I'm working on making a counter for a pickup coil winder for my guitars (I build electric guitars for a living) and have been relentlessly searching for a sensor that will be able to keep up with the RPM's of the spindle.

I was wondering if I could buy a MB part to use for the input device to my counter? I've used Reed Switches and they only get up in the low thousands. Obviously our car's tack gets in the 5k range which is closer to what I'm looking for. Is there a switch on the MB engine actually ticking away, or just a little generator somewhere creating a proportional voltage?

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-190d '89 Non Turbo 2.5 5cyl 240k (my first MB)
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http://www.schumanautomotive.com/forums/showthread.php?t=5269

Full write-up, some guy actually built his own replacement tach amp after reverse-engineering it.

The long and short of it is, the crank position sensor magnet generates a tiny tiny voltage pulse which is amplified and post-processed by the tach amp, and drives the actual tachometer.

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1983 Mercedes-Benz 240D
Automatic, A/C, Power Sunroof, Power Right Side Mirror
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The OM617A diesel engines use a switched reluctance sensor. This is a coil of wire that registers the passing of a ferrous metal pin through the imposed magnetic field. Each time the pin passes, the current in the coil changes. The tach amp registers this change in the current and outputs a single 3ms pulse - once per revolution.

If you're only interested in full turns, then the single pulse per revolution would be fine. If you want to register half turns, quarter turns, and finer, then an optical sensor might be better.

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Bob
'82 300D Petrol B-G Metallic

Yep, just turns but I'm not sure what type of sensor would be best. I've found that the magnetic Reed switch works but not fast enough. I've also tried, metal contact. That only worked at VERY low RPMs. I'm not sure what to buy.

__________________
-E300d '99 350k
-Suburban '93 220k
-TDI Jetta '03 350k
Sold
-F250 '96 7.3
-Dodge Ram 12V
-E320 '95 200k
-E320 Wagon 1994 155k
-300d Turbo '87 187k miles
-E320 1994 200k
-300d Turbo '84 245k (sold to Dan62)
-300d Turbo '84 180k
-300sd '80 300k
-7.3 Powerstroke Diesel 15P Van 500k+ miles
-190d '89 Non Turbo 2.5 5cyl 240k (my first MB)
Tom's Imports of Columbia MO Ruined the IP in changing leaky delivery valve O-Rings - Refused to stand behind his work. Mid-MO MB drivers-AVOID Tom's.

Here it is locally:

How the Tach Amp Works

That "some guy" was actually two guys - I reverse-engineered the amp, and techguy512 built a microcontroller version after that.

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1984 300TD

Do you want this just for a visual readout or do you want to control something with it. If you just want to be able to visually read the RPMs there is an inexpensive optical hand held unit on the market. It requires sticking a small piece of reflective tape (included) on whatever you are clocking, shaft, pulley, etc. I have used one for adjusting generator RPM and it seems to be very accurate in the 3 to 4,000 RPM range I use it for. I think you can find them on eBay for under fifty bucks.