Webmaster note:
(Since the 993's electrical system is very similar with the 964's system this DIY may help 993 owners diagnose some of the air bag warning light gremlins.  Of all the years I have worked on German cars one thing I did notice is that the soldering quality on the electrical circuits are terrible, mainly cold solder, crack solder problems.  I have seen this problem in cruise control modules, HVAC control units, radios, etc.)

If you are handy with a soldering iron, this is an easy, dirt-cheap repair that you can complete in an evening. The circuit board on the back of the clock module is actually very simple.
I’ve experienced the mysterious airbag warning light and seatbelt warning light gremlin in my '90 C2, that goes away when codes are cleared with the Bosch Hammer and mysteriously returns (in my case 2 months after the first clear, 5 days after the second clear). Sometimes the airbag and seatbelt lights would remain dark for awhile after starting the car, but the spoiler light also would be dark (it should stay lit after startup until the car exceeds several miles per hour). Also, the alternator often would not come online until the motor exceeded 3,000 rpm, the ABS light would remain lit until the alternator came online but the battery light never lit (and it should have with the alternator offline). 

So I pulled the clock module and checked it over. I found that most of the solder joints joining the ring of male connector prongs to the circuit board on the back of the clock module were “cold” solder joints. They looked good, but were functionally horrible with intermittent connections showing open circuit or high resistance conditions. If your clock module has problems, I'll bet this is the cause. 

I resoldered the joints, had the fault codes cleared, and so far all of the symptoms are gone (including the alternator not coming online immediately at startup). 

Below are a description of the circuit board, a list of tools, a sequence of repair steps, and finally a theory as to how the clock module could cause fault codes that will light the airbag and seatbelt warning lights.

The printed circuit board (PCB) on the back of the clock module is actually very simple from an electrical point of view. The PCB provides hard wire connections from a) the circle of male connector prongs soldered onto the PCB, to b) the various light bulbs arranged along the perimeter of the PCB. The PCB also provides three connections to the quartz clock module: a positive voltage connection +, a ground connection -, and a set signal connection V (which goes to the single large connector prong soldered to the board, the corresponding female connector connects to the clock set switch on the dash). There are also two large resistors on the PCB that relate to the battery light, and a small resistor and small diode relating to the quartz clock set signal V and the + connection to the quartz clock respectively. That’s it. 

TOOLS:

1. Flat screwdriver with tape on the tip, for gently prying the clock module loose from the dash. 
2. Phillips screwdriver sized to fit the two small screws fastening the PCB to the clock module housing.
3. Multimeter with ohmmeter function.
4. Soldering iron (I used a 30 watt iron from Radio Shack).
5. Rosin-core solder, with no silver content (available from Radio Shack).
6. Solder sucker, aka “vacuum desoldering tool” (available from Radio Shack).
7. (optional) Test lead with small alligator clips on the ends (Radio Shack).
8. Soapy water (apply sparingly between rubber seal & dash to smoothly reinstall the clock module in the dash).

REPAIR SEQUENCE:

1. Remove clock module from dash, by gently prying with the taped screwdriver until you can grip it with your fingers and pull it straight out, then disengage the two electrical connectors (small one first). The large connector takes patience to remove, work it loose without side-loading it if you can. For more detail see Tom Sharpes’ nicely written How-To on the PelicanParts website (link here). 

Here is what the back side of the clock module looks like. 

2. Remove the two small screws securing the PCB to the back of the clock module. Don’t try to pull the PCB off the clock module just yet. 

3. Locate the three evenly spaced solder joints directly adjacent the inside edge of one of the light bulbs along the perimeter of the PCB. These joints connect three posts to the PCB, the posts pass through a partition of the clock module housing and are soldered to a small circuit board in the quartz clock on the other side of the partition. The picture below shows where they are on the PCB, I took the picture after step 6 below. Note the blue solder sucker in the background.

