'85 300D/W123 tach stopped working

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Tom Plunket - 18 Feb 2007 00:01 GMT

Shortly after having the steering gear replaced in this car, I noticed
the tach had stopped working. My wife didn't notice whether or not it
worked at any point up to when I did, but I know that it worked at some
point before replacing the steering gear.

I didn't do the work, but I don't really want to bring it back to the
shop that did the work if it's an easy thing to troubleshoot and fix.

I looked under the hood, but of everything coming through the firewall,
everything appears to be connected. The CD-based manual and Chilton's
both are silent on the cable that actually makes the tach go, so I'm
wondering if anyone can point me to where on the engine it actually
connects, so I can see if it's still connected and then subsequently
diagnose it from there. My thoughts are that it is related to the
steering gear installation, but again, I'm not really sure where it's
going to and there are no obvious "dangling cables" to show me, "ah ha".

The car only has 185k miles on it, and the speedo and odometer work
fine. Seems odd that the tach would just "stop working" since I'd think
it mechanically similar to the speedo; just monitoring the crank rather
than the driveshaft.

Thanks for the help,
-tom!

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Karl - 18 Feb 2007 02:53 GMT

It reads off the flywheel and is mounted by the oil filter
housing.
The rpm signal goes from the sensor to the EGR control unit mounted in the right kickpanel. From
there it goes to the tach and to the klima relay mounted in front of the fuse box.
Does your a/c compressor engage? If yes, you have a bad tachometer. If you are going to replace it,
make SURE it is a 84/85. Not 81-83. The type of signal to
the tach is totally different.
If you have no compressor engagement, you have no rpm signal. Look for the 2 pin connector along the
firewall near the #5 cylinder injecter line. Get a digital meter and set it for a/c volts. At idle,
look for >0.5 vac.

> Shortly after having the steering gear replaced in this car, I noticed
> the tach had stopped working. My wife didn't notice whether or not it
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>
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Tom Plunket - 18 Feb 2007 07:33 GMT

> It reads off the flywheel and is mounted by the oil filter
> housing.

Thanks. I'll check this stuff all out, but the A/C compressor froze at
some point in the past so I'm thinking that checking to see its
engagement will be futile. ...although might that have caused the tach
to fail for some reason?

Thanks for the explicit directions, though. I'll take a peek again when
it's light outside.

-tom!

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