Body Control Module: Description and Operation

IDENTIFICATION OF SYSTEM
The vehicle systems that are part of the "body" system are the airbag system and vehicle communication.

AIRBAG SYSTEM
The airbag system is designed to provide increased driver and passenger protection if the vehicle is involved in a front end collision. The system is designed to be used in conjunction with the seat belt system.

The airbag control module (ACM) is an electronic module that monitors the airbag system for proper operation, stores diagnostic trouble code (DTCs), controls the airbag warning lamp, and contains the safing sensor and accelerometer that is responsible for airbag deployment. The ACM is located on the floor pan between the gear shifter and the parking brake assembly. The ACM provides diagnostic in- formation (DTCs) to the technician through the DRB III (R) via the CCD bus. Some circuits are tested continuously; others are checked only under certain circumstances. The warning lamp is hardwired to the ACM and there are no external impact sensors.

The AIRBAG warning lamp is the only point at which "symptoms" of a system malfunction can be observed by the customer. Whenever the ignition key is turned to the "RUN" or "START" position, the airbag control module performs a lamp check by turning the AIRBAG warning lamp ON for 6-8 seconds and then goes OFF. If the lamp remains OFF, it means that the ACM has checked the system and found it to be free of discernible malfunctions. If the lamp remains ON, there could be an active fault in the system or the wire that operates the lamp may be shorted to ground. If the lamp comes ON and stays ON for a period longer than 6-8 seconds, then goes OFF, there is usually an intermittent problem in the system.

Perform the WARNING LAMP CIRCUIT OPEN/SHORTED procedure in this book to find the cause of any customer complaint regarding the AIRBAG warning lamp, such as:
- warning lamp does not illuminate
- warning lamp stays illuminated

The deceleration or g-force resulting from the impact of a front-end collision causes the electronic sensor inside the ACM to be triggered. This causes the inflators to be actuated thus deploying the airbags. The total time between triggering to full deployment and deflation of the bag is usually 1/10 of a second.

WARNING! THE AIRBAG CONTROL MODULE CONTAINS THE IMPACT SENSOR, WHICH ENABLES THE SYSTEM TO DEPLOY THE AIRBAG. BEFORE ATTEMPTING TO DIAGNOSE OR SERVICE ANY AIRBAG SYSTEM OR RELATED STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENTS, YOU MUST FIRST DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE. THEN WAIT TWO MINUTES FOR THE SYSTEM CAPACITOR TO DISCHARGE BEFORE FURTHER SYSTEM SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE AIRBAG SYSTEM. FAILURE TO DO THIS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.

NEVER STRIKE OR KICK THE AIRBAG CONTROL MODULE, AS IT CAN DAMAGE THE IMPACT SENSOR OR AFFECT ITS CALIBRATION. IF AN AIRBAG CONTROL MODULE IS ACCIDENTALLY DROPPED DURING SERVICE, THE MODULE MUST BE SCRAPPED AND REPLACED WITH A NEW UNIT.

CCD BUS
The CCD (Chrysler Collision Detection multiplex system) bus is the means of communicating between the Airbag Control Module (ACM) and the DRB III (R) about the operation of airbag system components and circuits.

The Chrysler Collision Detection multiplex system (CCD bus) consists of a twisted pair of wires. These wires run from the airbag control module (ACM) to the data link connector and the DRB III (R). They receive and deliver coded information between the ACM and the DRB III (R).

The two wires (D1 and D2) of the twisted pair that make up the CCD multiplex system are called "bus+" (bus plus) and "bus-" (bus minus) respectively. Each wire has a measurable voltage level of roughly 2.5 volts, which is supplied by the DRB III (R). In order to maintain the 2.5 volts on each line and provide a means of transportation for the coded messages, there is a "voltage divider network." This network, contained in the DRB III (R), supplies bias and termination. Bias is the part of the voltage divider network that places both bus+ and bus- at 2.5 volts. Termination in the circuitry is required to complete the voltage divider network and also to provide some electromagnetic protection for the bus.

AIRBAG DIAGNOSTIC TROUBLE CODES
Airbag diagnostic trouble codes consist of active and stored codes. If more than one code exists, diagnostic priority should be given to the active codes.

Each diagnostic trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of the trouble codes. It is not necessary to perform all of the tests in this book to diagnose an individual code.

Always begin by reading the diagnostic trouble codes using the DRB III (R). This will direct you to the specific test(s) that must be performed.

Active diagnostic trouble codes for the airbag system are not permanent and will change the moment the reason for the code is corrected. In certain test procedures, diagnostic trouble codes are used as a diagnostic tool.

AIRBAG ACTIVE CODES
The code becomes active as soon as the malfunction is detected and becomes stored after one minute of occurrence or key-off, whichever occurs first. An active trouble code indicates an ongoing malfunction. This means that the Defect is currently there every time the airbag control module checks that circuit/function. Active codes automatically disappear when the reason for the code has been corrected.

With the exception of the warning lamp trouble codes or malfunctions, when a malfunction is detected, the airbag lamp remains lit for a minimum of 12 seconds or as long as the malfunction is present.

AIRBAG STORED CODES
Airbag codes are automatically stored in the ACM's memory after one minute of occurrence or when the ignition is turned OFF. The exception is the "Loss of Ignition Run Only" code which is an active code only and is not stored.

A "stored" code indicates there was an active code present at some time. However, the code currently may not be present as an active code, although another active code could be.

When a trouble code occurs, the airbag warning lamp illuminates for 12 seconds minimum (even if the problem existed for less than 12 seconds). The code is stored, along with the time in minutes it was active, and the number of times the ignition has been cycled since the problem was last detected.

