I just looked this up last week as I was troubleshooting an intermittent tachometer on my '84 190D-2.2 5-speed.

The tach signal comes from the A/C compressor control module. You don't need EGR to have a tach (but you do need the A/C compressor control module for the tach to work). My car came from the factory without EGR, so there is no EGR module. Unless your car is a california model, you won't have one either.

See page 118 of 290 in the Electrical Troubleshooting Manual (PDF copy). This page is marked 127. It shows the tach signal coming from a triangle marked 'B'.

Then, see page 264 of 290 (marked 309). It shows the "Engine RPM Output" coming from the A/C compressor control unit and going to a triangle marked 'B' (which refers back to the first drawing I mentioned).

The EGR module shown in the electrical drawings can be misleading since it may not actually be present in your car.

I can send you these PDFs if you don't have the manual I'm referring to.

The A/C compressor control module is located behind the battery (there should be a plastic barrier between the battery and the module). It plugs into a socket similar to what you have described and has ~9 wires going into it.

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1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD