1986 MERCEDES BENZ AIR BAG RESTRAINT SYSTEM

AIR BAG RESTRAINT SYSTEM
1985-87 MERCEDES-BENZ AIR BAGS
IDENTIFICATION

WARNING: To avoid injury from accidental air bag deployment, read and carefully follow all WARNINGS and SERVICE PRECAUTIONS.

All models equipped with an air bag have the letter "B" or "D" in the eighth position of the Vehicle Identification Number (VIN). The words AIRBAG or S.R.S. will be embossed on the steering wheel hub pad. Also, all models equipped with Supplemental Restraint System (SRS) can be identified by the SRS labels provided at different locations of vehicle, such as driver's door latch post.
DESCRIPTION & OPERATION

The following are main components of Supplemental Restraint System (SRS): inflatable air bag/gas generator module, slip ring, carbon brushes, SRS sensor/control unit, energy accumulator, voltage converter (or voltage transformer), SRS warning light and driver's knee bolster. See Fig. 1 . Emergency Tensioning Retractor (ETR) is incorporated within passenger's seat belt assembly. For SRS to operate properly, 3-point seat belts must be worn.

If vehicle's deceleration rate and "G" force recorded by SRS sensor/control unit is sufficiently high, SRS sensor/control unit generates a voltage signal. This voltage signal triggers gas generator, igniting a solid propellant to produce a quantity of non-toxic nitrogen gas, which fills the air bag in milliseconds. In about 6 milliseconds, after the air bag has inflated to maximum expansion, the gas is released via lateral ventilation slots in air bag, and the air bag collapses. The entire sequence of air bag inflation and collapse takes about 25-30 milliseconds.

Fig. 1: Locating SRS Components
Courtesy of MERCEDES-BENZ OF NORTH AMERICA
AIR BAG MODULE

Air bag module consists of a signal receiver, gas generator, and an inflatable air bag. The complete air bag module is attached to the steering wheel hub. The steering wheel-to-air bag module wiring connector contains a special plug to fit module. When steering wheel-to-air bag module wiring plug is disconnected, module is automatically short circuited by a shorting bridge wire inside module connector. This prevents accidental operation of system by static electricity or careless handling. The shorting bridge wire is automatically opened when wiring plug is inserted into connector.

When an electrical impulse is transmitted from SRS sensor/control unit to ignition pellet in the gas generator, the solid propellant will then burn off within milliseconds to produce a given quantity of nitrogen gas at a given pressure. During this process, gas is guided through a filter into the air bag. The folded nylon air bag on the gas generator will then rip open the vinyl of steering wheel hub pad. The air bag is tightly inflated by the gas, and will prevent head/facial impact of driver to steering wheel during a frontal collision.
EMERGENCY TENSIONING RETRACTOR (ETR)

The ETR passenger's seat belt assembly consists of gas generator, cable pulley, clutch, guide tube, automatic belt retractor reel, and piston with cable. The complete unit is installed in the right center door pillar, in place of standard belt reel unit. The ETR unit is electrically connected to SRS system by a double plug connector.

When ETR unit wiring connector is disconnected, unit is automatically short circuited by a shorting bridge wire inside connector. This prevents accidental operation of ETR system by static electricity or careless handling. The shorting bridge wire is automatically opened when wiring plug is inserted into connector.

The moment an electrical impulse is transmitted from SRS sensor/control unit to ETR capsule containing solid propellant, a high pressure burn-off gas is generated. This gas accelerates piston and cable in upward direction. The cable transmits a turning torque to pulley, causing a shear pin to break off, and pulley is then positively connected to wind-up shaft. The wind-up shaft is turned back to eliminate any slack. This pulls the seat belt snugly to front passenger in milliseconds.

Fig. 2: Identifying ETR Components
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
ENERGY ACCUMULATOR (CAPACITOR)

Energy accumulator is attached to underside of front foot rest. See ENERGY ACCUMULATOR (CAPACITOR) LOCATION table. Energy accumulator is connected to SRS sensor/control unit and voltage converter by separate SRS wiring harness from vehicle main harness. If vehicle battery fails immediately as a result of an accident, the SRS sensor/control unit and ignitor circuits are supplied voltage by this energy accumulator. Malfunction of the energy accumulator is indicated by SRS warning light remaining on.

ENERGY ACCUMULATOR (CAPACITOR) LOCATION
Model Series Location
190 Passenger's Side, Right Front Corner Of Floor, Under Carpet
300 (1) *
300SE, 420 & 560 Driver's Side, Left Front Corner Of Floor, Under Foot Rest
Convertibles (1) *
(1) Information is not available.
SLIP RING

Slip ring assembly is attached to bottom side of steering wheel. Slip ring unit ensures continuous electrical contact from SRS control unit to air bag module. This electrical contact is accomplished by carbon brushes, mounted in steering column, constantly rubbing against slip ring surfaces on bottom of steering wheel.
SRS SENSOR/CONTROL UNIT

SRS sensor/control unit houses 2 integrated circuits and an acceleration/deceleration pick-up. SRS sensor/control unit triggers air bag module inflation by measuring and recording deceleration and "G" force of vehicle. The acceleration pick-up in SRS sensor/control unit detects deceleration or acceleration of vehicle in a longitudinal direction, and generates an electronic signal. This signal travels through a by-pass filter to an amplifier, which adapts signal rate through subsequent processing stages within SRS sensor/control unit.

The SRS system will deploy only when sensor electronics have determined that the type and severity of impact is sufficient to require air bag deployment. An internal mercury switch disconnects the ignitor circuit of SRS when vehicle is in normal driving mode, preventing any errors in deployment system.

SRS sensor/control unit housing and electrical connections are shielded to prevent SRS deployment from system interference trouble caused by engaging large electrical consumers, or engaging normal electromagnetic radiations on public roads. SRS sensor/control unit also functions as a diagnostic monitor, which continuously monitors system operation and records any faults in SRS. The fault codes MUST be read with SRS Tester (126-589-10-21-00). See Fig. 3 .
SRS WARNING LIGHT

The SRS warning light is located in instrument cluster, below tachometer. With ignition switch in position No. 1 or No. 2, Red SRS warning light will illuminate. The SRS warning light illuminates and goes out after approximately 10 seconds. This indicates SRS is operational. A malfunction in SRS system is indicated if warning light does not come on up when ignition is first turned on, or if it does not go out after approximately 10 seconds. If a malfunction is indicated, the entire SRS system should be checked and serviced as soon as possible.
VOLTAGE CONVERTER (VOLTAGE TRANSFORMER)

Voltage converter is also known as voltage transformer. Voltage converter is located behind driver's left kick panel, behind passenger's right side kick panel, or behind left instrument panel, above brake and accelerator pedals. Voltage converter is connected to SRS sensor/control unit and energy accumulator by separate SRS wiring harness, apart from main wiring harness. The voltage converter supplies SRS sensor/control unit and energy accumulator with a constant 12 volts at very low battery voltage (above 4.5 volts). Malfunction of the voltage converter is indicated by SRS warning light remaining on.
SERVICE PRECAUTIONS

NOTE: The following precautions should be observed when working with air bag systems.

* When working around steering column and before any repairs are performed, disable air bag system. See DISABLING & ACTIVATING AIR BAG SYSTEM .
* Always wear safety glasses and gloves when handling a deployed air bag module. Air bag module may contain sodium hydroxide deposits, which are irritating to the skin.
* Never drop, strike or jar any sensor or SRS unit. All sensors and mounting bracket bolts must be torqued carefully to ensure proper sensor operation.
* Air bag module and steering wheel MUST be replaced following a collision in which air bag was deployed.
* Never apply power to SRS if any sensor is not rigidly attached to the vehicle.
* To avoid air bag deployment when troubleshooting SRS, DO NOT use self-powered electrical test equipment such as battery-powered or AC-powered voltmeter, ohmmeter, etc. DO NOT repair any portion of SRS wiring circuit.
* Always handle air bag module with trim cover away from your body. Always place air bag module on workbench with trim cover up, away from loose objects.
* Never expose any SRS system components to temperatures in excess of 212°F (100°C).
* Never expose any SRS system components to cleaning agents such as solvents, gasoline, lye, etc.