The minimum time shown for any code will be one minute, even if the code was actually present for less than one minute. Thus, the time shown for a code that was present for two minutes 13 seconds, for example, would be three minutes.

If a malfunction is detected a diagnostic trouble code is stored and will remain stored as long as the malfunction exists. When and if the malfunction ceases to exist, an ignition cycle count will be initiated for that code. If the ignition cycle count reaches 100 without a reoccurrence of that same malfunction, the diagnostic trouble code is erased and that ignition cycle counter is reset to zero. If the malfunction reoccurs before the count reaches 100, then the ignition cycle counter will be reset to zero.

If a malfunction is not active while performing a diagnostic test procedure, the active code diagnostic test will not locate the source of the problem. In this case, the stored code can indicate an area to inspect.

If no obvious problems are found, erase stored codes, and with the ignition "on" wiggle the wire harness and connectors, rotate the steering wheel from stop to stop. Recheck for codes periodically as you work through the system. This procedure may uncover a malfunction that is difficult to locate.

NOTE: A "STORED" CODE CANNOT BE ERASED IF IT IS ALSO AN "ACTIVE" CODE AT THE TIME THE ERASE FUNCTION IS BEING ATTEMPTED.

AIRBAG TROUBLE CODES
The airbag control module may report any of the following diagnostic trouble codes.

For the following (active or stored) codes, replace the airbag control module (ACM) even if set intermittently:
- AECM Accelerometer
- Internal Diagnostic 1
- Internal Diagnostic 2
- AECM Output Driver
- AECM Stored Energy Logic
- AECM Stored Energy Driver
- AECM Stored Energy Passenger
- Internal Diagnostic 3
- Internal Diagnostic 4
- Safing Sensor Shorted
- Warning Lamp Driver Error

For these active codes, refer to the appropriate diagnostic procedure:
Driver Squib Open
Driver Squib Circuit Shorted
Either Squib Terminal Short to Battery
Either Squib Terminal Short to Ground
Loss of Ignition Run Only
Loss of Ignition Run/Start
No CCD Communication
Passenger Squib Circuit Open
Passenger Squib Circuit Shorted
Warning Lamp Circuit Open
Warning Lamp Circuit Shorted

DRB III (R) ERROR MESSAGES AND BLANK SCREEN
Under normal operation, the DRB III (R) will display one of only two error messages:

- User-Requested WARM Boot or User-Requested COLD Boot

If the DRB III (R) should display any other error message, record the entire display and call the MDS Hotline, or call for information and assistance at 1-800-825-8737. This is a sample of such an error message display:

ver: 2.14
date: 26 Jul93
file: key_itf. cc
date: Jul 26 1993 line: 548
err: 0x1
User-Requested COLD Boot

Press MORE to Switch between this display and the application screen.
Press F4 when done noting information.

DRB III (R) DOES NOT POWER UP
If the LED's do not light or no sound is emitted at start up, check for loose cable connections or a bad cable. Check the vehicle battery voltage (data link connector cavity 16). A minimum of 11 volts is required to adequately power the DRB III (R).

If all connections are proper between the DRB III (R) and the vehicle or other devices, and the vehicle battery is fully charged, an inoperative DRB III (R) may be the result of faulty cable or vehicle wiring. Perform *No Response Condition test.

DRB Scan Tool:

DISPLAY IS NOT VISIBLE
Low temperatures Will affect the visibility of the display. Adjust the contrast to compensate for this condition.

VEHICLE DAMAGE WARNINGS
Before disconnecting any control module, make sure the ignition is "OFF". Failure to do so could damage the module.

When testing voltage or continuity at any control module, use the terminal side (not the wire end) of the connector. Do not probe a wire through the insulation; this will damage it and eventually cause it to fail because of corrosion.

Be careful when performing electrical tests so as to prevent accidental shorting of terminals. Such mistakes can damage fuses or components. Also, a second code could be set, making diagnosis of the original problem more difficult.

When replacing a blown fuse, it is important to use only a fuse having the correct amperage rating. The use of a fuse with a rating other than indicated may result in a dangerous electrical system overload. If a properly rated fuse continues to blow, it indicates a problem in the circuit that must be corrected.

Service and general information labels about the airbag system can be found on the driver's sun visor, the glove box door, and in the engine compartment.

ROAD TESTING A COMPLAINT VEHICLE
Some complaints will require a test drive as part of the repair verification procedure. The purpose of the test drive is to try to duplicate the diagnostic code or symptom condition.

CAUTION: BEFORE ROAD TESTING A VEHICLE, BE SURE THAT ALL COMPONENTS ARE REASSEMBLED. DURING THE TEST DRIVE, DO NOT TRY TO READ THE DRB III (R) SCREEN WHILE IN MOTION. DO NOT HANG THE DRB III (R) FROM THE REAR VIEW MIRROR OR OPERATE IT YOURSELF. HAVE AN ASSISTANT AVAILABLE TO OPERATE THE DRB III (R).

REQUIRED TOOLS AND EQUIPMENT
DRB III (R) (diagnostic read-out box)
Jumper Wires
Ohmmeter
Voltmeter

GLOSSARY OF TERMS
ACM airbag control module
CCD Chrysler Collision Detection (vehicle communication bus)
DAB driver airbag
DLC data link connector
EM entry module
MIC mechanical airbag cluster
PAB passenger airbag
PCM powertrain control module
PDC power distribution center
SQUIB also called initiator (located in rear of Airbag module)