Cruise control keeps turning off?

Diagnosis of this can be very simple, but it requires a Chrysler DRB3 scanner. 2004 was the first year it started going obsolete, but only on the Dakota / Durango. It still worked on some Jeep models up through 2008. Your truck falls in the middle there somewhere.

This scanner doesn't need any cartridge, however, with an additional plug-in card, it can do emissions-related stuff on any brand of vehicle sold in the U.S. Starting with '96 models. For that reason, a lot of independent shops bought them. Since they are obsolete now for the newer models, you can find a lot of these on eBay. If you want the extra card, look for the "Supercard 2" with a yellow label. That gives you the emissions capability for other brands, and it lets that scanner work on Chrysler models back to 1994. There's another Supercard with a white label that lets it work on all Chrysler models back to 1983. I have one of these and all of the cards for all of my older vehicles.

What this scanner can do is display the "Reason for last cutout" for the cruise control system. This is especially useful for intermittent problems. There are some things to be aware of. First, if you're reading the reason in the service bay or in your garage, the reason will be "vehicle speed too low". You'll need to be on a test drive where you can go fast enough without having to slow down.

At highway speed, set the cruise, then read the scanner right after it cuts out. Probably the most often seen reason is "Brake switch pressed". A brake light switch that's out of adjustment can bounce on bumpy roads causing this to happen. There will be three parts to the switch. As I recall, the switch that turns on the brake lights is not the one that cancels the cruise control, so it's unlikely the brake lights will be flickering. The switch that cancels the cruise is the same one that tells the Engine Computer to momentarily unlock the transmission's lock-up torque converter. You won't notice that because engine speed will drop right away, but if that switch is out of adjustment and you're driving at a steady speed with your foot on the accelerator pedal, each time the brake pedal bounces, you will see engine speed increase 200 rpm for a few seconds as the converter unlocks, then engine speed will go back down. You can force the same condition to see what I'm referring to. The van will have to have a tachometer in the gauge cluster, or you'll have to watch engine speed on the scanner. Drive at a smooth, steady speed above 45 mph, and note engine speed. Now lightly tap the brake pedal with your left foot. The engine will speed up by 200 RPMs, then drop back down a couple of seconds later.

The third cause can vary by year and model, but it provides a place to start looking. That is, "ignition power lost". We know you're not turning the ignition switch off because you're still driving, but the Engine Computer saw one of its four 12-volt supplies go away. One is a switched 12-volt supply to run that computer. Another 12-volt supply is constant. It's used to maintain diagnostic fault codes, fuel trim tables, and other data in memory when the ignition switch is off. A third supply comes from the automatic shutdown, (ASD) relay that powers the injectors and ignition coils. One of those last two supplies also powers the alternator's voltage regulator and the cruise control. Depending on the model, a break in that circuit will kick the cruise control off.

On other models, a similar reason can be "Cruise control switch turned off". The best suspect for that is a clock spring just starting to come apart. That's a wound-up ribbon cable in a plastic housing, right under the steering wheel. As it continues to break, eventually the "Air Bag" warning light will also turn on and the horn will stop working.

Besides reading these reasons for last cutout, the scanner will display the current state of each switch. Power will have an "on" or "off". The Set and Resume switches will be shown with a "Pressed" or "Released" next to each one. The state of the brake light switch will be shown the same way. There are also a number of tests you can run from another menu. Those can't be done with the engine running because activating the "Apply" solenoid would send the engine into wide-open-throttle. What this test is meant for is to let you hear and feel each solenoid clicking in the servo, or you can measure the voltages on the wires feeding those solenoids.

You might be able to view the same data with some of the better aftermarket scanners. I described the DRB3 because that's the one I'm very familiar with.

Also check the condition of the vacuum hose going to the servo. On older models that hose tapped off the vacuum check valve on the brake power booster.

When this problem is consistent, as in it kicks out after two seconds all the time, it tends to point to a vacuum hose issue or some other mechanical problem. The system is activating every time indicating everything electrical is okay, but the Engine Computer is seeing something it doesn't like. Electrical problems like corroded connector terminals or bare / touching wires will have a more varied series of occurrences. Sometimes the system might engage for a minute, and the next time not at all. With that kind of intermittent operation, but no scanner, you almost have to wait until the failure becomes permanent, then we can find the cause with a voltmeter.

Let me know if that's enough to get you started, then we'll figure out where to go next.

'95 and older models used the OBD1 emissions system. That's what the extra plug-in cards are for with the DRB3. Every manufacturer had their own unique computer language, diagnostic connectors, and locations for those connectors. '96 and newer, including your truck, do use the OBD2 system which the DRB3 works on directly. With this system, computer language and connectors are standardized, and that connector is always under or near the steering column. As I mentioned, the Dakota / Durango were the first Chrysler models to switch over to the still newer CAN Buss system. That's a different computer language. The DRB3 will not work with that system.

I should add a comment of value here for anyone considering a DRB 2 or DRB3. When used on a '95 or older models, there are two cables needed. The main cable connects to a large square plug under the hood. That's for use with the Engine Computer. A different cable with a smaller, blue, rectangular, six-pin plug connects under the dash for all the other computers such as Air Bag, Anti-Lock Brakes, and Body Computers. They talk on different networks, so different plugs are needed. All of those systems and computers are tied together through multiple networks on OBD2 and newer models so only one plug is needed.

When you look at other OBD2 units, the first to show up were simple fault code readers, and typically cost less than $50.00. They had their place, but they could not display sensor and other data. The next generation were able to show sensor values, but the screens updated painfully slowly, as in once every four or five seconds. Those were of no use when looking for rapid little glitches. Today, often for less than $50.00, you can get one that is "bidirectional", meaning you can talk back to a computer and command it to do things such as turn relays on and off. That lets you power up a circuit so it can be diagnosed. The problem here is unless you have a friend's to look at and play with, there's little chance to know what it can do. The advantage of the dealer-level scanner like Chrysler's DRB2 and DRB3, and GM's Tech2, is they will perform every function on every model for that brand, for the models they were designed for. Aftermarket scanners never do quite as much on any given model, but their advantage is one unit works on a real lot of brands. For independent shops that work on dozens of brands, they need an aftermarket scanner to cover all of them. The dealer just needs to be able to cover the brands they sell. They'll still invest in a less-expensive version of aftermarket scanner for their trade-in cars.

For your cruise control problem, we can't rely on simple voltage readings to figure out why it is cutting out. All communication between the switches and Engine Computer is digital. I could look at those signals with an oscilloscope I used in tv / vcr repair, but what I'd see would be meaningless. Instead, we have to let the computer interpret those signals, then look at what it does in response, or, in this case, let us tell us why it disengaged the system.

I have three aftermarket scanners. My first one is an old Monitor 4000 from the '90s. It was made by the same company that made Chrysler's DRB2 and looks very similar, but with a single cartridge, it works on GMs and Fords too. That one will not work on OBD2 systems and it never displayed the "reason for last cutout". My newer scanner is a Snapon Solus Edge. It works on all the systems, including OBD1 and OBD2, but I never tried to view cruise control data. I do know it will display switch status with a "True" or "False" which is confusing, but comparable to Chrysler's "Pressed" and "Released".

A huge problem with Snapon scanners that works to your advantage is the extremely high cost of annual updates. They're about $1000.00 per year, and you can't skip any years. Most people tell me since mine is updated through 2018, if I want to bring it current, I have to buy the 2019 update before I can buy the 2020 update, and so on. One person said that is not correct, that I can just buy the latest update and it will include everything I'm missing. I suspect the first story is the right one, because due to that high cost, you can find a lot of these on eBay for close to $700.00. They're commonly updated through 2012 to 2014, for example, which is fine for your needs. To bring them current would cost more than the cost of buying a brand new one with the latest updates. A brand new one costs around $4000.00, then you have to pay extra for Asian import coverage, and more for European coverage. That's far less than the cost of ten annual updates at $1000.00 each. Because of the cost, a lot of independent shops have just one scanner they keep current, and others they use until they get too old, then they put them on eBay. This is where you can find the good deals.

My latest scanner is made by Autel and it would be a hugely better deal for you. These can be found at Harbor Freight Tools, but I bought mine from an eBay seller who has hundreds of them listed. Steve W, one of our other experts, told us about his and how happy he is. The model I got can do a lot more than my Snapon, including programming keys and modules, but the entire cost was just three hundred dollars more than the cost of just one Snapon update. This Autel is basically a large tablet computer with lots of preloaded software, but that doesn't include any scanner software. The scanner function has to be downloaded over a wireless internet connection. I did that one evening at the local YMCA. The instructions were very easy to follow, and everything went exactly as described. It took about an hour, mostly because you can download files for any car brands you want, and I went for all of them from all over the world. You never know when I might be visiting Kazakstan. I guesstimated I have coverage for over 200 car and light truck brands I get two free updates, then I'm pretty sure the next ones won't be $1000.00. The seller on eBay had other models in all price ranges. It looks like a popular one runs just over $400.00. I don't have much experience with mine yet, but from what I've seen so far, this is the scanner I'd recommend. Be aware there are some things that require a wireless internet connection. Mine displays fault codes with their definitions in a somewhat easier-to-read fashion, but I haven't used it for other functions yet. Both have a Troubleshooter function that runs through diagnostic steps for each fault code. The Snapon has many of those in memory. The Autel retrieves that information over the web, as needed. My friend has the same Snapon scanner as mine, but he keeps it updated. Part of the last two updates requires him to have an internet connection too. For him that has meant, on multiple occasions, to put a truck back together so it can be towed out of the shop, so he can get the vehicle off the hoist and drive it across the yard to near his house where it can detect the wireless router. That's a major inconvenience, and the car manufacturers are seeing to it that becomes even more of a problem every year. With some of the latest models, he may be unable to even read fault codes without the wireless connection, and if he can, he still needs that connection to erase the codes after the repairs are done. I guarantee I will never buy or own a vehicle like that.

Besides the scanners I just described, I don't have any experience with the others that are out there. If you know any mechanics or shop owners, consider asking what they use and whether they would buy another one if they had to. Chrysler's DRB2 was easy to use and especially fun for having the ability for the first time to let the car tell you which circuit needed attention. The DRB3 was exciting when it was new because it did so much more, did it on more models, and could be updated by the dealer over a satellite hookup. While the DRB3 could look at emissions-related data on other car brands, '96 and newer, the Snapon scanner was a big step up in that it could work on all car brands, all systems, and like me, you could get them at a good price if they were out-of-date, as long as they were updated enough for your needs. The Autel will be a much better value to buy and to update, and I'm hoping the module and key programming capabilities will let me avoid future trips to the dealership.

I have one more parting thought for the moment. You might consider asking at the dealership if they still have their old DRB3 tucked away, and if you could buy, borrow, or rent it. Remember, you'll need to view the "Reason for last cutout" on a test drive, and only after it cuts out while driving. You can't do it while sitting in the parking lot because the reason will be "vehicle speed too low". Also, on that test drive, you must read the reason before using the brakes to slow down, otherwise the reason will be, "brake pedal pressed".

Now that I've shared all that wondrous information, it would be smart to look at the servo's throttle cable to see if the casing slipped in its mount or a bracket got bent. If the servo has to pull the cable too much to pull the throttle open, it might look like it's going for wide-open-throttle. Cutting out would be a designed-in safety measure.

Keep me updated on your progress or when you know something new.