2004 Mercury Mountaineer



January, 26, 2011 AT 3:33 PM

To "caradiodoc"--- thanks so much for the super answer to my question about airbag sensors on a 2004 Mercury Mountaineer. I have another question. Why would both side curtain airbags suddenly deploy at the same time while just driving down the road at about 55mph? The dealer is telling us that evidently one of the airbag sensors was deactivated when the SUV had a run in with a ditch two years ago and another sensor was jarred. The airbags did not deploy then. So he said it was just a matter of time before that second sensor deactivated and caused both airbags to deploy. He said at least two sensors have to be deactivated before airbags will deploy. I found info on the web that said that airbag sensors on Ford products reset thenselves after an accident or hard bump where the airbags did not deploy? They want to charge us $80 to run a diagnostic test to see why those airbags deployed, but we would still have to pay the bill for replacing both airbags and headliner. I don't have that amount of money.


Driver Airbag


Mustan Air Bags


3 Answers



January, 26, 2011 AT 7:50 PM

First of all, I have never worked on side air bags so I'm the wrong person to ask. Next, my expertise is with Chrysler products, but all systems basically work the same way, so let me describe the front air bags first.

There are some less expensive systems with just one sensor, but most systems have three. Two are in the front near each head light. They will detect an impact only a few degrees off-center from straight ahead, so the side systems have different sensors. The third sensor is called the "safing" sensor and lives inside the computer. That computer and the two front sensors must be mounted in the proper locations and angles to work properly. You don't want them to activate when you hit a shopping cart in the parking lot, for example. And if they are mounted at too high an angle, they will require a harder impact to activate.

Basically each sensor is an on / off switch. Imagine a steel roller bearing with a sheet of spring-metal wound up around it. That spring-metal is attached at the rear of the sensor housing. In an impact, the momentum causes that roller bearing to roll up a small incline and stretch out the ribbon of metal. When the bearing gets far enough, it makes a connection with two gold-plated contacts. That sensor has just turned on. Right after impact, the metal ribbon will wind up again and retract the roller bearing. That all happens in a fraction of a second, so timing is critical because either one of the front sensors AND the safing sensor have to make contact at exactly the same time. If one sensor is mounted at the wrong angle, one will make contact, then recoil before the other one makes contact. That won't deploy the air bags. The bags only deploy when two sensors make contact at the same time.

There's one problem with the story you got from the dealer, but keep in mind you may have gotten it from a service adviser who understands customers, and maybe a little about cars, or he could have gotten the story mixed up from what he was told by the mechanic. There is no such thing as a sensor that is stuck on from a previous crash. The computer will detect that. Remember my story about the 1000 ohm resistor in the rocket fuel pellet in the steering wheel air bag? The same is true of all crash sensors. When the roller bearing is just sitting there, the circuit is open, meaning turned off, just like the light switch in your house. The same condition would exist if a wire was cut or the sensor was unplugged. To self-test for that, there is a resistor across those gold-plated contacts in each sensor. The computer detects that value of resistance whenever the ignition switch is on. It can detect an open circuit, (sensor unplugged / cut wire, etc), it can detect either wire is grounded, and it can detect the two wires shorted together. That's the same thing as when the roller bearing makes contact. When it detects a shorted sensor, the air bag warning light will turn on and the system will be deactivated. It will not pop the air bags even if another sensor turns on. So the point of the story is if a sensor had been stuck turned on from a previous crash, the system wouldn't have worked. There are only three possible states the sensor can be in; open, shorted, and measuring about 1000 ohms. The first two conditions are defective and will turn the system off.

So now that we understand that all sensors must be working properly for the system to arm itself, it should also be understood that two sensors have to activate at exactly the same time for the air bag to deploy. That means running around the car with a rubber hammer is not going to set the system off. The exception is those few cars that just have the one sensor inside the computer. Smacking that computer might do it, but I seem to recall they also have a speed sensor signal going to the computer so it won't pop the air bag below a certain speed.

The only things I can think of that could have set your side air bags off would be if a side impact sensor was not mounted correctly or if only one needs to trip and you hit a pot hole large enough to do that. I find that hard to believe, otherwise they would all be popping up here in WI. There are so many safeguards built into these systems that it is near impossible to set them off intentionally. The warnings about disconnecting the battery when removing an air bag or working on the system is mostly for liability reasons. Most service manual instructions even have you disconnecting the battery to replace wiper blades! If there is anything you can possibly think of to cause injury, they are covered from liability because you didn't disconnect the battery like the service manual said.

My approach to your problem would be to request a meeting with the district representative. At the Chrysler dealership, he came around once per month to meet with anyone who had a complaint the dealer couldn't handle. They have the authority to warranty things the dealer can't. (One guy even offered to warranty the battery on my ten-year-old Dynasty that hadn't been driven in nine years)!

At the very least, they will likely make a record of the incident and do a basic investigation. That's where recalls originate. If they even think there might be a suggestion of a lawsuit, they will likely pay for any diagnostics. Even if you end up paying for repairs, if a recall goes into effect in the future, you will likely be able to get reimbursed for the previous repairs if you have documentation, meaning the repair receipt. This is where the dealer becomes your ally. They keep copies of all repairs on file, usually for five to ten years, and they will research the repair history for you if you don't have any paperwork.




January, 26, 2011 AT 7:54 PM

I should mention too that it is better to reply to this thread than to start a new one. When you do, I get an automatic e-mail directing me right to your reply so I'm sure to see it. I don't always check the Ford section unless I'm caught up with other stuff and I'm getting bored. It's just by luck that I came across this message. I don't want you to be ticked off if I don't reply.



Ernest Patton

September, 26, 2011 AT 11:27 PM

I was trying to find anyone else who has had this problem with side curtains(air bags)deploying for no reason. My daugher was driveing our mercury mountaineer and the air bags deployed for no reason. Our mountaineer has not been in a wreck. The insurance said they can't cover to fix the liner or the air bags because she didn't have wreck.I talked to Ford the said the moutaineer has over the warranty on miles so there is nothing they can do about it also. I'm just trying to see if anyone else has had this happen to them? If this has happen to you what did you have to do to get this fixed?



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