Tachometers in a W123.

[johan01]

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,

[Tref]

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.

[rudy]

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....

[johan01]

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,