Tips and techniques for supplemental restraint system service

Feb. 1, 2017
Let’s discuss the issue of fixing a vehicle that develops a problem and illuminates the SRS warning light, but has not been in an accident.

Keeping vehicle occupants safe during a collision is major concern for manufacturers and generates enormous costs for car companies in the area of crash worthiness testing and SRS system engineering. Vehicle collision restraint systems have undergone steady improvements over the last several decades since their introduction in the 1970’s. All passenger vehicles built after Sept. 1, 1998 were required to have driver and passenger front airbags. As system improvements continued, the number and location of inflatable restraint bags has grown considerably and modern vehicles can have 10 or more pyrotechnic charges around the vehicle to inflate airbags or tension seat belts or even disconnect battery cables in the event of a collision. This article will focus on SRS system service which can be somewhat intimidating to a service technician with little or no SRS experience or training. But fear not, SRS system repair can be done safely and profitably if basic caution and some common sense is followed.

A lot of letters

Let’s start by mentioning that SRS will be used in this article as an all-encompassing acronym for airbags, seat belt pre-tensioners and occupant detection systems. There is no shortage of acronyms for SRS systems and different manufacturers use different names for the same components. The airbag control module is called an SDM (Sensing and Diagnostic Module) by GM, an RCM (Restraint Control Module) by Ford, or an ORC (Occupant Restraint Controller) by Chrysler so just deal with the issue and we’ll move on. Most often a vehicle with an illuminated airbag light will have trouble codes stored to help identify the area of concern and point the technician in the right direction. Most of my SRS experience is in fixing codes in a vehicle or getting the warning light to go out after post-collision repairs are made by a body shop. I will not be discussing proper post-collision service procedures because this is usually done by the body shop. Often times a body shop will bring in a vehicle after repairs are performed because the SRS warning light remains on and they do not know how to clear the system, or they do not have the necessary scan tool to perform code clearing or re-learn procedures that must be done.

Let’s discuss the issue of fixing a vehicle that develops a problem and illuminates the SRS warning light, but has not been in an accident. Most of the problems I have seen are either inflator circuit issues or crash sensor problems. The good news is the inflator circuits are just twowire circuits that either develop high or low resistance faults and are not too difficult to diagnose. The SRS module constantly monitors each airbag or seat belt pre-tensioner pyro-technic circuit for the proper resistance to ensure the circuit will work if needed in the event of a collision. The normal resistance in these circuits is around 1-3 ohms. The scan tool capture from a 2007 Chrysler Sebring shown in Figure 1 lists many codes that were set when the vehicle was in a collision.
Figure 1

There are many codes for open circuits in the various inflator circuits. Chrysler calls the airbag inflator charge or pyrotechnic a “squib”, but it’s just another variation in naming preferences. If an airbag inflates the inflator becomes an open so the module now codes for an open circuit. Keep in mind that most SRS modules can only fire an airbag once, so if an airbag deploys the SRS module must also be replaced. This should be covered in post collision repair procedures but many body shops do not realize this and send the vehicle over to clear the warning light only to be told the SRS module must be replaced. If a vehicle comes in with a low or high resistance inflator circuit code the technicians job is to determine if the fault is in the wiring or the inflator (airbag or seat belt pre-tensioner charge). This will require gaining access to the component connector which may involve removing the airbag or any necessary interior trim to access seat belts or roof mounted airbags. Once you access the connector you can substitute the correct resistance into the circuit, clear the code and see if it returns after cycling the key. I have found it very helpful to keep handy in my toolbox ¼ watt resistors in 1, 2 and 3 ohm values to use as substitutes for inflator modules to test circuits. You can purchase SRS load tools as seen in Figure 2 to substitute the removed airbag inflator but when you service many different makes this is way too large an investment to cover all models.

