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