1985 W123 300D (Federal) Tachometer Issues - esp. paging Karl

[JMH]

Hi,

I recently acquired an 85 w123 300D. The tachometer does not work,
and so I desire to get it going again.

I understand that the tachometer does not use the typical signal path
with amplifier, etc., and rather comes from the flywheel via an EGR
computer. I would very much like to know how I can trubleshoot the
system to determine where my issues are.

As far as I can tell, I need to do the following, with questions
intersperced:

-Verify that I have the flywheel pickup. Where do I find this, and
where do either end go?

-Verify that the flywheel pickup works. How can I verify this
functionality?

-Verify that I have the correct transmission that allows the pickup of
signal from the flywheel. What transmission would an 85 w123 have,
and how would I tell what I have (in case parts were swapped in the
past)?

-Verify functionality of EGR computer that transfers the tach signal.
Any ideas how to do this?

-Verify connectivity of tach signal to tachometer in cluster. I
should be able to see this by pullng the cluster out, right? If so,
it ought to be a 3 pin tach connector, correct?

-Verify overvoltage relay operation. I believe that this relay is
what makes the EGR computer work at all, correct? If so, where do I
find it, and how do I test it?

-Verfy Klima Relay for A/C operation. Where do I find this relay and
how do I test it?

Given that the system in the 85 w123 is different, does this mean that
there are changes to the ACC unit in the 85 as well? Mine doesnt seem
to work, but I worry about buying parts that might not fit properly,
as most places generically lump engines, ACC uinits, etc. from 82-85
all together, though there are apparently differences.

Any insight and assistance would be most appreciated.

Thanks!

JMH

[Tiger]

I think mbz.org does talk about this matter... very common problem.

[Richard Sexton]

In article <K56Vh.8$vE...@newsfe12.lga>, Tiger <tige...@hotmail.com> wrote:
>I think mbz.org does talk about this matter... very common problem.
>
>

http://articles.mbz.org/electric/300tachamp/

--
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
1970 280SE, 72 280SE | Home pages: http://rs79.vrx.net
633CSi 250SE/C 300SD | http://aquaria.net http://killi.net

[JMH]

Thanks...

the 85 does not use the tach amp, but rather an inductive pickup on
the flywheel.

However, ti turns out that my car has a replacement engine, so there
is no pickup.

Now I need to convert the harmonic balancer pickup to be used in
connectivity to the cluster as the tach, like in the older cars (this
includes installing a tach amp and the right wiring).

The other issue is that the rpm sensor is related to the AC
compressor, so I need to bypass the klima relay too.

his is getting into advanced w123 operations now. Any advice is most
appreciated.

Thanks,

JMH

[Karl]

The tach will NOT work if you wire in the early speed sensor.
Totally different frequency.

The REAL fix is to get the 84/85 adaptor plate that bolts on back of the engine behind the flywheel.

"JMH" <jm...@yahoo.com> wrote in message news:1176857236....@y5g2000hsa.googlegroups.com...