David’s Procedure for Fixing the Airbag Warning Light and Seatbelt
Warning Light Gremlin
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
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
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
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.
|Flat screwdriver with tape on the tip, for gently prying the clock module
loose from the dash.
|Phillips screwdriver sized to fit the two small screws fastening the PCB
to the clock module housing.
|Multimeter with ohmmeter function.|
|Soldering iron (I used a 30 watt iron from Radio Shack).|
|Rosin-core solder, with no silver content (available from Radio Shack).|
|Solder sucker, aka ?vacuum desoldering tool? (available from Radio Shack).|
|(optional) Test lead with small alligator clips on the ends (Radio Shack).|
|Soapy water (apply sparingly between rubber seal & dash to smoothly
reinstall the clock module in the dash).
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.
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
|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.
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
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