'85 300D/W123 tach stopped working

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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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.

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Karl wrote:

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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