DISABLING & ACTIVATING AIR BAG SYSTEM
DISABLING

1. Before proceeding, follow air bag service precautions. See SERVICE PRECAUTIONS . Ensure ignition is off. Disconnect negative battery cable and insulate cable terminal clamp.
2. Disconnect Red SRS 10-pin harness connector located at right front corner of passenger's floor, beneath foot rest panel. See Fig. 6 . System is now disabled.

ACTIVATING

Ensure ignition is off. Reconnect Red SRS 10-pin connector. Reconnect negative battery cable. System is now activated.
POST-COLLISION AIR BAG SAFETY INSPECTION

POST-COLLISION AIR BAG SAFETY INSPECTION
Action Components
Replace After Deployment (1) Air Bag Module(s) (1) ETR Seat Belt Assemblies Which Have Deployed (1) SRS Control Unit
Inspect & If Damaged, Replace Component (Even If Air Bag Did Not Deploy) (1) Any Parts Which Fault Codes Show Are Faulty (1) Instrument Panel (1) SRS Mounting Brackets & Location (1) SRS Wiring Harness
(1) DO NOT attempt SRS wiring harness repairs.
DISPOSAL PROCEDURES
* PLEASE READ FIRST *

Several situations, including the following, may arise, requiring some form of disposal action.

* Scrapping A Vehicle With A Deployed Air Bag
* Scrapping A Vehicle With A Live Air Bag
* Disposing A Live But Electrically Faulty Air Bag Module
* Disposing A Deployed Air Bag Module

DEPLOYED AIR BAG

Deployed air bag module and/or ETR unit can be thrown away. None of its components are reusable. Deployed air bag module is not classified as hazardous material.
SCRAPPED VEHICLE

WARNING: An undeployed air bag module CANNOT be disposed without first deploying air bag. If this is not possible through procedures outlined below, contact vehicle manufacturer for further instructions. Perform remote deployment outdoors. Keep all personnel at least 20 feet away to ensure their physical safety from projected objects, and hearing safety due to loud noise from air bag deployment.

1. Place vehicle outdoors in a remote area, away from workshop and other personnel. Disconnect negative battery cable and shield cable clamp. Open all vehicle windows and doors. Ensure air bag module is secured to steering wheel. Remove loose articles from front seat. DO NOT allow any occupant inside vehicle.
2. Disconnect Red SRS 10-pin connector located at right front corner of passenger's floor, beneath foot rest panel. Using Ignition Trigger Device (126-589-00-90-00), connect ignition trigger device connector to SRS 10-pin connector. Move away from vehicle, as far as trigger device wiring will allow.
3. To deploy air bags, turn rotary switch on trigger device to position No. 4. Depress both push buttons at same time. Air bag material should be visible in vehicle. Trigger device warning light in upper right corner of box should illuminate if SRS air bag ignited. If air bag material is not visible, but trigger device warning light is illuminated, air bag module is faulty. Contact manufacturer for further disposal procedures.
4. To deploy ETR seat belt, turn rotary switch on trigger device to position No. 2. Depress both push buttons at same time. Trigger device warning light in upper right corner of box should illuminate if SRS ETR seat belt ignited. If trigger device warning light is illuminated, but passenger's seat belt is not very tight against seat, ETR is faulty. Contact manufacturer for disposal procedures.

UNDEPLOYED AIR BAG - FAULTY UNIT

After diagnostic testing and deployment procedures have confirmed that air bag module is faulty, contact vehicle manufacturer for disposal procedure.
REMOVAL & INSTALLATION

WARNING: Follow air bag service precautions to prevent accidental air bag deployment and personal injury. See SERVICE PRECAUTIONS .

