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Cruise control disengagement

My 2005 Mercury Montego has a weird issue with the cruise control. I have already taken it to a Ford dealer once. They claimed the switch on the brake pedal was the issue.

It isn’t.

At 70 mph, with cruise control OFF, take foot off accelerator and push on brake pedal. Works great.

At 70 mph, with cruise control On and engaged, the brake acts like the engine is off. It is very hard to push. It is like the engine is OFF and the vacuum assist is gone. Press really hard, and the car will slow down. Jab the brake once first (hard), then apply brakes, and it is fine. Push the cancel button on the steering wheel, and all is well. The few times I have looked, the tachometer has not moved. Car has CVT.

I bought this car new, and know it didn’t always do this. And certainly shouldn’t. A panic stop is scarier than it already would have been. Car has around 62000 miles on it.

Mechanic wrote that he observed the issue in a test dive and tested it following repair and it now works. I don’t get this at all.

This probably works the same as my 97 Ford. The brake pedal switch sends volltage to the brake lights & to the cruise control brain & the CC disengages. If this switch fails the backup is a pressure switch mounted on the brake master cylinder. Hit the brakes hard enough & pressure is detected by this switch & the CC is disengaged.

Check to see if your brake lights are working.

See if you have a fuse for the cruise control. If so, remove it and see if the brakes operate normally. If it has no effect, than you have some other cause.

I’m concerned that your CC may not be automatically disenegaging and you’re having to fight it with your brakes. That’s not a safe condition…and can destroy your pads quickly. I’m not so sure this isn’t hard on the drivetrain too.

Try another shop.

Post an update. We care.

I took the car back to the dealer today. My intent was to show a mechanic the issue. A mechanic went with me for a drive on the interstate. He was driving. Cruise on or off, he noticed no difference. We swapped seats. I made several slow downs from 65-70mph. I did not keep count, unfortunately. One some slow downs, the brake worked perfectly. On others, the pedal was initially hard to move, as if cc was engaged and fighting it. I failed to check the status of the cc while braking. This whole thing was just surreal. We swapped back. Getting out of the car, the smell of something burning was quite strong. Mechanic stated that was due to the number of times I applied the brakes. I did not apply them hard, and probably applied them maybe ten times in a four minute period (guessing). Car has 62000 miles on it, mostly highway. I don’t know if I mentioned this in the beginning, but the first time I can recall this happening was after the left front tire tread tore off while I was driving 70mph. My first thought was that road was really rough - like it had been milled far deeper than I have ever seen. I gently applied the brakes and pulled onto the shoulder. Left front tire tread was about 30 feet behind my car. The smell of something burning was also present. That smell was the same as today. I had assumed it was burning rubber. I can’t imagine the brake pads making that smell from a single stop. I never lost control of the car.

I remain very confused. There are no indicator lights on. Mechanic stated that if there had been an issue that a hard code would be stored and indicator light would be on. He also stated that if I took the car to an auto parts store, it would be unlikely that the code scanner used would access brakes and cruise control, even though the Ford scanner would, and through the OBD connector. He “offered” (not free) to perform a code scan, but felt that would not reveal anything.

I would take it to another dealer. In this day with the computer in control of most cc units, it’s important to get someone familiar it.

I did send an email to another dealer - what was left after Ford dropped Mercury. That resulted in a phone conversation. Result of the conversation was the suggestion to submit a report to NHTSA.

Can this issue really be this bad?

I meant to add: to diagnose.

I am wondering, and only that. Since brake pedal gets hard to push initially, but I have observed that it isn’t always hard - could vacuum be involved? I recall reading years ago (I’m not a mechanic!) that cruise control used a vacuum motor to control the throttle. Is that still true? Or has a stepper motor possibly replaced the vacuum motor? Now, if the throttle >is< controlled by a vacuum motor, that implies, to me anyway, a potential vacuum leak in that part of the system. With cruise disabled, braking is always great. Does this make any sense? Is there any other explanation for bake pedal to be hard to push, initially, on most tests?

If I am right, how would a garage diagnose this? Do they no longer look beyond the ODB II connector?


