Stage 2 Air Bag Diagnostics

Air bags are getting “smarter” all the time. That means technicians have to work smarter to diagnose problems with air bag systems.

Accidents happen fast, which means air bags have to “happen” faster in order to prevent serious injury or even death. In most cases air bags must deploy in milliseconds in order to do their job. Not surprisingly, it takes a tremendous amount of force and energy to accomplish that.

First generation air bags, in some instances, while preventing injury and death, were deployed with such a force that certain occupants were being injured by the air bag. To solve this problem, the Occupant Classification System, or OCS, was introduced. This system determines the weight classification of a front passenger seat occupant and deploys the airbag accordingly.

Sensors in the seat act as inputs to the Occupant Classification Module, or OCM. The OCM sends a message via the bus to the Occupant Restraint Controller, or ORC, to report the conditions detected in the front passenger seat. The ORC will then make the determination whether to deploy the airbag and at what force. Thus, the deployment force is based on the weight of the passenger and the distance the passenger is sitting from the dashboard.

When performing any service work on a passive restraint system, it’s important to know which system you are servicing. Devices used and the number of control modules varies from model year to model year.

For example, the 2005 Chrysler and Dodge minivans have one airbag control module, the ORC, and it deploys the driver and front passenger air bags, the knee airbag, both seat belt tensioners and the side curtain airbags. On the other hand, the 2003 minivans have a control module to deploy the driver and front passenger airbags and the seat belt tensioners, but a separate control module is utilized to deploy the side mounted airbags. Verify which system you are working on before you begin.

Air Bag and OCS Basics

There are two basic types of Occupant Classification Systems. One type uses a seat cushion mounted bladder and pressure sensor to determine information about how front passenger seat is being used (see Figure 1). The second type uses four strain gauge sensors mounted at the seat corners to retrieve this information (see Figure 2).

In both cases, the system uses an Occupant Classification Module (OCM), which receives weight-related inputs as well as other inputs and sends this information over the data bus to the Occupant Restraint Controller (ORC). Other inputs might include the seat position, as some vehicles are equipped with seat track position sensors.

The ORC decides whether to deploy the passenger air bag for frontal impacts. When necessary, it also illuminates the Passenger Airbag Disable Lamp (PADL). This lamp alerts vehicle occupants that the characteristics of the front passenger seat occupant prevent airbag deployment.

The bladder-type system is designed to suppress air bag deployment and turn the PADL on for all occupants the size of a typical six-year old child and smaller. This is in contrast to the strain gauge system that is designed to suppress the airbag deployment and illuminate the PADL for a child seat. The key difference is that the strain gauge system is used in vehicles in which the air bag system is a low risk to a six-ear old child and in which the system might not be suppressed.

Servicing Air Bag Systems

Regardless of the type of air bag system being serviced, there are a few tasks that must always be performed. Most importantly, the battery must be disconnected. The proper method to do this is as follows: (1) turn the ignition off, (2) disconnect the battery and (3) wait 2 minutes before proceeding. This waiting period will allow the system capacitor sufficient time to discharge and make the system safe for service work.

Usually a warning lamp that remains lit after the bulb check test is run (from the driver’s viewpoint, the light remains lit after you have started driving) is the only indication that something is amiss in the passive restraint system. This light is the Air Bag Warning Lamp (see Figure 3).

Anytime the Air Bag Warning Lamp illuminates after the bulb check, it is time to check the Diagnostic Trouble Codes in the ORC using the DRBIII® or equivalent scan tool. Besides the ORC and its inputs and outputs, the codes can lead to investigating the OCM, the cluster or bus communication. The published diagnostic procedures are an excellent guide to direct you to your next step in diagnosing and solving the problem. After a while, you will begin to understand the general types of codes that control modules used to indicate faults and this will help you better understand the strategy behind the diagnostic procedures.

The majority of trouble codes indicate a problem with something other than the control modules. For example, the problem can be with a sensor, squib or seat belt tensioner circuit. In such cases, the key is to investigate the entire circuit and not condemn a component just because its name appears in the trouble code. With this in mind, let’s take a look at a sample problem.

Passive Restraint System Sample Problem

The subject vehicle for this example is a 2005 Jeep® Grand Cherokee. It arrived at the shop with the Air bag Warning Lamp and PADL illuminated. Numerous Diagnostic Trouble Codes (DTCs) indicated a Loss of Communication with the Occupant Classification Module (OCM). The StarSCAN ECU View verified that the OCM was Not Active.

The technician observed that several conditions could cause a communication problem with the OCM. As a result of this information, the OCM was replaced since it was not communicating over the bus.

The new OCM was installed, but did not correct the problem. What went wrong with the diagnosis? The technician was too hasty in judgment. As mentioned previously, just because the name of a component appears in the DTC, it does not mean that component is bad.

After replacing the OCM, the technician began to check the individual circuits, as outlined in the service information. All was well until the Driver Seat Position Sensor Voltage-Driver circuit was checked. The test indicated continuity between the sensor and ground. This means that the circuit is shorted to ground.

The service information stated that the Driver Seat Position Sensor Harness should be replaced.

Since the technician was working on the passenger side of the vehicle, the conclusion was reached that the service information was wrong. The passenger side seat wiring harness was replaced, not the driver side seat wiring harness. Guess what? The problem was still not fixed.

By noting the relationship in the circuitry and understanding the role that the driver’s side plays in the overall system operation, the technician was then able to get on the right track and fix the problem. As it turned out, a section of the body harness, of which the Driver Seat Position Sensor Harness is a component, was pinched. Moving the harness revealed that several other circuits were damaged. Also, the OCM became active when the harness was shifted. Replacing the harness corrected the problem.

The moral of the example is that determining the root cause of a DTC, at times, takes a little more effort. In this case, paying careful attention to the wiring diagrams and having confidence in the service information would have prevented some unnecessary, and costly, repair steps.