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In August, I became a Smart car owner when I bought a used 2009 Smart Fortwo Passion Cabriolet. Just a few days later, however, I heard an extremely loud humming sound that caused a severe vibration in the car. It lasted for only a few seconds, so, since the car had no warranty, I hoped for the best and didn't look into it. Unfortunately, it kept happening from time to time, and I had no clue what was causing it. When it happened three times during a single trip into town, I really started getting concerned. And then, on the very next day, the hum began the second I turned the key to the on position and did not go away. I had an appointment, so I had no choice but to drive the car -- it being my only mode of transportation -- but the hum was so loud and the vibration so bad that, just five minutes in the car left me nauseous and with a splitting headache.

I was able to determine that the hum was coming from inside the car, and I thought that perhaps it was being caused by the door speakers, but then I found that it was coming from underneath the driver's seat. Some investigation online revealed that's where the factory-installed Active Subwoofer was, so I got my answer as to the culprit of the noise that had made my car undrivable.

So, of course, then came the question of how I was going to resolve the problem. Online discussions were saying that the seat had to be removed to get the subwoofer out, but that wasn't an option for me, so I went looking under the seat and found where the wires plugged into the subwoofer. With the aid of a screwdriver, I got the plug out, and blessed silence ensued.

I decided to share my experience here just in case someone else ever has the same problem. The location of the plug is under the front right corner of the driver's seat. It was clearly visible and accessible in my car, although one discussion here said that the plug is located underneath the carpeting and can't be reached unless you remove the seat and lift the carpet. I've also included a photo.

I have no plans to replace the underseat subwoofer, but perhaps, someday, I'll see about putting one under the dash, which I understand is an option.

The one question that remains is if the dealer where I bought the car was aware of the problem when they sold it to me or if the person they got the car from chose to keep hidden the fact that their car was occasionally making a dreadful humming sound. I'm considering sending an email to my salesperson to let him know about the issue.
 

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That was one scary headline! But I'm glad you figured it out!!! :)

And welcome to our cozy forum!
 
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I'm surprised the stock 451 sub could produce enough volume to be that traumatic! When I had one I could scarely tell it was there.

Glad to hear you found a solution.
 

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You could most likely replace the under seat woofer with another like it from various places on line or even here in the "Smart Parts Marketplace'. They do show up from time to time. It sounds like the amplifier section of the woofer went south and hence the loud hum....It's not worth trying to repair....
 

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Discussion Starter #7
Thanks for all the warm welcomes.

Neonspinnazz, it was a pretty scary experience when I thought that the car I haven't even had for three months had something seriously wrong with it. Realizing that it was just a speaker problem was a big relief. By the way, I had to smile at your name since the car I had before this one was a 1997 Dodge Neon, which I bought new. Yes, it had been 18 years since my last car purchase.

Mister_smart_LA: Yes, there is no doubt that the subwoofer is blown and needs replacement. What amazes me is that one that's only 4" in diameter could produce a vibration that intense.

NCC1701: It may be that not everyone would have been as affected by the sound and vibration as much as I was. I've always had very sensitive hearing. The vibration, however, was intense enough that my voice would vibrate when I talked, and music from the radio playing through the door speakers also vibrated (the music did not play through the subwoofer at all). I could touch the door and feel the vibration. So, yeah, it was quite intense.

kheran: I already figured that the speaker could not be repaired. I don't listen to music very often in the car, so laying out the money to replace the subwoofer is very low on my list. I'd also have to pay someone to take the old one out and put the new one in. I have no one to do something like that for me, and removing the seat myself is not something I could tackle on my own, especially since I live in an apartment and have no garage or carport. Now, if there was a way to get it out without removing the seat, then, when the dry weather returns, I might consider it, but there is a definite lack of room in the back for maneuvering something of that size out from under the seat, so it might not be possible.
 

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Discussion Starter #9
Portland? :)

See what the dealer sez, maybe they will replace it...

All they can say is no :(
Nope, Salem, Oregon's capital. :)

The car was purchased "As is," so I doubt that the dealer would offer to replace it, especially since it's not a serious issue.
 

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You mention removing the seat. Be sure whoever does it knows that they will be unplugging the airbag. If at any time the key is turned on (with the battery connected) the airbag warning light will come on and requires the dealer to reset it, at considerable cost. So be sure the battery is disconnected as long as the airbag is unplugged.
 

