Got a problem with my cruise. When I enable it the light comes on, but when I go to set it it goes off and it won't come on again until I turn off the vehicle and start another trip.

when it originally stopped working, I enabled the cruise then set it, but it seemed to struggle to maintain the set speed (about 50 mph), then it just dropped out and I've never been able to set ever again since.

if anyone can tell Me what the problem might be, I'd happily give you my first born child as payment.

thanks

John.

 

Any lights on in the dash?

 

no engine management lights but the cruise control light does come on but then goes off when you attempt to set it.

not sure it's relevant but mine is a 2007 3.7 version

thanks

 

That's telling me there is a fault in the system. You either have a bad sensor or your servo is bad. Do you have a scanner to read codes?

 

no I don't, but I've ordered a servo anyway, are they easy to replace?

 

Yeah just a few nuts and a vacuum line. It's turning it off because it detects a fault somewhere, just have to figure out where. Could be the brake switch, servo, abs. There are a big number of things

 

This may or may not be helpful. A bit of a story for background to setup. Hopefully this comes through and makes sense.

I have an '04 Ram, 5.7, 1500. I too occasionally have cruise control functionality problems. It has been intermittent. Many times it works perfectly fine (80%). Sometimes it isn't (20%). Damn you Pareto! It has not been consistent so it is really hard to say it is this or that. When there are problems, the symptoms can be ANY of the following. The symptom will be consistent within that trip or that key cycle, and may change the next time you start the truck. Ie not always the same: Symptoms:
- cruise sets and works perfectly fine below a certain speed. When road conditions clear and increase speed set point all is good up to a certain speed range. Then the cruise it turns itself off, and it cannot be turned back on until speed is reduced below the range. On my truck that is about 121 km/hr. The cruise light goes off. It cannot be turned on until speed is reduced.
- cruise light is on in dash but cruise cannot be set at all. It does not engage and hold speed at all. Stop the truck and go again and it works fine.
- no problems whatsoever for most of the trip, cruise turns itself off and then cannot be turned on again or it turns on but does not engage when speed set button pushed.

Causes of problems with the cruise control function can be; electronic/digital in the PCM, electric in the wiring or sensors, and perhaps vacuum line.
I replaced the vacuum line and the PCV awhile ago. The cruise control problem disappeared completely for quite awhile. So that was a fix? Lately the problem has been occurring, again it is inconsistent.

Background and new developments:

This is a multipurpose vehicle for me. Sometimes it is a pleasure driver, sometimes a daily driver, sometimes a workhorse hauling loads, sometimes a heavy towing rig. I have a diablo sport inTune programmer that I use on this truck to change the program depending on what is going on. The current program is basic aggressive roundabout driving. However, I have some teenage drivers who take this truck. To protect the old truck from their antics I have changed a couple parameters in the program. Specifically the rev limits. There are two rev limit options, one for when in park/neutral and one for when in drive. I have the neutral rev limit set at 3800, so the teen does not over rev in the parking lot and blow rods out of the side of the block while making catchy look-at-me noise for their friends. The drive rev limit I have set at 5250, again to keep them from popping parts under full throttle when wot is not necessary. Remember these numbers, they are important for what's next.

Here is the story to that sets up another possibility for cause of cruise control functionality issue(s)? Yesterday was an interesting and enlightening experience. I was on a long 5 hours trip. I pulled out to make a pass and gunned the Hemi. It rumbled and ramped up with authority exactly as expected until it hit 4000 rpm then abruptly cut back. "What the heck!" I said to me. Tried again, it did it again. Then it dawned on me. Here is where the different rev limits programmed helped and comes into play. I am bombing down the highway at 100kph and pull out to make a pass. I gun the engine and it caps at 4000 rpm. All during this trip for 3 hours the cruise control can be turned on, light on, but it will not engage at all when set. Hmmmmm. ..... I am in the middle of nowhere a hundred of kms from civilization, there is no way I am stopping at this point and end up being stranded at a turnout. I carry on for another luxurious and uneventful hour of driving while thinking over the truck's systems. I stop at a convenience store and turn off the truck. I hook up the Diablo and verify my thinking to the set points that in the program. I did not reprogram, just checked the settings. I restart the truck and carry-on with the drive. Get onto the highway and gun it. Zooom! ramps to 5200 easy and off we go rip roaring tearing up the asphalt. Cruise control works fine and rpms are fine entire rest of the trip home. *** --> Here is where all of that is leading to ..... somewhere somehow even though the truck is driving fine down the road and the gear shifter is in drive, perhaps there is a flaky sensor signal telling the PCM that the transmission is in NEUTRAL. (?). There is NO other explanation for why the rpm capped at 4000 earlier in the drive. It would be like driving down the road and gently pushing the shifter from D to N and coasting while revving the engined. It also makes perfect sense that the program inhibits setting the cruise control speed when there is a signal telling it that the truck is in neutral. Perhaps this is also an explanation for what is happening a those times when the cruise control is dropping off at a certain speed. Perhaps that speed coincides with a perfect vibration level that is affecting the contacts of the shift position sensor and bounces it between Drive and Neutral?
( An afterthought, as I have 4wd this could be a significant problem if I (or the young drivers) had a brain fart and switched to 4Lo on the highway = Ka-BOOM! )

Based on this experience and observation yesterday. My next steps in figuring out this cruise control issue is to get under the dash and under the truck to investigate the what where and how does the PCM get the signal of what position the gear shifter is in. To try to determine a cause for how that could be intermittent. Specifically the points between D and N on the linkage and whatever sensors is on that. At this point, it appears that sometimes that signal is not indicating properly or changes on the go which is obviously affecting the functionality of the cruise control program loop and probably some other as of yet discovered program functions. Finding physically where on the truck the gear shifter indicator to the PCM is, is a new one to me so if someone can quickly point me on where to start looking and troubleshooting that circuit, I just may get this cruise control thing finally and completely resolved.

Sorry for the long description, but I do hope it presents the observations which may point to the problem being something other than the vacuum circuit or the cruise control solenoid - PCM circuit. I would have never thought of this neutral signal had I not programmed the separate rev limits.

I came on today to open a new thread and share this observation to help out others having cruise control issues. Seeing it is a recent/current for Johnaward, I put the information here.
If my logic and my troubleshooting proves out, this should put put up somewhere as a one of those "sticky" posts for quick answers to a baffling problem.

Hope that helps!
FaceDeAce