Airbag and seat belt detonator assemblies don’t use a separate removable resistance sensors, 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 potentiometer (pot’) in place of the actual airbag assembly in order 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 or the module itself.
I typically use 0 – 25 Ohm range pot to test airbag systems. You only need two of 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 Ohms. There are some that are as high as 7 Ohms but again they are rare. Either way you can dial your pot from 1 – 10 Ohms, stepping in 1 Ohm increments to cover your bases if the bag resistance value is not known.
Figure 1: Equivalent Airbag Circuit
Many of the newer vehicles offer airbag module scan date that will read the actual circuit resistance values up to a programmed limit, see Figure 2. Having scan date 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 mounted side airbag assembly circuit. Perform a wiggle test on all suspect harnesses.
Figure 2: Scan Data
Try graphing the resistance values while wiggling or manipulating the 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 ohms or “Open”. Most of the time the scanner will read around 25 ohms when in fact the circuit is open. This is simply due to the Airbag modules software programmed limits.
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. Install the meter in place of the airbag module to test the entire harness.
When installing your pot’ in place of a removed airbag or seat belt detonator you must either consult the vehicles wiring diagram or visually inspect the connector you are to install your pot for the presence of connector “Shorting Bars”. See Figure 3. Shorting bars are a safety devise that connects or “shorts” the airbag terminals together when the connector is seperated. This is a safety devise employed to prevent an open harness connection from simultaneously being shorted to voltage and ground accidentally deploying the airbag. If the connector half of which you are connecting your pot’ has a shorting bar assembly then you must first carefully disengage the shorting bars from the connector terminals before making your measurements. I use flat plastic toothpicks for this purpose. The old cut off ends of plastic zip ties also work well. If the shorting bars are left in their default “open connection” shorted position then the circuit resistance will read almost zero ohms due to the mechanical short. This is obviously not a true resistance value of the circuit.
Figure 3: Shorting Bars
Never bypass shorting bars while the airbag unit is still attached to the harness. An accidental airbag discharge causing severe personal injury is always possible.
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. Typically the only time you 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.
If you do determine that an airbag connection has slightly high resistance by just a few Ohms I have had good success applying a product called “Stabilant 22A” liquid improver to connectors. Just a tiny drop applied with a wooden toothpick cures many intermittant high resistance connection problems.
As always, make sure the airbag system “proves out” after