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 our potentiometer (pot) in place of the actual airbag assembly to determine if the airbag resistance itself is out of specification or if there is a defect within the harness going to the suspect airbag circuit.
I typically use 0-25 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 2-4. There are some that are as high as 7, but again they are rare. Either way you can dial your pot from 1 to 10 ohms, stepping in 1 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 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 or 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 harness.
As you watch for incorrect resistance values on the scanner, don’t expect absolute true readings. I have never seen scan data read 100 or open. Most of the time the scanner will read around 25 when the circuit is in fact open. This is simply due to the airbag modules 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.
PAGE 2When installing your pot in place of a removed airbag or seat belt detonator, you either must consult the vehicles wiring diagram or visually inspect the connector for the presence of connector “Shorting Bars” (see Figure 3).
Shorting bars are a safety device that connects or shorts 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. 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 of the types 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 system proves out after testing and repair.
