I was able to acquire a tachometer/clock for a W123 280E. How does the wiring of these things go? There are three terminals on the tach itself. One is marked with a + (+12V I assume), the center pin is grounded to the tach's body, and another one marked with a W-.

Any ideas? I tried hooking up the + terminal to a 12V source and the W- terminal to the negative side of the coil. Nothing.

However, when the center terminal is hooked up the negative side of the coil, and the W- terminal is not hooked-up, the tach works (although a few hundred rpms low).

My wiring doesn't seem right. Anyone?

Thanks and regards,

I am experiencing a similar problem with my 240D. Apparently the sensor comes off of the fly wheel (or somewhere around there). The feedback that I received was that if the car is not designed for it is is difficult to add it.

I am interested to see if you receive other feedback.

john, ..try removing and cleaning the sensor located in the transmission. Minor dirt can give you false reading in the tach.....I heard that there's a good MB techs in Makati.

Will you let me know....

Got it to work.
 

First off, I believe the tach on a Diesel engined car is different from a Gasoline engine. The 240D has a mechanical tach and there is a tach sending unit somewhere in the engine (not sure where, or what it is -- I haven't worked with diesels).

I haven't heard of the sensor on the transmission before. Could it be possibly for the ABS to measure road speed?

Anyway -- here's the fix. I connected the terminal on the tach clearly marked with a + to the negative side of the coil. The terminal on the tach marked with a -, was connected to the positive side of the coil. Huzzah! It now works. I figured this out by accident, BTW.

Why? I do not know... my first thought was that my coil was wired backwards -- but using a voltmeter, there is 12V+ on the positive side of the coil. So its wired correctly.

So there it is, its either I got a tampered/repaired tach and the terminals were soldered in reverse, or that really how MB/VDO did it -- to confuse the general public???

Oh BTW, the tach I got was for a 4-cylinder engine, so the tach read a bit high when used in my 250T. So I installed in my 4-cyl 230TE -- works just dandy.

regards,