I am going to quote a message one of our members sent to another related post:

[QUOTE=rcounts;1833709]For those wondering about the 84 and 85 models and the tach amp, my 300 CDT (production date 01/84 - must've been one of the first one off the line that year) there is NO tach amp under the cover on the assembly mounted inside the DS fender. If you remove the cover and look at the sockets, the #3 socket (they are numbered) doesn't have the metal female pin in it.

On this style the tach gets its signal from a different sensor mounted at the BACK of the engine in the bellhousing of the transmission - just behind and below the bottom of the oil filter assembly. The sensor is held in place by an allen-head screw and it takes a 5mm allen key to remove it.

My sensor is bad and needs to be replaced. You can test it by unplugging the 2-wire cable connector at the center of the hood opening just in front of the cowl. If you follow the wire from the sensor up to the connector it is easy to find.

I ordered and installed a new sensor wire to no great change. I used an OHM meter set to 2K to read the resistance in the new sensor and got a digital reading of 573, the OHM reading on the old sensor is 1900 +- some. With the engine running I can't get any kind of voltage reading with the old or new sensor installed. The OHM reading (1900) on the old sensor was made with the sensor installed, the reading on the new sensor wire was with the wire not installed, just a test out of the box.
I know it is a pulsed signal from the flywheel, something akin to an ABS sensor type pick up right? I am pretty sure my tach issues are related to not getting a signal from the sensor. Tach used to work, then intermittently worked, then failed. Are there different versions of this sensor wire? Am I using the OHM meter correctly? Set to lowest DC volt setting, I never see anything with the engine running. Did I maybe receive a bad-out-of-the-box sensor wire (BERU)?
Thanks Much

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1985 300D 197K - Semi-Daily Driver Diesel
1998 Volvo V70 AWD 226K - Daily Driver 2
1998 Volvo S70 140K - Wife's DD
2003 GMC Sonoma ZR2 Option - Rusty Truck
THE BABY 1958 220S Sedan 66K All original, never restored and never will be.