The Tachometer on my 190D was working and now it has quit.
Need to know where it gets the signal from... I am used to gas engines that get it from the coil, so kinda confused.

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2009 Mercedes ML320 Bluetec

There is either a sensor on the bellhousing that picks up the starter ring gear, or one on the front that picks up the harmonic balancer. As I recall all 60x engines use one at the back of the motor.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

What type of output is it?

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2009 Mercedes ML320 Bluetec

In '84 the 617's went to a pick up on the transmission.

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83 SD

84 CD

On the 190D-2.2, the tach signal runs through a control unit located behind the battery before it goes to the tachometer gauge in the instrument cluster.

I posted some schematics, etc. from the service manual on the following thread which may help you trace out the tach signal.
'84 190D, EGR Module..???

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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

Thanks Cornblatt!
Seems I had not connected the Tach failure with the fact that I took the Klima Relay out cos I was testing the A/C!
That did it!

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2009 Mercedes ML320 Bluetec