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Removing the Seat

You mention removing the seat. Be sure whoever does it knows that they will be unplugging the airbag. If at any time the key is turned on (with the battery connected) the airbag warning light will come on and requires the dealer to reset it, at considerable cost. So be sure the battery is disconnected as long as the airbag is unplugged.
Good advice.

I read online that removing the seat(s) through the rear hatch is easier and safer, rather than through the door (possible paint damage to door jam).
 

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Discussion Starter #12
You mention removing the seat. Be sure whoever does it knows that they will be unplugging the airbag. If at any time the key is turned on (with the battery connected) the airbag warning light will come on and requires the dealer to reset it, at considerable cost. So be sure the battery is disconnected as long as the airbag is unplugged.
As I said earlier, I have no plans to replace the subwoofer under the seat. It is not worth the hundreds of dollars it would probably cost to buy one and pay to have somebody remove the old one and put the new one in, especially if the seat has to be removed. It would likely be cheaper to have someone install a subwoofer behind the dash. Like I mentioned before, I rarely listen to music in my car, so having a subwoofer simply is not on my must-have list. I didn't even know there was one in it when I bought the car.

I read the post about unplugging the airbag when I was researching how to resolve the subwoofer issue, so I was aware of that. Thanks for the reply, though.
 

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When I bought my used 2008 Cabrio, the sub was making very loud random pops and sometimes long, agonized-sounding squeals for no apparent reason. The drive home from the dealer before I could disconnect it was.... Awful.
 

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As I said earlier, I have no plans to replace the subwoofer under the seat. It is not worth the hundreds of dollars it would probably cost to buy one and pay to have somebody remove the old one and put the new one in, especially if the seat has to be removed. It would likely be cheaper to have someone install a subwoofer behind the dash. Like I mentioned before, I rarely listen to music in my car, so having a subwoofer simply is not on my must-have list. I didn't even know there was one in it when I bought the car.

I read the post about unplugging the airbag when I was researching how to resolve the subwoofer issue, so I was aware of that. Thanks for the reply, though.
Mareen T you will find that there should be 4 wires going to your subwoofer. One should be red for the power to it. One should be Black for the ground. You know that both of those are working because it was vibrating. It has it's own amplifier built in so power and ground are both working. But there should be 2 more wires and forgive me but I don't know the colors of them, but they both go to the back of your radio. One of them is designated as ground and one is designated as your "hot side" input wire and that wire is broken somewhere between the connector you unplugged and the back of the radio. When that hot side input wire from the radio loses connection the amp starts humming and buzzing. So your sub woofer is probably okay it's just the broken wire to the back of your radio. You don't have to remove your seats if you decide to remove the sub. You just have to take the 4 bolts out of the seats where they fasten to the floor and lean the seat backwards or forwards enough to unfasten and remove the offending speaker. And always unhook the battery because disturbing the seat sometimes causes the air bag light to come on as you have already read. I went through that mess when I removed my seats to put heaters in them. But it's not a faulty sub woofer, just a faulty wire. DCO.
 

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Thank you! I had the same problem with my Smart 2009 (it was so loud and noisy that I couldn't drive it) and the dealership was going to charge me $500. I don't need a subwoofer in my Smart. I did exactly what you did, and now it's fixed! I'm very grateful!
 

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A big thank you to you, Maurene T. I bought an 09 for my grandaughter at an auction last year and have been maintaining it for her. She came to me last week with exactly the same symptoms . I looked everywhere, not knowing exactly what I was looking for, not knowing the car had any options. It wasn't until I read your post , saw the picture and got a flashlight that I found it. She's always thought I was a genius....now she thinks I'm a Smart-genius!
 

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My 2008 Smart had developed a noise described by amused passengers as either a 'grumbling' or 'farting' when starting the ignition, stopping it and sometimes at red lights. Mercedes thought that would be loose engine mounts, and $1700 to fix, thanks very much. I replaced the front mount for $40 parts and free labour from a mate. Not the issue. Then I thought to myself, 'that noise almost sounds a little electronic'... Guess what? Subwoofer connection removed, problem solved!
 

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I know this post is a few months old but I just purchased a used 2008 Passion with 23k miles on it (for $4K mind you :)). I thought that it had all the options that you could get - pods, driving lights, light/wiper sensor, leather heated seats, radio 10 (with silver trim). But I noticed that there isn't a sub under the driver's seat (or other seat either). I thought that if you bought the Radio 10 that it came with the sub? It does have the door speakers and dash tweeters though. Did you have to specify the sub if you wanted it when you ordered?
 
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