NOTE: After component replacement, check system to ensure proper operation. See TESTING - SYSTEM OPERATION CHECK .
AIR BAG MODULE

NOTE: Air bag module and steering wheel MUST be replaced following a collision in which air bag was deployed.
Removal & Installation

1. Before proceeding, follow air bag service precautions. See SERVICE PRECAUTIONS . Disable SRS. See DISABLING & ACTIVATING AIR BAG SYSTEM .
2. Using Torx TX-30 bit with long shank, remove 2 Torx screws from rear of steering wheel. Lift off air bag module enough to disconnect plug connector from rear of module. Place module away from work area, with pad facing upward.
3. To install, reverse removal procedure. Tighten Torx screws to 53 INCH lbs. (6 N.m). Activate SRS. See DISABLING & ACTIVATING AIR BAG SYSTEM. Check air bag system to ensure system is functioning properly. See TESTING - SYSTEM OPERATION CHECK .

CARBON CONTACTS
Removal & Installation

1. Before proceeding, follow air bag service precautions. See SERVICE PRECAUTIONS . Disable SRS. See DISABLING & ACTIVATING AIR BAG SYSTEM . Remove air bag module. See AIR BAG MODULE. Remove steering wheel. See STEERING WHEEL .
2. On some models, remove steering column cover plate retaining screws and remove cover. Replace SRS carbon contact assemblies.
3. On other models, carbon contacts are held in place by plastic retainers. Pry out retainers, unplug wiring and replace each contact/retainer as a complete unit. To install, reverse removal procedures.

ENERGY ACCUMULATOR (CAPACITOR)
Removal & Installation

Information is not available. Energy accumulator is attached to underside of front foot rest. See ENERGY ACCUMULATOR (CAPACITOR) LOCATION table.

ENERGY ACCUMULATOR (CAPACITOR) LOCATION
Model Series Location
190 Passenger's Side, Right Front Corner Of Floor, Under Carpet
300 (1) *
300SE, 420 & 560 Driver's Side, Left Front Corner, Of Floor, Under Foot Rest
Convertibles (1) *
(1) Information is not available.
SRS SENSOR/CONTROL UNIT
Removal & Installation

Information is not available.
VOLTAGE CONVERTER (VOLTAGE TRANSFORMER)
Removal & Installation

Information is not available.

NOTE: Air bag module and steering wheel MUST be replaced following a collision in which air bag was deployed.
Removal & Installation

1. Before proceeding, follow air bag service precautions. See SERVICE PRECAUTIONS . Disable SRS. See DISABLING & ACTIVATING AIR BAG SYSTEM . Remove air bag module. See AIR BAG MODULE. Remove steering wheel retaining bolt.
2. Reference mark steering wheel to shaft. Pull steering wheel off with hands, or if necessary, use proper steering wheel puller to remove steering wheel. When removing steering wheel, use care not to break carbon contact brushes mounted to top of column.
3. To install, reverse removal procedure. DO NOT lube slip ring contact surfaces. Tighten steering wheel retaining bolt to specifications. See TORQUE SPECIFICATIONS table.

SLIP RING
Removal & Installation

1. Before proceeding, follow air bag service precautions. See SERVICE PRECAUTIONS . Disable SRS. See DISABLING & ACTIVATING AIR BAG SYSTEM . Remove air bag module. See AIR BAG MODULE. Remove steering wheel. See STEERING WHEEL .
2. Remove horn contacts and support springs. Inside steering wheel hub opening, disconnect horn wiring connector, and remove 5 Phillips head screws retaining slip ring to steering wheel. Remove steering wheel hub metal assembly. Remove slip ring and guide wiring through steering wheel opening. To install, reverse removal procedure.