Figure 2

You only need to simulate the correct resistance in the circuit to see if the code clears or changes from an active code to a history code. If the same code returns, then the problem is in the wiring from the SRS module to your test point. If you are working on a driver’s side airbag inflator, you can disconnect the wiring to the steering wheel clockspring and connect the resistor there. If the code does not return then the clockspring is defective, a pretty common problem on many vehicles. Figure 3 shows a picture of a resistor inserted into the connector of a seat mounted side airbag to simulate the airbag inflator.

Figure 3 This type of circuit testing can be simplified even more when the inflator connector has a shorting bar in the connector. Many SRS system connectors have shorting bars that short the two harness wires together when the connector is dis-connected. This is done to prevent accidental airbag deployment. If there is a high resistance code set for the deployment circuit and you dis-connect the inflator and a low resistance code is set due to the shorting bars in the connector, then you know the circuit is good and the high resistance is the inflator module. The picture in Figure 4 shows a replacement SRS system wiring connector body before the wire terminals are inserted and the shorting bar is easy to see.
Figure 4

This leads to one of the more common problems I have seen when a body shop has done post collision repairs. Many SRS deployment loop inflator connectors have small connector pin assurance (CPA) locks which have to be unlocked to disconnect the connector. Often technicians loose or break these plastic locks and then a low resistance or shorted circuit code sets because these locks also release the shorting bars in the connector. These may be difficult to find and are not available separately so be careful when disconnecting inflator module or seat belt pre-tensioner connectors.

The code scan seen in Figure 5 is from a 2013 BMW 528i that was brought in by a body shop to have the replacement airbag module programmed. After successful programming the SRS light remained on and the code causing the light is shown.

Figure 5
Figure 6

The code is for low resistance in the “Safety Battery Terminal”. This BMW has a pyrotechnic charge at the positive battery cable clamp to open the battery cable in the event of a collision. The body shop has replaced the battery cable but the technician left the connector lock out so the connector shorting bar has shorted the wires together and is causing the code. The shop found the connector lock and once re-installed the problem was fixed. Shown in Figure 6 is the battery cable with the missing connector lock.

Real-world issues

These are all very common SRS service problems. During research for this article I spoke with Bob Heipp, a lead mobile diagnostic technician working at Mobile Auto Solutions located in Chicago, Illinois. Mobile Auto Solutions has seven service techs and services body shop and repair shop clients throughout Chicago and the suburban area. Bob stated about 75 percent of their business is with body shops so Bob has seen it all when it comes to SRS system repairs. Mobile Auto Solutions services Domestic, Asian and European vehicles so Bob has a wide range of experience. When asked, what are the most challenging SRS diagnostics, Bob said without a doubt that man- made problems top the cake. Wrong parts installed, connectors crossed or poor wiring repairs can take the most time to sort out. His most common issues are what was shown above, connectors not mated properly or missing CPA locks causing airbag lights to stay on. Bob also pointed out the most important items to have for SRS service are the correct scan tool, usually a factory tool, and good service information. With these two items and some resistors Bob can handle almost any SRS issue. Module programming, code clearing, and Occupant Classification System seat weight calibrations are the most commonly performed jobs and constitute 40-50% of the jobs he performs each day. Bob felt a technician needs a good understanding of electrical system diagnostics and the ability to follow service procedures laid out in service manuals along with proper tooling to be successful servicing SRS systems.
(Image courtesy of Mitchell 1) Figure 7
(Image courtesy of Mitchell 1) Figure 8

The deployment loops are the output side of the SRS control module. The crash sensors are the input side of the SRS module and they fail as well. Armed with a scope it is possible to look at the signals, or data that these crash sensors send out and determine if the sensors have failed. In my experience, when a crash sensor fails the module will properly code the offending sensor and the sensor itself is usually the bad component. Many frontal crash sensors are mounted in harm’s way and in the Midwest where my shop is located there are always issues with corroded wiring and terminals. These failures are often found with a simple visual inspection. It is interesting to note here that some manufacturer’s such as Ford still use airbag warning light “flash” codes which can be very helpful. A 2006 Ford Freestyle came in with the airbag light illuminated. A scan test with the factory Ford IDS scan tool revealed a code B2296, “Front Impact Sensor Circuit Failure” stored. Unfortunately, the code description does not reveal which crash sensor is at fault. The wiring diagram seen in Figure 7 shows there are 6 impact sensors on this vehicle. Code B2296 covers multiple flash codes as seen in Figure 8.

