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Have you ever measured resistance on an airbag with DVOM

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4 years 1 month ago #37811 by Dylan
There seems to be a lot of controversy about measuring resistance with your ohm meter on an airbag. Manufacturers don't want you to do this. I recently saw a video from Ivan (Pine Hollow) and from Leon (Bodgit & Leggit) where they both perform this test. And some people are freaking out in the comments :) Personally I've done it. I don't believe the very small current from your DVOM can set the airbag off. Now I admit I usually just use a potentiometer or a fixed resistor to 'replace' the airbag unit. But how many of you have measured resistance on an airbag?
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4 years 1 month ago #37812 by Tyler
Never. Always wanted to.

I did put a scope on a pretensioner circuit before! Got a lot of flak from others for that, too. :silly: Oh well.

Speaking of the ohm meter, has anyone measured how much current one generates? :huh: I feel like Paul may have at one point...
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4 years 1 month ago #37814 by ScannerDanner
I see know need to, to be honest. Substitute the bag in question with a resistor of equal value and if your code goes away, the wiring and computer are fine.
On a separate note. I've recently learned the hard way to NEVER rotate an airbag module with the key on. Smh
The rollover sensors are in the module and it blew the side curtain bags. Yes, I have it on video too

Don't be a parts changer!
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4 years 1 month ago - 4 years 1 month ago #37829 by Dylan
The same thing has happened here. A guy from one of the dealerships here in Belgium rotated the airbag module with key on and blew a few airbags. Definitely need to be aware of that.
I'd love to see the video Paul :)
Last edit: 4 years 1 month ago by Dylan.
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4 years 1 month ago #37831 by Dylan

Tyler wrote: Never. Always wanted to.

I did put a scope on a pretensioner circuit before! Got a lot of flak from others for that, too. :silly: Oh well.

Speaking of the ohm meter, has anyone measured how much current one generates? :huh: I feel like Paul may have at one point...


I'd also like to know how much current the ohm meter generates. I think Paul has mentioned in one of his videos it's in the milliamp range.
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4 years 1 month ago - 4 years 1 month ago #37837 by Noah
I've done it, didn't bite me :)

I didn't know about the module thing though! I'll be cautious when it comes to those from now on.

"Ground cannot be checked with a 10mm socket"
Last edit: 4 years 1 month ago by Noah.
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4 years 1 month ago - 4 years 1 month ago #37840 by Andy.MacFadyen
A modern Digital Multimeter on ressistance won't set an airbag off --- air bag or pretensioner squib is basically like a tungsten light bulb fillament of two ohm resistance.
The plug connector on an airbag has a shorting bar so if you try and measure the resistance unless the the shorting bar is lifted clear or put out of action the airbag or pre-tensioner squib with read a dead short. Substituting 2 ohm resistor is still probably the easiest test.

My experience sugests where there hasn't been a crash impact 95% of air bag faults are due to bad contacts in connectors, 4% are down to water ingress into the impact sensors.

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



Last edit: 4 years 1 month ago by Andy.MacFadyen.
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4 years 1 month ago #37842 by Andy.MacFadyen
I figured out how to roughly measure the current from DVOM measuring the resistance of an airbag my guestimate from rough working it will be less than 1ma I will do it over the weekend to find out.

The other question is how much curent is needed to set off an airbag squib

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4 years 1 month ago #37845 by JeffBirt
The problem comes down to what sort of multi-meter is some one using and are they using it correctly. A good quality basic DMM can source very little current in ohms mode. If it happens to be a meter that can measure very high resistances it can source more current. If you accidently put it on the diode check function then it might source 10ma or more. If you have a 'who-flung-dung' meter from Alibaba who knows what it might actually do? Most likely it would be OK but given the quality standards you just don't know.