TESTING - SYSTEM OPERATION CHECK

Turn ignition switch to ON (No. 1) position. SRS warning light should illuminate then go out after 10 seconds if no fault codes are present. The following conditions indicate that SRS is malfunctioning, and an SRS fault code will be stored in control unit memory:

* SRS light does not illuminate.
* SRS light does not go out after 10 seconds.
* SRS light does not go out after engine is started.
* SRS light comes on while driving.

If SRS system is malfunctioning, go to DIAGNOSIS & TESTING.
WIRE REPAIR

DO NOT repair SRS wiring. If SRS wiring is faulty, replace wiring harness.
TORQUE SPECIFICATIONS

TORQUE SPECIFICATIONS
Application Ft. Lbs. ( N.m)
Steering Wheel Retaining Bolt 59 (80)
INCH Lbs. (N.m)
Air Bag Module Screws 53 (6)
DIAGNOSIS & TESTING

WARNING: Follow air bag service precautions to prevent accidental air bag deployment and personal injury. See SERVICE PRECAUTIONS. NEVER use ohmmeter or other self-powered meter to test resistance of air bag module, or any SRS component while system is activated. Use only approved SRS test equipment to perform tests. The following test procedures are performed using SRS Tester (126-589-10-21-00).

NOTE: After component replacement, check system to ensure proper operation. See TESTING - SYSTEM OPERATION CHECK .

Fig. 3: SRS Tester (126-589-10-21-00)
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
SRS TESTER PREPARATION

WARNING: If SRS tester is to be used for external voltage and resistance measurements, DO NOT connect SRS tester to SRS 10-pin connector.

1. To set-up SRS tester for voltage and resistance measurements only, go to step 2). To set-up SRS tester for SRS system testing, turn ignition off. Disable SRS. See DISABLING & ACTIVATING AIR BAG SYSTEM . Connect SRS Tester (126-589-10-21-00) to SRS 10-pin connector, and connect battery voltage supply to tester's external voltage supply terminals. See Fig. 4 and Fig. 7 . Reconnect negative battery terminal.
2. If SRS tester is used for voltage and resistance measurements only, plug tester leads into test sockets. To measure voltage, turn test selector switch to position No. 8. Meter will show volts. To measure resistance, turn test selector switch to position No. 9. Meter will show ohms.

SRS WARNING LIGHT TESTS

NOTE: The following test procedures are performed using SRS Tester (126-589-10-21-00).
SRS Light Does Not Illuminate

1. Turn ignition switch to position No. 1 or No. 2. If SRS warning light does not illuminate along with other panel lights, check SRS fuse in fuse panel. If fuse is not blown, remove instrument cluster and check SRS bulb. If bulb is defective, replace bulb and retest light. If fuse is blown, replace fuse and retest light. If warning light still does not work, go to step 2).
2. Check voltage supply to bulb. Set up SRS tester for external voltage measurement. See SRS TESTER PREPARATION . Turn tester selector switch to position No. 8. See Fig. 3. Turn ignition switch to position No. 1. Touch tester lead probe to Black/Red wire of SRS warning light bulb socket, and touch other tester lead to good vehicle ground. See Fig. 5 and Fig. 6 . If voltage exists, go to step 5). If voltage does not exist, go to step 3).

Fig. 4: Locating SRS Warning Light Bulb Socket
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig. 5: Locating SRS Electrical Supply
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
3. Disconnect battery and remove SRS fuse. Turn SRS tester selector switch to position No. 9. See Fig. 3 . Using tester, check continuity of Black/Red wire between bulb socket and fuse panel terminal No. 15. See Fig. 5 . SRS tester should indicate 0-1 ohm. If reading is not 0-1 ohm, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS . If reading is 0-1 ohm, go to step 4).
4. Disconnect negative battery cable and SRS 10-pin connector. See Fig. 6 . Using SRS tester, check continuity of Pink wire from pin No. 10 of SRS 10-pin connector to bulb socket. SRS tester should indicate 0-1 ohm. If reading is 0-1 ohm, go to step 5). If reading is not 0-1 ohm, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS .
5. Using SRS tester, check continuity of Brown wire from pin No. 4 of SRS 10-pin connector to vehicle ground. SRS tester should indicate 0-1 ohm. If reading is 0-1 ohm, replace SRS sensor/control unit. Retest SRS light system. See SRS WARNING LIGHT TESTS. If reading is not 0-1 ohm, there is an open in Brown wire circuit from pin No. 4 of SRS 10-pin connector to main ground connection on steering column support bracket. Replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS.

