[W126 Coupe] electrical expertise needed

Wed Sep 14 12:09:30 EDT 2005

Is the light behind the HVAC temp dial also burned out? I know, I've
looked at the wiring diagram and there is no reason for this to be the
case, but the clock in my car stopped working for about 6 months. The
light behind the temp dial also didn't work. One day my bro got mad at
the temp dial for some reason, smacked it, the light turned on and the
clock started working. This was about 4 years ago. Still works.

Richard

________________________________

From: mbcoupes-bounces at mbcoupes.com
[mailto:mbcoupes-bounces at mbcoupes.com] On Behalf Of Markus Meyer
Sent: Wednesday, September 14, 2005 12:02 PM
To: figstir at yahoo.com; mbcoupes at mbcoupes.com
Subject: Re: [W126 Coupe] electrical expertise needed

Hi "figster"...

I am trying to test my annoying clock, that sometimes works, sometimes
doesn't. If you remember the recent post by Dick Spellman pointing me to
the BenzStore that rebuilds them, they told me that clocks are testable
by hooking 12v to the spade connector on the back of the clock and
grounding the clock. Also see Dick's email earlier today talking about
connecting the clock with wire leads to + and - of the battery for
testing purposes.

Now, why test it this way? Well, because the clock acts flaky, that's
why. I don't want to waste money shipping it somewhere to have them tell
me its not broken and then ship it back. So I am trying to test it on my
own first to see if it really works. If it does, I know the problem is
in the wiring in the car, either the power to the clock wire is flaky,
or the ground that it uses is flaky. As to the clock itself, there
aren't that many components. I've been told the capacitors go bad, but
when I talked to the BenzStore they said that if those go, "usually" the
clock just stops working and that's that. But mine works fine for a day
or two, then suddenly stops, then starts up again when it feels like it,
and so on. Dick suggested maybe the adjustment mechanism is sticking but
that only makes sense if I touch it and adjust it. I can do NOTHING and
the clock will work or stop as it feels like. So I'm trying to debug
this situation without spending tons of money and do things logically.
I'm not trying to hook up anything permanently, I wouldn't go that far
on my own for sure! Just debugging.

Thanks,
Markus

--- On Wed 09/14, a figment of the imagination < figstir at yahoo.com >
wrote:

From: a figment of the imagination [mailto: figstir at yahoo.com]
To: mfmeyer at iwon.com, mbcoupes at mbcoupes.com
Date: Wed, 14 Sep 2005 08:39:33 -0700 (PDT)
Subject: Re: [W126 Coupe] electrical expertise needed

Hey Markus:

Electrical stuff is my specialty, so I'll try to help.
This is a simple, abbreviated explanation.

Basically, there are two types of ground. First,
there is ground, such as the chasis. Second, there is
electrical ground, which is closer to a negative lead
or terminal. In these W126 bodies, Mercedes-Benz
engineers have done a great job of color coding all
the wiring. Brown wires without any stripes are
generally ground, which I would call closer to an
"electrical ground" or one that goes directly to the
negative terminal of the battery. 

I've tried both types of grounds when debugging,
diagnosing, and re-wiring or modifying tons of
electrical stuff in my SEC. The best ground to use is
one off the brown wires. There are several common
contact points for this throughout the car. One great
one is right behind the instrument cluster. Another
one sits behind the stereo deck.

Regarding direct connections to the battery. This is
a "no no"! 

First, you are playing with 12 volts DC. While this
may not seem dangerous to you, it can be. It is
direct current and you can pull as many Amps as your
resistance/impendance will allow you to. As an
example, this could be as high as 15-25+ Amps (hence
the fuse ratings), which could theoretically kill you,
if not give you a great big jolt. I have only to tell
you that I have experienced some level of it and I'll
tell you that it hurt bad and had me using rubber
gloves from that point further, despite my experience
and talent. 

Second, you don't want to directly connect to the
battery because you can not only short-circuit the
electronics you are working with but you could also
create a hazard for a nice electrical
fire!...obviously, right in the engine compartment
with its rich sources of fuel, air, etc... 

Third, it's simply bad etiquette to wire anything
directly to the 12-volt battery; it looks bad...like a
serious hack job.....of which I would immediately shy
away from, if you were to ever resell your car. 

Fourth, the wire you may be using may not be able to
handle the current directly from the battery. Many
people don't understand wiring ratings (thickness,
length, conductivity, strands, shielding, etc..). If
you use the wrong one, you can create a source of an
electrical fire. 

Fifth, you would always want to make sure you have an
in-line fuse rated for the max amps of your testing
device and in some cases an electrical relay to trap
fast spikes! This is why there is a fuse box with
fuses and relays! 

Consequently, I highly recommend that you pick off
12-volts from somewhere within the W126 body and that
you use the solid brown wires as you're ground. What
I have found is that the bright red and white striped
wires are a hot continuous 12-volts. The
yellow-black stripped wires supply "ignition key"
switched-on 12-volts; that is, after you have put the
key in the ignition and turned it one notch, without
cranking over the engine, you get current. I would
recommend that you use this wire, but test it first
with a volt-meter.
Always, use proper testing equipment before directly
connecting to any device. 

Finally, in general, I don't recommend that you play
with electronics. It would be best to find a
specialist to do the work for you, because you will be
extremely upset if you were to damage anything due to
either my suggestions or work of your own, even by
accident, which can happen easily. It is frustrating
and often hard to tell what has gone wrong and
requires great debugging and problem-solving thinking
skills. Again, this is electronics and it takes years
of experience and physics/engineering study and
practice to really understand what you are doing, how
the system was designed, and to use test equipment and
to diagnose and debug problems you encounter along the
way. Most kids at audio shops, as well, do not have
this experience and may tell you things that are
blatantly wrong, by-passing common sense or physics,
leading to a short-circuit or electrical fire later on
after the work is done or something else. I know this
also from experience in believing these kids were more
"experienced" and knowledgeable" than me, even despite
playing with electronics since I was a 7-years old and
studying it formally at a pre-eminent school. 
Sometimes they are correct, but they stay within
limited bounds, which allows them success, as an 
apprentice, and that's good for the limited stuff they
do. This may be all that you need depending on what
you are doing. In general, if you are extremely
careful and have great understanding of everything,
you should be successful.

Please tell us what you are testing or trying to
hook-up. If there is a way, I can help let me know. 
I can be briefer, if I know more specifics.

-figment of high-tech electronics
aka, fig

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