Then, what is the cost of the failure you might cause vs. the value of the time saved? There was one piece of equipment I worked on where you needed to clean an optical sensor when doing a service. Getting to the sensor by the book took 20-30 minutes. But, you could bend a metal flap slightly, pull it out and have access to the sensor; which took about 20-30 seconds. If you slipped you could damage a $200 part though. Given the time saved doing several of these jobs a week it was worth the risk but if it was a potentially very expensive part (like airbags) it would not have been worth it (IMHO).
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4 years 1 month ago - 4 years 1 month ago #37853 by Andy.MacFadyen
Okay going back to basics the basic input circuit on a DMM has in voltmeter mode has an AC impedance & DC ressistance of 10 miillion ohms (NB not a Power Probe which has a much lower ressistance).


From what I think reliable sources the amperage used for firing off a single air bag pyrotechnic is 1 to 3 amps ---- which implies they should fire reliably at as low as 2 volts.

Measuring my varied collection of meters testing the open circuit voltage accross the test leads on the ohms scales none showed more than 300mv open circuit voltage.

Putting 3 ohm load on and the voltage accross the load dropped below anything I could measure with any of my meters and current was measured in all cases at less than 0.3ma

So know your DMM and make your own judgement at your own risk --- I would not advise using a Power probe or old style analogue meter (aka an "AVO" meter) Having said all that airbag pyrotechics are dangerous and substituting a 2 ohm ressistor is still the weapon of choice.

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Last edit: 4 years 1 month ago by Andy.MacFadyen.
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4 years 1 month ago #37861 by Dylan
Thanks for the feedback Andy. Wow less than 0.3 mA that's even way lower then I expected.
Totally agree here. I do NOT encourage anyone to measure resistance on an airbag. Even if the chance is pretty small for the airbag to deploy and there's a shorting bar to bypass don't risk it. A loose airbag unit that explodes is lethal.
We used to set off airbags at the Toyota dealership after replacing them because of some TSB. Can't remember what the issue was. But man the first time I did that I'll never forget. I squeezed it very tight in the vice, took some wires and a battery. The thing wouldn't even go off just by putting battery voltage to it. So I started pulling some sparks at the battery and that was the loudest bang I've ever heard. Boss ran into the shop. Poor guy thought his garage was gone. I'm laughing now but I wasn't at the time :lol: Should've taken it outside and blow it on the roof.
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4 years 1 month ago #37893 by Noah
We play with them in the junk yard. I sent a 55 gallon drum 30 feet in the air with one.
I know it was at least 30 feet because it landed on top of the car stripping tent and bent one of the supports...

"Ground cannot be checked with a 10mm socket"
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4 years 1 month ago #37913 by Andy.MacFadyen
In Europe airbags are less powerful because wearing seat belts is mandatory and fairly strictly enforced in most country.

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4 years 1 month ago #37936 by Noah
Seat belts are mandatory in every US state except New Hampshire with various degrees of enforcement. In some states it is a primary offence, meaning if spotted you can be ticketed, while in other states it is a secondary offense. So you would have to be stopped for another offense and then subsequently be fined for not wearing a belt.

"Ground cannot be checked with a 10mm socket"
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1 year 2 months ago - 1 year 2 months ago #59664 by scopeman
is a resistance decade box ok to use to diagnose airbags.cheers
Last edit: 1 year 2 months ago by scopeman.

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1 year 2 months ago #59676 by juergen.scholl
The shown resistance box seems to be capable to increments of 0.1 ohm. Most of (good) airbag circuits show resistance values with a scantool of ~2 ohms, give or take.

So the resistance box is suitable for checking out the air bag circuit.

An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
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1 year 2 months ago #59693 by scopeman

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11 months 3 weeks ago #60865 by George Uk

I see know need to, to be honest. Substitute the bag in question with a resistor of equal value and if your code goes away, the wiring and computer are fine.
On a separate note. I've recently learned the hard way to NEVER rotate an airbag module with the key on. Smh
The rollover sensors are in the module and it blew the side curtain bags. Yes, I have it on video too

WOW !!

Never knew or thought that could happen, for sure never mentioned in the BOSCH (UK) Airbag Training course. In their (Bosch) defence they do teach you to handle Airbags with Ign OFF & Battery Disconnected and allow time for Caps to discharge.

Im so glad I read this - Lesson Learnt, Disaster Averted !!

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