When service information was accessed, the diagnostic test menu showed the LFC or “light flash code” table as seen in Figures 9 and 10. The table reveals that light flash code 42 is for the left front crash sensor mounted behind the grille, code 42 was the light flash code this Freestyle output.

(Image courtesy of Mitchell 1) Figure 9
(Image courtesy of Mitchell 1) Figure 10

This seemingly simple issue illustrates the need for following service information and using all the resources available. Much time can be wasted trying to determine which sensor is responsible for the code when all the while the blinking airbag light can point you in the right direction.

The waveforms seen in Figures 11 and 12 were captured from a 2005 Chevy Silverado with a stored SRS code for the left hand front crash sensor. Figure 11 is the key on capture of the failed sensor and Figure 12 shows the right-hand sensor that is working properly. The difference is obvious. Also, note the very low voltage scale needed to see this activity. I should mention here that correct terminal orientation is critical. The correct wire must go to the correct terminal to the crash sensors and often times the harness may have been repaired after a collision so don’t overlook wires being crossed if you are diagnosing crash sensor codes after collision repairs. Also, remember that incorrect parts can really make you chase your tail and these are man-made faults so verify the vehicle you are working on and the correct part numbers of replacement parts. One last item is crash sensor mounting. Make sure the mounting location is clean and the bolts are properly torqued to prevent comebacks or codes resetting.

Figure 11
Figure 12

The second issue I want to mention is SRS module replacement and set-up. This is a common procedure because most cars require the SRS module is replaced after airbag deployment. This is where the correct factory scan tool is really important. Some modules require only a configuration or setup procedure and some modules will require programming to be performed so check service information before agreeing to take in a car so you are certain you have the capabilities to complete the repair. Most vehicles with a passenger front airbag will have some form of occupant classification system that can detect whether or not there is a person in the passenger seat during a collision event. This system will prevent passenger side airbag deployment if the passenger seat is un-occupied. This system can save a lot of money in post collision repair costs. Often times after a collision occurs, the passenger seat weight measuring system will need to be re-zeroed or calibrated. If this is not performed a code will be set in the main SRS module and the dashboard airbag warning will remain on. Procedures to perform this function vary greatly among manufacturer’s so consulting service information is necessary before beginning. The pictures in Figures 13 and 14 show the weight set needed to perform a calibration on many Chrysler vehicles. This 3- piece assembly will simulate a rear facing infant seat, (RFIS), six-year-old child, and a fifth percentile woman weight levels and are installed on the passenger seat one at a time while the scan tool performs the calibration. Without this special tool, you should not attempt to perform the calibration.

Figure 13
Figure 14

One issue that is somewhat common is SRS modules being replaced on Ford vehicles without following Ford’s scan tool procedure called PMI, or Programmable Module Installation. The PMI procedure which should be performed when replacing most modules on a Ford product involves having the factory IDS scan tool copy or “inhale” the module configuration files and reinstall or “exhale” the file in the replacement module. If this is not performed you have two options, first you could reinstall the original module and do the PMI, or using the scan tool write in the original “as-built” data for the module. The IDS scan tool may have the as-built data on the computer’s hard drive or you can find it on the Ford service website at https://www.motorcraftservice.com/QuickGuides/QuickGuides. Another problem I have seen is on Hyundai\Kia vehicles when a replacement SRS module is installed and there is still a failed component on the vehicle. Installing a new SRS module on these newer vehicles requires something called “variant coding” which tells the module the SRS components that are installed on the vehicle. If there is a bad component such as a deployed seat belt pre-tensioner, the variant coding will fail without any indication as to why. Because the replacement module is not coded, it cannot set fault codes but with the factory GDS scan tool you can get a screen as seen in Figure 15.

Figure 15

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