SRS Light Remains Illuminated Or Flickers While Driving

1. When ignition switch is turned to position No. 1, SRS warning light should illuminate and go out within 4-10 seconds. If warning light does not go out, or flickers occasionally while driving, turn SRS tester selector switch to position No. 1. See Fig. 3 . Ensure vehicle battery is in good condition. Connect SRS tester to SRS 10-pin connector located at right front corner of passenger's floor, beneath foot rest panel. Turn ignition switch to position No. 1. Read voltage on SRS tester. If battery voltage exists, go to TESTING VOLTAGE TRANSFORMER/CONVERTER under SRS COMPONENT TESTING . If battery voltage does not exist, go to step 3).
2. Disconnect negative battery cable. Remove fuse panel mounting screws, and lower fuse panel. On back of fuse panel, check Black/Red wire for proper connection. See Fig. 5 . Disconnect SRS tester from SRS 10-pin connector located at right front corner of passenger's floor, beneath foot rest panel. Set up SRS tester for external resistance measurement. See SRS TESTER PREPARATION . Go to step 3).
3. Turn SRS tester selector switch to position No. 9. See Fig. 3 . Check for open circuit in Black/Red wire from fuse panel terminal No. 15 to pin No. 1 of SRS 10-pin connector. See Fig. 6 and Fig. 7 . SRS tester should indicate 0-1 ohm. If reading is 0-1 ohm, go to step 4). If reading is not 0-1 ohm, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS .
4. Check resistance of Brown wire circuit from pin No. 4 of SRS 10-pin connector to vehicle ground. See Fig. 6 . SRS tester should indicate 0-1 ohm. If reading is 0-1 ohm, go to TESTING VOLTAGE TRANSFORMER/CONVERTER under SRS COMPONENT TESTING . If reading is not 0-1 ohm, an intermittent open exists in Brown wire circuit from pin No. 4 to main ground connections at steering column support bracket. Locate and repair faulty circuit connections if possible, otherwise replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS.

Fig. 6: Identifying SRS 10-Pin Connector Pins
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig. 7: Locating SRS Electrical Grounds
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
SRS COMPONENT TESTING
Testing Voltage Transformer/Converter

1. Turn SRS tester selector switch to position No. 2. See Fig. 3. Connect SRS tester to SRS 10-pin connector located at right front corner of passenger's floor, beneath foot rest panel. Turn ignition switch to position No. 1. SRS tester reading should be about 12 volts, with minimum testing time of 30 seconds. If voltage reading is as specified, go to TESTING AIR BAG CIRCUIT 1. If voltage reading is not as specified, go to next step.
2. Turn SRS tester selector switch to position No. 6 to test energy accumulator charge. Energy accumulator is charged by SRS tester until battery voltage is reached (or minimum of 9 volts). If voltage is okay, go to step 3). If voltage is not okay, go to step 4).
3. Turn SRS tester selector switch to position No. 7 to test capacity of energy accumulator. When SRS tester capacitor warning light goes out, read value on tester. SRS tester reading will depend on SRS sensor/control unit-to-energy accumulator (capacitor) match up. See SRS TESTER READINGS table. If SRS tester measured value is not as specified in table, replace energy accumulator. Retest SRS light system. See SRS WARNING LIGHT TESTS . If SRS tester measured value is as specified, go to step 6).

