[W126 Coupe] Paradise by the dashboard light?

[a figment of the imagination]

Hi,

About your dimmed dashboard lights.....mine don't work
at all!  I've read that this is a common problem among
the W126's.

I took apart my instrument cluster and tried hard to
isolate this problem. 

In my opinion, I would make the following flow diagram
to debug.

The #1 culprit would be your interior dashboard
bulbs---again, pretty easy to find and replace.

#2. Check your alternator and battery....a weak
alternator and/or battery can easily cause a dim set
of lights.

#3. Make sure all the contacts on the rheostat are
good.  Generally, rheostats shouldn't go bad....most
commonly the connections to it wear out or break. 
There is a spring that is used as the contact point on
the rheostat for the 126 instrument cluster.  This
spring is what you feel as friction when you are
turning the dimmer switch knobby...  A metal contact
from the knobby makes contact with this spring as it
is turned; the resistance is changed like a variable
resistor (if you will) as you turn the knobby.  If the
contacts are otherwise good, simply replace the
rheostat (or chuck it and short the connections to
make a permanent contact).

#4. Make sure you know what "dim" really means; in
general, the MBZ instrument cluster lights are dim to
begin with, especially compared to newer cars.  Don't
expect these to be the equal in intensities, as the
W126, W123's generally have dimmer instrument cluster
lights.

#5.  Make sure the round multiple-point contact cable
to the instrument cluster (left-most cable) is
connected well.  If this is loose or if the terminals
are oxidized, you may not be getting enough current to
power up your dashboard lights.  The SEC service
manual has a page with a diagram about the terminal
contacts  which feed power to the instrument cluster. 
If I remember correctly during my debugging, pin #5
and 6 are connected to the exterior headlight switch
and receive a split 12 volts to send current to the
instrument cluster lights.  When you turn the
headlight switch off, you zero the current to these
instrument lights.

#6.  Make sure your two main instrument cluster light
bulbs are screwed on well and making good contact with
the instrument cluster's circuit board's contacts.

#7. Check for faulty grounding.  This right away can
fix the problem, but it should normally accompany
problems in other areas on the cluster or else where
in the car, as well.

#8. Make sure you don't have any shorts any where in
your car and that all your fuses are good and making
good contact with the terminals in the fuse box, under
the hood.  

#9. Check  the sending unit from the gas tank and make
sure there is no open circuit here.

#10. Check that all your power windows are connected
properly and work fine.  A short here can light up or
dim down your instrument cluster.

#11. Check the seat belt guide assembly; a bad relay
behind the rear seats side panels, a short circuit or
other faults can also affect instrument lighting.

These are my suggestions for now.  I hope this helps.

Cheers!

-figment of inadequate instrument lighting and total
blindness during night-time navigation....not even
enough to find paradise by the dashboard light. ;)

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