4. Remove the light bulb so you can access the three solder joints with the soldering iron without burning anything.

5. Remove the solder from the three joints using the soldering iron and the solder sucker, so that the posts are free of the PCB and the PCB can be lifted away without pulling on the posts. Note, the solder joints connecting the posts to the PCB are stronger than the solder joints connecting the posts to the quartz clock. If you tear the posts loose from the clock you will have to remove the bezel on the front of the clock module (Tom Sharpes’ writeup explains how) to access the quartz clock and make the repair. 

6. Lift the PCB away.

Here is what the inner face of the PCB looks like:

7. Check the solder joints that connect the connector prongs to the PCB with the ohmmeter. (On mine, 10 of the 15 prongs arranged in a ring for the large connector had bad connections, including connections to traces for the airbag, seatbelt, battery and spoiler lights). For example, place one test lead in contact with the tip of a prong, and then place the other test lead in contact with a bare portion of a trace that connects to the prong, for example where a light bulb connects to the trace. Most of the prongs connect to traces on the inner face of the PCB, some to traces on the outer face, and some connect to traces on both sides. You should see at most a fraction of an ohm of resistance. You can also remove and check the light bulbs too. Light positions are numbered this way:
Light 1 - spoiler
Light 2 - cabrio
Light 3 - airbag
Light 4 - not used 
Light 5 - “toothed belt indicator” (fan belt)
Light 6 - side marker light
Light 7 - trailer turn signal
Light 8 - automatic transmission
Light 9 - seat belt
Light 10 - catalyzer
Light 11 - battery
Lights 12, 13 illuminate the clock face.

8. Resolder any faulty connections. I resoldered all of solder joints for the main connector prongs, and I soldered on each side of the PCB where there was a trace connecting to the prong. I got the joints nice and hot, and applied lots of solder (I used the solder sucker to remove excess). I used lots of solder so there would be plenty of flux (rosin) to ensure a good, clean joint. I also put a coat of solder on the inside edges of the hollow prong openings on the inside face of the PCB, because the prongs appear to be riveted to the PCB and I wanted to ensure that the solder made contact with all components of each prong. 

Be careful not to get the two large resistors too hot for too long, when you solder the connector prong solder joints connected to them. Cool them afterwards by blowing on them and/or applying a damp rag or paper towel (which you will already have on hand to clean the soldering iron tip with). The small resistor and the diode were far enough away from the connector prongs that they did not seem to heat up significantly. 

9. Retest the connections.

10. Replace the PCB back on the clock module housing, and fasten with the two screws. 

11. Solder the three posts to the PCB to restore the power (+, -)and set signal (V) connections to the quartz clock.

12. Put clock module back in the car. You’re done.

13. If your airbag and seatbelt warning lights are on continuously, go to a shop that has a Bosch Hammer and have them clear the fault codes. Once set, fault codes remain (even after the fault that set them goes away) until cleared.

THEORY:

Circuit diagrams in the Factory manual show that the airbag light is connected on one side (via the connector prong 15) to a positive voltage source, and on the other side (via the connector prong 9) to a pin of the airbag controller. The airbag controller turns the light on by connecting the pin to ground. 

The airbag controller pin will see the source voltage of the light when the connections are good and the light is out. This is because the pin is connected to the positive voltage source through the light bulb, and when there is no current flowing through the light bulb (when the pin is not connected to ground), both sides of the light bulb will be at the same, positive voltage. 

It appears that the airbag controller expects to see a positive voltage on that pin, and that’s the kicker; If an intermittent connection at either prong 9 or prong 15 opens and disconnects the pin from the positive voltage, then the airbag controller will record a fault that causes the airbag light to come on when the connection is restored. Others have pointed out that the seat belt light is activated as a safety feature when the airbag light is activated. 

There seems to be a consensus on this forum that if a 964 is fired up when the clock module is not connected (e.g. when a car is in the shop for new gauge faces), then fault codes will be set that cause the airbag and seatbelt warning lights to come on. This behavior is consistent with the theory that the airbag controller expects to see a positive voltage on its pin leading to the airbag warning light.