Disclaimer: The content of this document relates to the Porsche 993 (911 1995 to1998) I am not a professional automotive technician and some of the opinions expressed in this document are just that, opinions. Also, the airbag/SRS system contains explosive devices and therefore potentially dangerous to handle or work on therefore, please feel free to use this document at your own risk. Many times a lot can be learned by simply watching qualified professionals work on your 993.
Please read this entire document before working on the airbag/SRS system. The information further down in the document may give you added insights into how to disassemble components in the system or suggest additional root causes of SRS system problems.
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Symptoms: The Air Bag (SRS) warning lamp lights at start-up, and goes out after a while.
This indicates a non-deployment (non-crash) related system fault, according to the 993 shop manual diagnostic section. Below are things to do before replacing any components. OR The AirBag (SRS) light in the clock remains on continuously and never goes out with the ignition on where no crash has been experienced:
- First, try to clear the fault and airbag warning light using Durametric or other OBDC Readers that can access the Supplemental Restraint System (SRS - Air Bag) and see if the warning light returns.
- General items to check:
- Check the fuses powering the system by inspecting and testing the fuses with an Ohm meter. If the fuse contact tabs are not clean and shiny just replace the fuse.
- Use a voltmeter to measure the voltage on the supply fuse top for the presence of power.
Disconnecting the Battery:
Cleaning the connection to the control module:
- Disconnect the negative battery terminal and tie it out of the way so it can't inadvertently fall back on the battery post. This is very important as disconnecting any aspect of the SRS system with the battery attached will create an OBD Fault requiring a reader that can access the SRS system to clear it.
Remember by disconnecting the battery you are also disarming a detonator on an explosive device! The battery remains disconnected for all the following procedures!
Keep in mind the SRS (Airbag) system is very simple in the 993. It has only 4 components, the two airbags, the controller, and the contact unit in the steering column. The factory 993 writing diagrams do not show any seatbelt buckle switches, seat pressure sensors, or external accelerometer sensors. The contact unit in the steering column is the only moving part subject to mechanical wear and connects the airbag/horn pad to the wiring harness in the dash. Therefore most non-deployment faults have to do with the connections to these components where the fix may simply involve cleaning them.
- The Airbag (SRS) Control Module is under the dash in the center of the car and is orange in color. It has a release lever on the back of the connector that flips up to release the plug. The connector is very delicate, very little force was needed to unplug the module. Clean both the plug & socket of the controller with spray electronic cleaner such as CRC QD Electronic cleaner and reassemble the connection.
- loosen the two screws through the back of the steering wheel to release the airbag/horn pad unit. The screws remain inside the steering wheel once loosened. Use the appropriate star drive wrench to unmount the airbag/horn pad. You will need to cock the steering wheel slightly to the left and right so your tool has a clearance between the wheel and the dash to get it in position. Once the screws are loose recenter the steering wheel.
- For safety touch a metal ground on the car such as the barrel of the cigarette lighter socket to ground out any accumulated static charges on your body, then unplug and set aside the airbag/horn pad unit making it easier to inspect and clean the wires behind it. This can also help in avoiding damaging the wiring harness behind the pad.
- Check all the wire connections behind the horn pad/airbag unit. The female spade connector that attaches the brown ground wire to the airbag mounting frame can be worn so check it carefully.
- The wiring pigtail to the contact unit (clock spring) in the center of the steering wheel behind the horn pad/airbag looks like it is attached to the contact unit with a plug. It does not unplug from the contact unit housing so don't try! Do not remove the steering wheel for this procedure.
- Repair any suspect wires and reconnect and reassemble the horn pad/airbag to the steering wheel.
