service manual covers cruise control.

 

[mibars]

I have a vacuum system. I've actually replaced two hoses just next to actuator that looked old and cracked and nothing has changed. Other hoses did pass the visual inspection so I didn't bother to change them. I need to get a vacuum gauge... Or actually I do have an electronic gauge, but it's installed in my other car.

HVAC controls does work, that is vents can be switched and they are powered from same vacuum source. However my cruise control almost works, so maybe what's enough for vents is not enough for cruise control.

 

[gmbellew]

I'd just replace all the vacuum lines under the hood as cheap insurance, verify the check valve off the manifold line is working, and make sure the vacuum reservoir holds vacuum. if that all checks, you will have to look closer at the servo pod to make sure it holds a vacuum. I've seen YouTube videos about rebuilding the internals. there may be an internal filter that has rotted and is plugging up the servo..

 

[Snoots]

Roger,
What year did they start putting digital cruise modules in the regular production vehicles?

I know '88's used vacuum. I don't know what year they switched. I agree with @gmbellew about the vacuum lines. Also check your vacuum cannister. See if this is of any help . . .

 

[Joshua Keith]

I’m pretty sure I have an extra cruise control diaphragm if you need it.

 

[HotWheelsBurban]

I know '88's used vacuum. I don't know what year they switched. I agree with @gmbellew about the vacuum lines. Also check your vacuum cannister. See if this is of any help . . .

I think the change was made with the generation change. All the OBS trucks I've seen have the electronic box on the firewall, with cables coming out. Then with the NBS it's all electronic.

 

[Poodlehead]

The cruise on my 77 C10 functions similar with losing speed going uphill. If I help it by manually pressing on the accelerator, once I get over the hill, it picks up speed like I've got a new 'setpoint' for the cruise to maintain.

I rarely use cruise so I've lived with it, but this thread has perked my interest to look into the problem...

I don't mean to hijack this thread, but any suggestions where to start would be greatly appreciated or should I start a new thread?
Thank you!

 

[Frankenchevy]

All y’all with your fancy pants cruise control…

 

[Goldie Driver]

All y’all with your fancy pants cruise control…

Probably has ac and a cloth seat - fancy city slicker truck.

Goodie's cruise plays games, too. It will suddenly lose the set speed but will also sometimes regain it when you push back down on the pedal. Our 77 back in the day had issues maintaining speed on hills - you would have to help it. The cruise would also shut off if you hit a big bump in the road.

Goldie has a reman cruise module under the hood.

I think some of my issues are with the cruise stalk but I have not replaced it.