Switching Off an Airbag

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.