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quico

Diy: Disable 450 Passenger Airbag Without Error Light

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Posted (edited) · Report post

Hi all,

First of all, let me introduce myself.

I'm from Portugal, and my mother in law owns a 2004 Smart fortwo Pure 450 model, petrol.

This EU model only has 2 airbags (driver & passenger), but doesn't have a passenger airbag deactivation button.

I found this forum because I needed to turn of the passenger airbag because my 8 month old son stays with her during the day, and she could not transport him on the car because the car doesn't have an airbag deactivation button. But I wanted to do it in a way that no error lights appear on dashboard, and therefore, the driver airbag keeps working.

If the airbag error light is ON, none of the airbag works.

In the Mercedes dealer, they wanted me to buy a "special" chair that comes with a module for disabling the airbag.

Well, I already bought a chair after a big research online for security tests and recomendations in security from specialties technicians, and don't feel like buying a 400€ Mercedes brand "weaker" chair just because they "demand".

I bought one of the most secure chairs for rear facing (Britax Max Way), and safety is first for me!

I found a topic here regarding disabling the side airbag with a resistor, and I registered to the forum in order to ask here if someone knew the exact ohm value for the passenger airbag.

Did a few more researches regarding this issue, and most of the times I read about a resistor between 2 ohm and 3 ohm was generaly what people use to disable airbag without error light.

But since the registration process required an activation from the admins, and it took some time to someone activated my account, and I had the available time on saturday, so I gave it a try on my own risk.

Well, just opening this topic to explain how I did, and it's now working fine, no error light and driver airbag working fine.

Things you'll need:

a) Multimeter (or voltmeter, not sure how you call it)

B) 2.2 ohm 1w resistor with 5% tolerance (strip code: red|red|gold|gold)

c) car type 3 amp fuse it's probably best to use a 1/2 amp fuse in order not to delay the driver's airbag deployment in case of a collision, but be careful to select a different resistor to achieve a total resistance of 2.6 ohm because de 1/2 amp fuse has more resistance than the 3 amp one. Probably a 1.7 ohm resistor combined with a 1/2 amp fuse, but always check the resistance before proceding.

Posted Image

d) some wire

e) soldering iron

f) terminals to connect to fuse

Posted Image

g) heat shrink tube

How to do it:

1) Cut about 2 x 15 cm of wire.

2) Connect one side of each wire to the terminal, and insert the terminals on each side of the fuse

3) On the end of 1 wire, solder the resistor.

4) On the end of the other wire, solder a pointy metal thing, such as piece of a paper clip.

5) Seal all connections with the heat shrink tube.

You'll end up with something like this:

Posted Image

With the multimeter, test for continuity. Bend and stress the wires a little while doing this, so you have absolutely sure that every connection is stable and never lose continuity.

If your gadget passes the continuity test, change the multimeter to resistance test.

With everything put together, you should get a total resistance of about 2.6 ohm

Ok, let's pass on to the car.

On the passenger side, lift the carpet, disconnect the battery terminals. You should wait about 15m after you disconnect the battery before removing the airbag connector.

Remove the 3 screws of the plastic piece immediatly bellow of the passenger airbag. This screws are on the bottom side of the plastic piece. Remove the 4th screw that is on the small coin tray that sits on the outside corner of that plastic piece.

Locations:

Posted Image

After you remove all screws, pull towards you the plastic piece, and bend it slightly down so you can unveal the yellow airbag connector.

Remove the connector. You can see that there are 2 small holes, about a needle wide in the connector. The smart connector is exactly like this, except in the colour. This one is orange, the smart's is yellow:

Posted Image

Insert the 2 pins of your gadget on the connector's holes.

Place some isolating tape over and around the connector and the wires so you make sure that it stays in place.

Plug the battery terminals and turn the car on.

The light should light up for diagnostic, and turn off just as it would do before you unplug it.

Sucess!

Best regards, and sorry for my english!

Edited by quico

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I have a concern with the size of the fuse. At your 2.6 ohms resistance, if an actuation is called for you will be passing a bit over 5 amps. The 3 amp fuse will blow, but could take some noticeable time, and that could delay activation of the other airbags. I would think a 1/2 amp fuse would be better. The monitoring current is really low, well under 50 milliamps as a rule so no worry about stressing the 500 milliamp fuse.

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I have a concern with the size of the fuse. At your 2.6 ohms resistance, if an actuation is called for you will be passing a bit over 5 amps. The 3 amp fuse will blow, but could take some noticeable time, and that could delay activation of the other airbags. I would think a 1/2 amp fuse would be better. The monitoring current is really low, well under 50 milliamps as a rule so no worry about stressing the 500 milliamp fuse.