SRS TESTER READINGS
Sensor/Control Unit No. Capacitor Unit No. Measured Value
1268201410 0008210062 4.0 mF
1268201410 0008210262 7.0 mF
0028209710 0008210262 7.0 mF
(1) Measured value depends on sensor/control unit-to-capacitor match up.
4. Turn ignition switch to OFF position. Unplug SRS tester from SRS 10-pin connector. Set up SRS tester for external resistance measurement. See SRS TESTER PREPARATION . Measure resistance at SRS 10-pin connector, between pins No. 4 and No. 9. Resistance should be 0-1 ohm. If resistance is not 0-1 ohm, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS . If resistance is 0-1 ohm, replace energy accumulator. Retest SRS light system. See SRS WARNING LIGHT TESTS.
5. Turn ignition switch off. Unplug SRS tester from SRS 10-pin connector. Set up SRS tester for external voltage measurement. See SRS TESTER PREPARATION. Turn SRS tester selector switch to position No. 8. See Fig. 3 . Disconnect voltage transformer/converter 12-pin connector. Turn ignition switch to position No. 1. To check battery voltage power supply to voltage converter, connect SRS tester positive lead to voltage converter connector socket No. 12, and connect negative lead to voltage converter connector socket No. 1. If battery voltage is not present, go to step 7). If battery voltage exists, go to step 6).
6. Turn ignition switch off. Turn SRS tester selector switch to position No. 9. Check for open circuit by connecting SRS tester leads at voltage converter socket No. 6 and at SRS 10-pin connector pin No. 9. SRS tester should indicate 0-1 ohm. If resistance is not 0-1 ohm, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS . If resistance is 0-1 ohm, replace voltage converter. Retest SRS light system. See SRS WARNING LIGHT TESTS.
7. Turn ignition switch to OFF position. Turn SRS tester selector switch to position No. 9. Using SRS tester, check for open circuit in Black/Red wire from voltage converter connector socket No. 12 to fuse panel terminal No. 15. See Fig. 5 . Resistance should be 0-12 ohms. If resistance is 0-12 ohms, go to step 8). If resistance is not 0-12 ohms, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS.
8. An intermittent open exists in Brown wire circuit from voltage converter connector socket No. 1 to main ground connections at steering column support bracket. Locate and repair faulty circuit connections if obvious, otherwise replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS .

Testing Air Bag Circuit 1

1. Connect SRS tester to SRS 10-pin connector located at right front corner of passenger's floor, beneath foot rest panel. Turn ignition switch to position No. 1. Slowly turn steering wheel completely to the left and completely to the right several times while reading SRS tester value. SRS tester should indicate 2-20 ohms. If reading is 2-20 ohms, go to step 2). If reading is not 2-20 ohms, go to step 3).
2. Push SRS tester grounding button for testing isolation resistance (wiring harness to ground connection). SRS tester should indicate 20,000 ohms to infinity. If reading is as specified, go to TESTING AIR BAG CIRCUIT 2 (ETR). If reading is not as specified, replace wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS . If SRS light system is now okay, this testing sequence is completed.
3. Disconnect battery. Locate and unplug slip ring 2-wire connector behind left side of instrument panel. Slip ring 2-wire connector uses a Purple wire and a Green wire. With slip ring connector unplugged, SRS tester should indicate infinity. If reading is less than infinity, SRS wiring harness is shorted to ground. Replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS. If reading is infinity, go to step 4).
4. Jump wire slip ring 2-wire harness connector and read tester resistance. Resistance should read 0-1 ohm. If reading is not 0-1 ohm, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS . If reading is 0-1 ohm, go to step 5).
5. Ensure battery is disconnected. With slip ring 2-wire connector still unplugged, remove air bag module from steering wheel, and disconnect module plug connector. Jump wire air bag module plug connector at steering wheel. See Fig. 8 . Reconnect 2-wire slip ring connector behind instrument panel. Read resistance on SRS tester.
6. Resistance should be 0-25 ohms. If resistance is 0-25 ohms, replace air bag module. Retest SRS light system. See SRS WARNING LIGHT TESTS. If resistance is not 0-25 ohms, remove steering wheel and jump air bag carbon contacts. First, jump the contact closest to steering shaft with the adjacent contact nearest the outer edge, then jump the second set of contacts. See Fig. 9 . 7) Read resistance on SRS tester. Resistance should be 0-1 ohm. If resistance is 0-1 ohm, replace slip ring assembly on steering wheel. Retest SRS light system. See SRS WARNING LIGHT TESTS. If resistance reading is not 0-1 ohm, replace carbon contacts. Retest SRS light system. See SRS WARNING LIGHT TESTS .

