[W126 Coupe] electrical expertise needed

[a figment of the imagination]

Hey Markus,

It sounds like a voltage problem right off the bat.

Dick's suggestions can be confirmed by myself, as
well.

I would like to add the following.

1. The terminal off the back of the instrument cluster
behind the tach is attached to a reddish wire which
provides 12-volts.  Test this wire to make sure you
are getting the proper voltage; it should be around 12
volts, although the clock should probably be able to
work with as low as 9-volts.  

2. If the voltage is low, then you have other obvious
issues, with regards to the battery, alternator, or
other.

3. With the clock and/or instrument cluster off the
car, simply connect 12-volts directly to this same
lead from a separate 12-volt source. Use the negative
terminal as your ground; you should be able to find
the common terminal to the clock easily on the clock
itself or on the back of the instrument cluster.  I am
sure that this is what you were thinking and were
asking about.

My only concern here is that you make sure you are
connecting directly to the clock.  There are indeed
capacitors, IC chips, transistors, and resistors in
the back of the instrument cluster that could be
damaged, by accident, if the are connected to 12-volts
directly when they were not designed to be.  The
terminal mentioned in #1 should be fine but check it
just in case.

4. Your ground on your instrument cluster could be
faulty.  While for me the jury  is still out, I am
aware of others with SEC's having problems with faulty
grounding on their instrument cluster.  I currently
have possibly a faulty ground to my instrument lights,
causing them to fail.  My comment about a faulty
ground is that, if this was indeed the problem, then
you should see other problems with other electronics. 


5. Based on this logic in #4, which may not be fully
complete, I would suggest that you may not have the
proper voltage being sent to the clock.  As stated in
#1, check it.  Check the battery level, check the
alternator, check for other problems with other
devices or shorts.  

6.  In general, a faulty capacitor should have nothing
to do with intermittent operation of your clock.  A
capacitor merely stores a set amount of voltage, after
being in properly adjusted with resistors and other
elements.  You seem to be describing a run-down type
of phenomenon, kinda like a "subclavian steal" in
medicine or even "sun-downing" in psych, or some
better analogy.  [Hence, alternator, battery, etc..
issues]. I highly doubt a faulty capacitor would allow
for intermittent operation, but a faulty and
non-constant voltage supply would.

I'm sure your clock is good!  You may need to take
care of a loose connection somewhere.  You can check
continuity and resolder broken or faulty leads or
terminals.  Make sure you check voltage, as I
mentioned in #1.  Another analogy is your analog clock
in your home/work, which slowly runs down or stops
working as the AA battery dies off.  If you are
draining the battery as you go throughout the day,
your clock will be affected.  If this is indeed the
cause, how this drain is occuring needs to be
investigated.

7. Regarding a good direct grounding point, I
recommend  the common ground source behind the
instrument cluster.  There are at least 3 big bolts
holding down the terminals of numerous brown leads.

If this is helpful, that's great.  If not, keep trying
with these and others' suggestions.  I am certain it
is very simple.  Please keep me informed.

Cheers!

-fig

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