- The contact unit has proven to be very robust, however, it can be tested without removing it or the steering wheel by using a Philips screwdriver to remove the center, left and right clamshell covers
over the turn signal/hi-beam/cruise control stalks that surround the steering column. Once removed the plugs to the car's wiring harness from the contact unit can be reached through the hole in the dash at the base of the steering column where they enter the dash. Cut the tie-wrap that holds the plugs and sockets inside the hole if present and pull the two connectors out of the hole. By unplugging them one can now use an Ohm meter to check the connection from the plugs through the contact unit to the wires in the center of the steering wheel behind the steering wheel horn/airbag pad.The contact unit behind
the steering wheel
Testing the Contact Unit:
The two harness plugs attached to the wiring harness in the dash lead to the ground wire, horn, and airbag plugs behind the horn/airbag pad. Use the set of leads on an ohm meter and try one of the pins on one of the wiring harness plugs by touching one of the meter's leads to it then cycle through the ground/horn and airbag plugs/wires with the other lead until you see continuity to one of them from that pin. Then do the same for each pin on the two connectors to the contact unit that came out of the dash. They should all show continuity to one of the pins/wires behind the horn pad. If any of them do not it is an open connection inside the contact unit. If the problem is intermittent, you may just need to replace the contact unit as it may test OK when static but when the steering wheel is rotated it loses continuity. A leading indicator of this may be if your SRS trouble light seems to come on and extinguish randomly while driving the car.
If the contact unit is not showing continuity in one or more of its connections here is how to replace it:
- Unplug the contact unit's wires connecting it to the steering wheel horn frame.
- Center the steering wheel so the front wheels of the car point straight forward and the steering wheel is level.
- Remove the large steering wheel hold-down nut with the appropriate socket tool.
- With a permanent marker mark the position of the string wheel shaft to the steering wheel body before removing the steering wheel so it can be reinstalled in the same position over its mounting splines.
- Lift off the steering wheel
- Unthread the contact unit's pigtail wires from the steering wheel and the steering column
- Remove the contact unit's mounting screws and lift off the unit.
- Install the new contact unit,
- Once the new unit is in place re-clock the contact unit according to the directions printed on the unit and in the shop manual. This is a very important step as an improperly clocked unit will be broken when the steering wheel is turned once the steering wheel is mounted. When re-clocking the contact unit a good trick is to remove the grossly over-engineered small metal spring-loaded “Rotation Lock” (call-out 3 in the image below). Instead, just use a long piece of electrical tape to hold the contact unit in the proper rotation position. Thread the tape through the steering wheel using the same hole the wires from the contact unit pass through. Once the steering wheel is placed over the correctly clocked contact unit and the contact unit’s pins (2) are in their slots on the back of the steering wheel at 12 o’clock and 6 o'clock just pull the tape out through the wheel.
- Place the steering wheel over the steering column splined shaft lining up the pen marks made earlier during its removal.
- Replace the washer(s) and the steering wheel hold-down nut and reconnect the contact unit's wires to the horn frame in the steering wheel.
- Reinstall the horn pad/airbag.
The Airbag (SRS) Contact Unit
- To get the clock out I first pulled out the speedometer and pushed the clock out from the back by reaching through the speedometer's hole.
- Pull the clock. The pins on the clock's connector looked tarnished. I used a little CRC QD electronic cleaner on a Q-Tip to clean the pins and then buffed off the tarnish with a pencil eraser. I then sprayed out the harness plug to the clock with a spray CRC QD electronic cleaner. Clear the fault and see if it returns if not,
- Disassemble the clock to verify all the warning light bulbs
in the clock are the original style incandescent bulbs, not LED aftermarket upgrade bulbs, and are tested as good with an ohmmeter. Bend the bulb contact tabs out slightly as needed to ensure good electrical contact to their sockets. Using a LED bulb in the SRS lamp position will trigger a fault warning light. The same goes for the seatbelt warning light.The back of the clock - Remove the internal circuit board in the clock and reheat all solder joints on it with a 25 Watt pencil-style soldering iron and if needed add some 60/40 Lead-based rosin core solder. Do not use lead-free solder as it has a higher melt point requiring greater heating of the circuit board that can damage it. Failed solder joints in the clock are a known issue on a 993 that may cause an SRS fault. There are three solder points on the board that attach the clock motor that need to be unsoldered for the circuit board to release from the clock so it can be inspected, reheated, and cleaned with spray electronic cleaner.
- Reassemble the clock, spray the plug and socket on the clock's back with an electronic spray cleaner, and reinstall it.
- Clear the diagnostic fault and see if it returns.
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