84 W126 - How does Tach Work Internally
 

I know there's a pickup on the bell housing (though I've yet to figure out where) and that this signal is fed to the EGR computer, which then sends a signal to the tach. But what is the nature of that signal? I've measured the resistance of the sending pickup - 1.9K-ohms so I assume that's good, and I swapped out the EGR computer from my parts car, and still no tach. So I'm assuming my tach is bad, but before I go through the effort of swapping it out, I want to make sure that this is indeed going to fix my problem, so I want to devise a test to check it out. I either want to measure the output of the EGR computer, or apply a signal to the tach to mimic the EGR computer output.

BTW, I've been under the parts car looking for the tach pickup sensor, and can't figure out where it is. Looked on the left side around the oil filter (from underneath) and didn't see anything like the picture in the manual. Where on the bell housing is it?

Do you have an oscilliscope? If they did not change the design from the earlier versions. There is just a pulse amplifier in the computor. You should see the pulse output with a scope. If there and reasonably clean I would suspect the tach unit in the dash itself. It has to incorporate the counter device. I am racking my brain to think of a common item to generate the pulses to check the dash unit but it does not come to mind. Perhaps someone else has a better ideal.

See this thread - I posted a schematic for a simple pulse generator you can build from RadioShack parts (if your local store actually still carries parts). The pulse generator will let you test your tachometer.

How the Tach Amp Works

If your engine is the OM617, I believer the tach (magnetic) pickup is on the harmonic balancer. It's resistance is 80 ohms per post #45 of this thread when I was working on it.

http://www.peachparts.com/shopforum/diesel-discussion/189424-tach-amp-repair-attempt-2.html?highlight=tach+amp+repair

my '84 has the pick up on the balancer in the front. There is a small round looks like a garbage can module on the drivers side front fender under the hood. This unit has been known to fail. I have taken a bunch apart and after getting the hot glue off, resoldered the connections and they work fine. I havent seen an actual tach unit go bad. Basically there is a square wave thats fed into the tach's circuit, which converts the number of pulses into a dc voltage. This voltage is then applied to the actual tach meter itself and the pointer then shows the rpm.

Your '84 has an amp under the "garbage can"? Mine does not. My '83, on the other hand does, and they are prone to failure, I have replaced the '83's with a new amp.
Both have that crank pickup, but only the '83 is connected to anything via said amp.

Not all tachs have the pickup in front. The sensor was moved to the bell housing starting some time in 1984.

where exactly on the bell housing is it, how big is it, and what's it look like?

Find the post about repairing with cigarette butts.Sounds crazy but it works.