Yes, I believe you're quite right.The objective of the fuse is to blow in case of an airbag deployment, so that the driver's airbag could work.In the electronic's store, when I bought all the items, they didn't had a 2 or 1 amp fuses, but instead they told me a 3 amp "fast blow" fuse will to the trick.Honestly, my electronics knowledge is very very limited, but I am a basic and curious DIY'er, but is true that people who make this mod, use 1 ou 2 amp fuses the very best, so I will buy what you say and look for a 1/2 amp fuse for a future upgrade of this mod.Will the replacing of the 3amp fuse for a 1/2 amp fuse change the overall resistance?Thanks!

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Posted (edited) · Report post

:welcome: Thanks for the write-up, quico. Your English is very good.

I did this about a month ago. I use it at the connection under the passenger seat instead of the dash bag, so I can leave the seat out of the car. But I can see that there are more people who would deactivate the dash bag for the same reason as you! In that position, do the wires need to be longer like yours, or could it be more compact like this one?

Posted Image

Bil :sun:

Edited by bilgladstone

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Will the replacing of the 3amp fuse for a 1/2 amp fuse change the overall resistance?

It will increase it it slightly. Actual fuse resistance for a 1/2 amp should be around 600 milliohm, compared to only 23 for your 3 amp.

I don't know what the tolerance on the airbag monitoring circuit is before a trouble code.

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:welcome: Thanks for the write-up, quico. Your English is very good.

I did this about a month ago. I use it at the connection under the passenger seat instead of the dash bag, so I can leave the seat out of the car. But I can see that there are more people who would deactivate the dash bag for the same reason as you! In that position, do the wires need to be longer like yours, or could it be more compact like this one?

Posted Image

Bil :sun:

I think that it's irrelevant and it's a matter of choice of each person!

In your photo, it seems like the resistor is connected directly to the fuse.

In my case, since I don't have much experience soldering, I choose to make it step by step.

1) connect the wire to connector;

2) insert the connector in the fuse;

etc etc

It's possible to cut steps, but I build it the way I thought it was correct.

The only thing that possibly changes, is that in my case, the extra wiring may increase by 0,01 ohm the total resistance value.

I'm saying this because when I measured the resistance of the resistor, I was getting 2.4 ohm, and when I measured the resistance of the final product, I ended up with 2.6 ohm.

Also, the fuse adds up a little resistance.

BTW, the photo I attached is not mine, and the fuse and resistor used is not the same that I used.

Also, this build in the photo is just an example of a user that changed the default steering wheel for an OMP aftermarket, and used this to "cheat" the ECU. He used 1.7 ohm resistor and 1 amp fuse.

It's from another forum where I researched about this issue and one of the various forums I read to get inpiration to make this mod:

http://www.smartz.co.uk/showthread.php?142...ssable-question

Thanks for the welcomes guys!

Kind regards

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Posted (edited) · Report post

Will the replacing of the 3amp fuse for a 1/2 amp fuse change the overall resistance?

It will increase it it slightly. Actual fuse resistance for a 1/2 amp should be around 600 milliohm, compared to only 23 for your 3 amp.

I don't know what the tolerance on the airbag monitoring circuit is before a trouble code.

Hum, in that case, in order not to have the risk of just changing the fuse and KABOOM, error code on because of resistance too high (and a trip to the MErcedes to reset the error light), maybe the way to go is a new build with a 1.7 resistor, 1/2 amp fuse, plus wiring, that should get me just about the 2.6 ohm value... I hope... LOL

=)

Thanks!

Edited by quico

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Just improved a little the DIY, and changed the instructions so that someone that want to do it, uses a 1/2 amp fuse...Glad to help!Kind regards

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Excellent! Thank you for the additional images showing the location and type of this connector!Next step would be to devise a way to switch between the two states without removing the panel and plugging/unplugging the bypass. Apparently smart makes a twist knob or keyed switch, that somehow accomplishes this by recoding with their Star equipment. Bil :sun:

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Excellent! Thank you for the additional images showing the location and type of this connector!Next step would be to devise a way to switch between the two states without removing the panel and plugging/unplugging the bypass. Apparently smart makes a twist knob or keyed switch, that somehow accomplishes this by recoding with their Star equipment. Bil :sun:

Yes, the switch would be golden, however, the first thing I did was contact a Smart dealer here in Portugal, asking how much would cost me to install an airbag toggle switch in the 450 model.The response was "In your model, is not possible to install the switch. The 2 options are:1) Either you buy the smart chair with the device that switches of the airbag: Cost 400€;2) Or you can come here, sign a term of responsability and we'll disconnect the airbag for you, error free: Cost 75€;"The 75€ option must be pretty similar to this procedure. Just waaaayyyy mooooooreeeeee expensive.I spend about 3€ in all the items that I used.They were basicly aiming for a brutal 2500% profit! What a sneaky bastards!!!! Mercedes, shame on you!!!!I could try and install a switch, but I was afraid that it would trigger the error in the exact moment that you toggle from off/on or vice versa, because I think the airbag ECU is independant of the engine ECU. Edited by quico

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