Testing airbags

Jan. 1, 2020
Airbag and seat belt detonator assemblies don't use a separate removable resistance sensor. However, they do have an intrinsic resistance value that the airbag ECU uses to sense a defective airbag circuit. With that in mind, we can substitute a pot i
scope & scan potentiometers pots air bag airbags diagnosing vehicles vehicle diagnostics repair shop training technician training automotive aftermarket I typically use a 0-25Ω range pot to test airbag systems. You only need two of the three potentiometer leads for airbag testing: one of the end connections and the adjustable center tap as shown in Figure 1. I have not run across very many airbag systems on which any of the bags circuits measured a normal 2-4Ω. There are some that are as high as 7Ω, but again they are rare. Either way, you can dial your pot from 1 to 10 ohms, stepping in 1Ω increments to cover your bases if the bag resistance value is not known.
Many newer vehicles offer airbag module scan data that will read the actual circuit resistance values up to a programmed limit, as seen in Figure 2. Having scan data and multiple airbags on each vehicle can give us the known good airbag resistance values to compare to the airbag circuit that is setting a DTC for a resistance value circuit error. Once this good value is known, we can use our pot in place of the suspect airbag to determine if we have a bad bag assembly, a harness issue, or both.

If possible, graph the resistance value data while doing things like turning the steering wheel and adjusting the tilt column. Move seats back and forth and up and down if you are testing a seat airbag assembly circuit. Perform a wiggle test on all suspect harnesses.

As you watch for incorrect resistance values on the scanner, don't expect absolute true readings. I have never seen scan data read 100Ω or open. Most of the time the scanner will read around 25Ω when the circuit is in fact open. This is simply because of the airbag module's software programming.

If resistance values are not available in scan data, you can always use your potentiometer and an Ohmmeter set to min/max mode to do your dynamic testing. A graphing meter is even better if you have one.

When installing your pot in place of a removed airbag or seat belt detonator, you either must consult the vehicle's wiring diagram or visually inspect the connector you are using to install your pot for the presence of connector shorting bars.

Shorting bars are safety devices that connect or short the airbag terminals together when the connector is separated. It's employed to prevent an open harness connection from being shorted to voltage and ground accidentally deploying the airbag. If the connector half you are connecting your pot to has a shorting bar assembly, then you first must carefully disengage the shorting bars from the connector terminals before making your measurements. I use flat plastic toothpicks for this purpose. If the shorting bars are left in their normal open connection shorted position then the circuit resistance will read almost zero ohms due to the mechanical short. This is not a true resistance value of the circuit.

NOTE: Never defeat the shorting bar system while an airbag is still connected to that circuit. Severe personal injury can result,

Typically, shorting bars are located on the airbags themselves, on the lower steering column connector for the clock spring and on the main airbag module harness connector itself. The only time you usually would need to defeat the shorting bars is when measuring for open circuits from the top of the clock spring (with airbag module removed) and the lower connection at the base of the steering column or when checking continuity to the ECU harness from one of the airbag connections. Again, the airbag must be removed for safety before these continuity/resistance tests are performed.

When installing your test pot into female harness connections, appropriate size test terminals must be used. I have seen way too many damaged female harness terminals in my diagnostic travels to recommend anything but the correct test terminals like those listed in August's column.

If you do determine that an airbag connection has slightly high resistance by just a few ohms, I have had good success applying Stabilant 22A liquid contact improver to connectors. Just a tiny drop applied with a wooden toothpick cures many intermittent high resistance connection problems.

As always, make sure the airbag systems proves out after testing and repair.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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