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 two wire 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.
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.