On trip tonight on the interstate, I tried an experiment. Set cruise for 70. On uphill areas, brake pedal was hard to push. On downhill areas, brake pedal was normal. So, if cc still uses a vacuum motor, there is a vacuum leak, somewhere. Any ideas where such a leak might be?

Tried a simpler test. Set cruise for 70. Disengaged and slowed to 60. Resume, but before vehicle reaches 70, try brakes. This works every time. Brake pedal is hard to push, but then returns to normal.

I put that procedure on paper and went back to the Ford dealer. Left car there. Hours later, I learned that they STILL could not replicate.

Next step was to contact the dealer via email. I explained the problem. I received an interesting reply. It came from the service manager, but had a page from AllData on the cruise control. The service manager has stated several things that do not make sense, to me. First, from the AllData page, an electronically controlled throttle body. On several replies, I asked him just how the throttle is operated. He says “drive by wire”. Second, there is absolutely no connection between cruise control and power brakes. But, that goes back to his statement that it is electronically controlled. Even though I can reproduce this effect every time, he still does not seem to understand. This, by , the way, was the person who rode with me and could not detect the problem.

He has to answer to my question as to why this issue happens every time when the engine is under load - such as accelerating back to previous set speed.

I even sent him a page from howstuffworks that specifically describes how cruise control works. I also informed him that three other Ford vehicles I have driven did not have this issue, and neither did this car until fairly recently.

I am not real thrilled driving with my finger parked on the Cancel button.

I was under the impression that service managers had a good understanding of how systems worked.

Nagging thought in my mind is whether he is blowing me off as the genuine repair is insanely expensive.

I am just tired of this. Years ago, I took a car to another dealer. That car had a noisy sunroof. When I went to pick it up, the panel that operated the aunroof was not properly attached to the ceiling. Just hanging down. Noise came back. Took it back two more times. Finally, I was told “that roof has 141,000 miles on it”, and “the glass is warped”. I have not been back to that dealer since 2001. I don’t want anything else broken on the Montego. Are all car dealer service departments full of incompetent people?

You should try to replicate the problem with the cruise control off, this will take the focus off the cruise control. Under mild acceleration at highway speed apply the brake. Under load there is reduced vacuum, however the brake booster should hold enough vacuum for at least one brake application. I suspect the vacuum check valve in the brake booster is leaking and you don’t need a cruise control repair.

@ibm1130: I would also suggest that you have a vacuum leak or a problem with the vacuum actuator that moves the throttle when the cruise is on. Since these take a considerable amount of vacuum “pressure” to move the throttle, it may be hooked in to the brake booster’s vacuum reservoir as its vacuum source. I would expect the problem would be worst if you resumed cruise from a slower speed, such that the car had to accelerate using the cruise system alone.

If your car uses “drive by wire”, and not a mechanical throttle linkage, then you’re barking up the wrong tree, but I think yours is old enough to use vacuum to run the cruise control.

Anyway, check the vacuum lines that run to the actuator. There may also be a check valve somewhere in the system that is failing. If you can’t find the problem, I’d at least suggest the hypothesis that there is a problem with the vacuum system to your mechanic.

To answer to your earlier question regarding the vacuum motor to control the throttle is no. In modern cars the throttle is actually controlled by a throttle motor that gets a signal from the ECU (the engine’s control computer). The ECU gets its signals primarily from the vehicle speed sensor and the throttle. When criuse control is on, the ECU simply commands the throtle motor to open the throttle plate a bit more whenever the VSS signals tells it that the speed has varied even by a small amount.

Your cruise control is supposed to disengage whenever you push the brake pedal. Any diagnostician worth his salt would be looking into what causes the cruise control to disengage when the brakes are applied. I don;t think your guy cares. Or perhaps your rides were with the “service representative”, most of whom know almost nothing about how cars actually work.

It’s also possible he was what I call a “trained monkey”, a guy who learned to follow the manufacturer protocols but not to think. I looked at my own manufacturer trouble table (below)
and, interestingly, failure to disengage isn’t listed.

I don’t have access to a repair database for your car. If I did, I’d take a peek at the system.

While I can’t even begin to tell you what the cause of this problem is with any certainty, a wild stab in the dark might be a throttle plate that is not releasing for whatever reason; carbon deposits, throttle body fault, PCM or wiring fault, etc, etc.

Most service managers have never worked as mechanics so anything they say should be taken with a grain of salt. Many know nothing more than what they’re heard or run into on occasion.

That being said, I’ll play devil’s advocate about incompetence. The fact that a car develops problems which may require much hand wringing to sort out does not necessarily translate to incompetence. It’s high technology for which there is often no black and white answer.
Customers love bells and whistles; until the hiccups start.

Shop rates are high. If a customer enters with an odd problem (especially electrical) what would be the customer’s reaction if they were told from the get-go that, “it may take 20 hours @ X dollars per hour to track this down”? Customer would run screaming from the premises.

I own a Lincoln Mark and also have the factory wiring book for it. Tracking down a problem with something so seemingly insignificant as a dome light can be mind-numbing at best. The same can be said for other systems on the car because at my last count a few years ago that car had 24 computers and modules on it. My fuzzy memory seems to recall running across a few more since that original count.
The PCM wiring alone encompasses 11 full pages.

In your case, it might help to know if the car has been scanned for any codes that may exist; and keep in mind that there can be problems even with no codes present.
At one time cruise control systems were pretty simple but high tech took over and high tech = complexity and money, both in the engineering and upkeep phases.

I agree with Nevada on this one, but I would not restrict my search to just the check valve. It could be anywhere in the vacuum hose or even in the vacuum booster.

I like this. Simple. But, how does this work on a car with no problem? Would brake always be hard to push?

While I was filling the gas tank, I looked around under the hood a bit. Not easy to see much. Even if I did, knowing what to do is next. I had not considered that a vacuum check valve was even present. The question now, is what is my next step. My last email to the service manager asking what pulls the throttle is still unanswered. I can go to a dealer in another city. But, I am reluctant as these things seem to take all day and I have no idea how I would pass the time.

Is there a procedure to follow when service manager fails to rectify the issue?

Big bonus is I need to get coolant changed - still has original in it. Do I go back to this same dealership?

Also, any idea why owner’s manual states that after original coolant is replaced, the change interval reverts to old number. I can’t remember if that was 2 or 3 years.

Big issue, though, is what to do next.

Ibm, the way it works on a car with no problems is simple. The brake booster is a canister through which the rod that pushes the pistons in the master cylinder runs. The canister contains a diaphragm. When the brakes are not being used, vacuum is applied equally to the entire canister, to both sides of the diaphragm. When th erod is pushed by the brake pedal, the rod moves a valve assembly that vents the rear of the diaphragm to ambient and closes the front of the diapgragm such that engine vacuum is applied. That diaphragm assists the driver in pushing the rod. It is only when the booster is inoperative that the brakes become hard.

The key as it relates to the posted roblem is that the vacuum is taken from the engine, Any leak in the system will present as a vacuum leak to the engine.

I’ve attached a few links with illustrations that might help you to understand the way this all works. As regards a procedure you can follow when the service manager fails to rectify the issue, you can ask to speak with the actual shop manager/supervisor. He’ll be far more knowledgable.

You are looking at the problem backwards. Your cruise control is working just fine. The reason the brake pedal is getting hard when the cruise control is applying throttle is that opening the throttle drops the vacuum wether you do it or the cruse control does. You probably think it only happens with the cruise on because you usually lift off the gas when you step on the brake.

Your power brake booster needs vacuum to work, you either have a small hole in the diaphragm ( if it was a large hole the booster wouldn’t work at all ), or low vacuum caused by something else. It could be a leak, timing or a worn out engine or even worn out spark plugs that only miss when they get under enough load.

40 years ago any decent mechanic would have confirmed this with a vacuum gauge. You need a mechanic who has one and is willing to use it. A scanner won’t tell you everything.

Oldtimer, I absolutely agree with you on the vacuum description. The reason I felt (and still feel) that the CC isn’t disengaging properly is that the problem disappears when the CC is disabled via the control stalk on the steering wheel.

Man, in many of these cases it sure would be nice to get my hands on the vehicle.