CAUTION: Contact surfaces between slip ring and carbon contacts MUST be clean. DO NOT touch carbon contact surfaces with fingers, as static electricity may activate air bag system inside SRS sensor/control unit.

Fig. 8: Locating SRS Air Bag Module Plug Connector
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig. 9: Locating SRS Carbon Contact Brushes
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
Testing Air Bag Circuit 2 (ETR)

1. This test checks the resistance in deployment circuit 2, which is for the Emergency Tensioning Retractor (ETR) seat belt circuit. Connect SRS tester to SRS 10-pin connector located at right front corner of passenger's floor, beneath foot rest panel. Turn ignition switch to position No. 1. SRS tester should indicate 2-7 ohms. If resistance is 2-7 ohms, go to step 2). If resistance is not 2-7 ohms, go to step 3).
2. Test isolation resistance by pushing grounding button on SRS tester. Resistance reading should be 20,000 ohms to infinity. If resistance reading is as specified, go on to TESTING ENERGY ACCUMULATOR CHARGE. If resistance reading is not as specified, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS .
3. Remove passenger's side seat belt trim panel located at passenger's door rear pillar. Disconnect ETR electrical plug connector. SRS tester resistance reading should read infinity. If resistance reading is less than infinity, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS. If resistance reading is infinity, jump the 2-wire ETR harness plug connector.
4. SRS tester resistance reading should read 0-1 ohm. If resistance reading is 0-1 ohm, replace complete ETR seat belt assembly. Retest SRS light system. See SRS WARNING LIGHT TESTS. If resistance reading is not 0-1 ohm, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS.

Testing Energy Accumulator Charge

1. With SRS tester connected to SRS 10-pin connector for over 30 seconds, the energy accumulator is recharged by SRS tester until maximum battery voltage is reached inside energy accumulator (minimum of 9 volts). If SRS tester reading for energy accumulator is 9 volts or greater, go to TESTING ENERGY ACCUMULATOR CAPACITY. If SRS tester reading is not as specified, go to step 2).
2. Disconnect SRS tester from SRS 10-pin connector. Unplug energy accumulator 2-wire electrical plug connector. Jump energy accumulator harness plug connector terminals. Set up SRS tester for external resistance measurement. See SRS TESTER PREPARATION . Using SRS tester, measure resistance between pins No. 4 and No. 9 of SRS 10-pin connector. See Fig. 6 . SRS tester should indicate 0-1 ohm. If reading is 0-1 ohm, go to step 3). If reading is not 0-1 ohm, replace SRS wiring harness. Retest SRS light system. See SRS WARNING LIGHT TESTS .
3. Replace energy accumulator and repeat step 1). If SRS tester resistance reading is now 0-1 ohm, go on to TESTING ENERGY ACCUMULATOR CAPACITY. If SRS tester resistance reading is not 0-1 ohm, replace SRS sensor/control unit. Retest SRS light system. See SRS WARNING LIGHT TESTS.

Testing Energy Accumulator Capacity

If SRS tester energy accumulator (capacitor) indicator light goes out, read measured value on SRS tester. This measured value should be greater than or equal to measured value specified in SRS TESTER READINGS table. If measured value is as specified, replace SRS sensor/control unit. Retest SRS light system. See SRS WARNING LIGHT TESTS . If measured value is not as specified, replace energy accumulator (capacitor). Retest SRS warning light system. See SRS WARNING LIGHT TESTS.
WIRING DIAGRAMS

Fig. 10: SRS Wiring Diagram (1985-87)