PeachParts Mercedes-Benz Forum - View Single Post - Adding late model w123 tachometer (3-pin) to earlier model car

Maxbumpo · #24 12-18-2017, 10:48 AM

How did you ground the tach?

Typically the RPM sensor provides an AC signal, which varies in frequency with the rotation speed of the engine because it is sensing and responding to either metal target or a magnet passing by. Your sensor can probably be tested with a digital multi-meter set to measure AC voltage, and with the meter leads connected to the two wires from the sensor. I don't know how the '85 sensor works exactly (magnet or metal target) but either way you got lucky and it works. I suspect that the sensor is pointed at the starter ring gear, and as each tooth passes by, it generates a signal.

The Tachometer uses that signal by measuring the frequency, doing some math to determine the number of signals that correspond to one rotation of the engine, and then moving the needle appropriately.