There are a few reasons why you might want to
switch off an airbag.
The most important is if you have a young child
riding in a front seat of an airbag-equipped car. It’s not generally stated but
any child under the age of about 12 should not ride in the front seat of an
airbag equipped car. That’s not something you’ll find trumpeted by car companies
but if you talk to engineers involved in crash testing, they’ll quietly make the
point that anyone smaller than about a typical 12 year old will be badly placed
should the airbag inflate. The primary problem is in the location and height of
the head. It’s for this reason that some cars have as standard a passenger side
airbag deactivation switch.
Another reason that you might want to disable an
airbag (permanently in this case) is if you fit a new seat to a car that
originally had airbag equipped seats. In the past we’ve covered a modified car
where this occurred: the driver had a disability and the standard seat was not
appropriate for his needs. A new seat was needed, and it didn’t have airbag
capability. Of course, he could have just unplugged the side airbag, but that
would have caused an airbag system fault light and probably prevented the other
airbags from inflating in a crash.
So how do you switch off an airbag without
triggering a fault code?
Information
Firstly it is absolutely vital that you have
access to the full manufacturer’s workshop manual. This is the case for two
reasons: (1) the manual contains safety information including the
appropriate measures that need to be taken before working on the airbag system,
(2) the electrical characteristics of that system need to be known if the airbag
is to be disabled without a fault code being logged.
In many car modifications, technical systems (eg
the engine management system) can be modified without full information being
available. The modification described in this story does not fit into this
category!
In the case of the Honda Insight on which the
airbag disabling modification was undertaken, the primary ‘safing’ procedure is
to disconnect the negative terminal of the battery and wait 3 minutes for the
internal energy storage capacitors to discharge.
Airbag System Sensing
The airbag Electronic Control Unit (ECU) in all
cars is designed so that it can sense when there is a problem with the integrity
of the wiring leading to the airbag, or the airbag itself.
The ECU obviously can’t fire the airbag to see if
it works, so it looks instead for two main fault conditions: the airbag is
showing zero resistance (ie there is a short circuit across the airbag and its
wiring), or there is infinite resistance across the airbag and its wiring (ie
the wiring or the airbag has a break).
So, in between the values of infinite and zero
resistance, there must be a resistance that the ECU sees as correct. In the case
of the Honda, this is 2 ohms. This value can be deduced from the fact that, as
shown in one of the workshop manual fault-finding sequences, when testing the
integrity of the wiring leading to the airbag, the airbag is unplugged and a 2
ohm resistance substituted. If the fault code disappears, there’s no problem
with the wiring between the 2 ohm resistance and the ECU.
However, in some workshop manuals that we have
looked at, such information isn’t readily apparent. That is, a substitute
resistance is not clearly stated. The apparent solution would be to measure the
airbag resistance directly with a multimeter. But be very careful if doing
this! If the procedure of measuring airbag resistance with a multimeter is not
shown in the workshop manual, don’t do it! This is because multimeters test
resistance by applying a current to the tested object and then measuring the
voltage drop. In other words, when you measure resistance with a multimeter, you
are applying some power to whatever you are measuring. In the case of an airbag,
if that current is too high, the airbag may inflate. The Honda workshop manual
states that the test multimeter should output a current of less than 10
milliamps on the lowest value resistance scale; however, it also states that you
shouldn’t measure airbag resistance directly.
So why do you want to know the resistance the
airbag poses to the ECU? The answer is that if the airbag is disconnected and a
substitute resistance supplied instead, the ECU won’t see any problems with the
system.
Substituting a Resistance
If the resistance that the airbag normally poses
to the ECU is known, a double pole double throw (DPDT) switch can be used to
switch the resistance in as the airbag is switched out.
If following this route, maintain the correct
polarity of the airbag and ECU wiring; that is, make sure that when the airbag
is connected to the ECU by the switch, the polarity of the connections remains
unchanged.
The appropriate resistance can be created by using
a normal electronics ¼ watt resistor (which will probably then burn out should
the airbag inflation signal be sent during a crash) or by using 5 watt or 10
watt resistors (which will probably withstand the current burst).
Switching to select or deselect the airbag should
only be done with the ignition off, and as a matter of course, no-one should
be positioned in front of the airbag whenever the switch is operated.
The wiring diagram for the modification is
deliberately not shown here: if you don’t have the skills to wire in a DPDT
switch, we don’t want you doing it!
Switch
A good quality switch must be employed. This is
because when the airbag is switched on, the integrity of the switch is important
in making sure the airbag fires in a crash. And, when the switch is in the 'off' position, you need to be sure it is in fact off. Switches with silver-plated
contacts are available from suppliers like RS Components and Farnell; we don’t
suggest you use the cheap switches available from more common electronics
retailers. A good approach is to use a key-operated switch. Don’t use a switch
designed for just small currents – a 5 amp rating should be a minimum.
The best approach is to attach all the wires to
the switch on the bench. Use a multimeter to measure continuities and
resistances, making sure that when the switch is in one position, the
appropriate substitute resistance is shown on the ECU connection wires, and when
the switch is in the other position, the ECU wires are connected to the airbag
wires. Make absolutely certain the ‘airbag on’ and airbag off’ switch
positions are clearly marked.
Wiring Connections
As shown in
Airbag Safety & Wiring Repairs, special crimp terminals
are available for when working on airbag wiring. However, at the end of the day,
the wires are just insulated copper wires so other, more traditional, techniques
can also be employed. If soldering, make absolutely certain that the wiring
cannot flex anywhere near the solder joint, where it will be more brittle than
elsewhere.
Note that even with the airbag on/off switch in
place, both short and open circuits will still trigger the fault warning light
as it did with the system standard.
Conclusion
The warnings contained in this article should not
be underestimated or ignored. However, the actual modification is quite simple:
it’s just not something that you can afford to get wrong.
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