Recent testing of airbag ignition circuits revealed a detection method we can apply to assist with diagnosis based on current flow using the TA496 current clamp https://www.picoauto.com/products//curr ... rent-clamp
One of the common SRS DTC’s surround erroneous airbag circuit resistances for side airbags fitted to driver and passenger seat assemblies
Often this can be attributed to seat/harness movement, especially where drivers of different sizes use the vehicle in question
Below we have:
Channel A connected around one of the side airbag wires under the passenger seat using the TA496
Channel B connected across the side airbag wires under the passenger seat using TA057 (Diff probe)
Channel C connected around one of the side airbag wires under the passenger seat using a K100 Milliamp clamp
Note the test above only requires the use of our TA496 current clamp and not the TA057 Differential probe or the high precision K100 clamp. These additional probes are included to reinforce the potential of the TA496 whilst also providing an indication of circuit behavior under varying fault conditions.
So, let us analyze “what a good one looks like”. Above, there are no faults, notice the passenger airbag is pulsed with current of approx. 40 mA at a rate of 5 Hz (5 pulses per second)
Pulsing an airbag with current does sound “uncomfortable” but this is essential to constant monitoring of the air bag circuit in order to detect a fault (Faults and failures in these circuits are like aircraft landing gear, they are unacceptable)
Notice in the image above I have highlighted a momentary change in the pulsed frequency which occurs every time the ignition is turned on. It would appear another pulse is inserted after the 8th pulse and this occurs after every ignition on event with a good airbag circuit. At this stage I am not sure of the relevance but worthy of noting!
So, why use voltage when we have current?
My first thought was to add voltage to indicate SRS ECU strategy when circuit resistance errors occur, however, given we now have voltage and current we can calculate circuit resistance
Using the maximum voltage values above 0.09529 V divided by the maximum current value of the TA496 (Channel A) 0.04157 A we conclude the circuit resistance of 2.292 Ω
This is to be expected as substituting an airbag with a 2.2 Ω resister is a technique we can apply to exclude the wiring during diagnosis
A word to the wise about measuring airbag circuit voltage; note how a TA057 differential probe has been used across the airbag. This technique minimizes the influence upon the circuit by the test equipment and ensures the relevant protective impedance and resistance to return a “stealth” like measurement from the perspective of the SRS ECU.
Moving onto failure modes:
Open circuit (Scan tool may display “High Resistance”): SRS warning light on
Below we have the passenger side airbag disconnected and therefore “open”. Note the current flow is 0 A as expected, however the SRS ECU pulse signal is still present at 5 Hz but now with a peak voltage of 8.467 V
High Resistance: SRS warning light on
Below our airbag circuit resistance has been deliberately increased to approx. 6.6 Ω by substituting the air bag for a resister of this value (3 x 2.2 Ω resisters in series)
Note how the voltage has increased from our good example (see image 1) to 257.3 mV and the average current is approx. 32.58 mA (I have used the average of channels A & C)
0.2573 / 0.03258 = 7.897 Ω
Once again, note the additional pulse inserted after the 8th pulse after the ignition on event! This pulse is therefore not linked to my original thought above that the pulse indicated a "good" airbag circuit!
Low Resistance: SRS warning light on
Below our airbag wiring has been shorted together with airbag still connected to the harness.
Note below the current flow continues at a peak of 37.59 mA @ 5 Hz but our voltage has dropped to near 0 V as we are now measuring the voltage drop across our shorted wires
To calculate the resistance, 0.006715 / 0.03759 = 0.179 Ω
The keen eyed will have noticed the TA496 is missing from the capture below and I am not 100% sure why! I know several months have passed since this research and this may have been part of my initial testing prior to using the TA496
Short to ground: SRS warning light on
Below we have deliberately shorted one of the airbag wires to ground at approx. 2.5 seconds
Note the voltage and current have both halted as the SRS ECU has recognized the short to ground and prevented current flow by removing any voltage differential across the airbag. The results below are applicable to either airbag wire shorting to ground
High resistance: SRS warning light off
I thought I would throw this one in as I have tried to determine the threshold at which the SRS ECU triggers a fault code for high resistance
Below our airbag circuit resistance has been deliberately increased to approx. 4.4 Ω by substituting the air bag for a resister of this value (2 x 2.2 Ω resisters in series)
Note how the voltage has increased from our good example (see image 1) to 174.1 mV and the peak current is approx. 37.59 mA (Sorry no TA496 again)
0.1741 / 0.03759 = 4.632 Ω Note, this value did not trigger a fault code or illuminate the SRS warning light!
We can therefore conclude a resistance value of between approx. 4.4 Ω and 6.6 Ω is the threshold required to trigger the DTC “High resistance”
Based on the above captures, I think we can agree how the relationship between voltage and current through an airbag ignition circuit can help with diagnosing a variety of typical faults.
If we think back to the seat airbags and how elusive they can be from a diagnostic point of view, monitoring voltage and current whilst rocking, sitting, sliding, reclining and raising the seat will capture any intermittent events that are not immediately flagged by the SRS ECU (i.e., due to response times)
Committing to the dismantling of a seat requires evidence and I believe the above techniques will help obtain that evidence
A note regarding the K100 current clamp
Whilst this clamp has exceptional stability and resolution for low current measurements (100µA to ± 4.5 A DC) its bandwidth peaks at 2 kHz which is why I have chosen to add a 2 kHz low pass filter to our TA496 clamp
Add into the mix the tiny jaw size and huge price tag of approx. £550 (not a Pico product) I think we can all agree the TA496 has done an exceptional job in terms of accuracy for a fraction of the price with 20 kHz bandwidth, 60 A peak current capability and a universal automotive jaw
All the data above was acquired from a 2002 Audi A2 1.6 FSI, which got me thinking about other vehicle manufacturers and how the above results would compare
Below we have a 2015 Honda CRV 2.2 Diesel passenger seat airbag with no SRS fault
Note the peak current is 22.04 mA and the frequency 2 Hz (Both lower than the Audi A2)
Below we have a 2013 BMW 320d F31 passenger seat airbag with no SRS fault
Note the peak current is 40.73 mA and the frequency 2 Hz
Moving now to other SRS components (No SRS faults on these vehicles)
Below we have the RH front seat belt pretensioner ignition circuit for a 2016 VW e-Golf Mk VII
Note the peak current is 40.73 mA and the frequency 2.5 Hz
Finally, back to our Audi A2 but this time the Drivers airbag at steering wheel
Note the peak current is 38.19 mA and the frequency, 5 Hz
It would therefore appear from all the above captures, the current, voltage and frequencies tend to change between Vehicle Manufacturer SRS systems and therefore, testing a number of known good and adding them to the Reference Waveform Library will help with further SRS diagnostic challenges
I hope this helps, take care.......Steve