For 85 and up diesels you get the tach pick up off the ring gear. This has two wires. How does this translate to RPM? I am trying to read this with an arduino and it's not clear if it voltage or resistance or what....

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http://superturbodiesel.com/images/sig.04.10.jpg
1987 300D Sturmmachine
1991 300D Nearly Perfect
1994 E320 Cabriolet
1995 E320 Touring
2001 Eurovan FOR SALE
1985 300D car, sold and missed.
OBK #42

It will make an AC waveform. Higher frequency = faster. It will work very much like an ABS wheel speed sensor.

http://www.youtube.com/watch?v=6dCoYv511AI

-J

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

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Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
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2013 Fiat 500E.

Also, make sure your input can handle the sensors max voltage + some head room otherwise it might start clipping and you will end up with eratic readings. Don't try to measure the voltage and convert it to RPM, you will end up having non repeatable readings. Frequency ( analog ) is all you care about, duty cycle won't get it done either.

The sensor is a coil of wire wrapped around a rod magnet, the end of the rod looks at the flywheel teeth.

This type of system is very comon on industrial diesels, look at " Murphy " brand for off the shelf tachs, speed switches.

so no, not with a multimeter. the waveform is just a sine wave, but you need to be able to count the periods, so an oscilloscope is the right tool for visualizing it.

are you trying to replace the EGR computer? i put an old-style tach amp on my '85 engine.

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'77 240D, 504H, OM617.952, etc.

Although a bit technical, it's not actually a Hall sensor, it's a VR sensor (varible reluctor). As has been pointed out, the signal is an AC signal that is read as one pulse every time the voltage goes from positive to negative.

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I just thought of something, some super multi meters have a frequency measurement function, this should work for what you want to be. However digital meters are useless for fast changing values.

The sensor should have an ohm reading of 750-1000 ohms cold. If the sensor reads 1000 to 1200 ohms cold it is marginal and should work with the engine cold. 1200 ohms and up is a bad sensor. The later models that use the sensor for ignition firing use the same style of sensor. Just in a different package. John

That explains why there is no such thing as a 2 wire hall sensor....

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1987 300D Sturmmachine
1991 300D Nearly Perfect
1994 E320 Cabriolet
1995 E320 Touring
2001 Eurovan FOR SALE
1985 300D car, sold and missed.
OBK #42

I guess I can figure out the max with my multi meter thanks!

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1987 300D Sturmmachine
1991 300D Nearly Perfect
1994 E320 Cabriolet
1995 E320 Touring
2001 Eurovan FOR SALE
1985 300D car, sold and missed.
OBK #42

I'm working on round 2 of this:

ESM Amarino project part 1 - YouTube

I lost all the code but I now have the arduino working with thermocoupler and the OEM bosch boost sensor from the 91 300d. I will probably get the BT and android interface working again but I am going to put a 1.8" lcd where my broken tach is.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1987 300D Sturmmachine
1991 300D Nearly Perfect
1994 E320 Cabriolet
1995 E320 Touring
2001 Eurovan FOR SALE
1985 300D car, sold and missed.
OBK #42

On which engine? From my notes the RPM sensor on an OM-617 (the one near the harmonic balancer) should be around 80 ohms. It's resistance should not change much.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked