This is a shoutout to Funola (and techguy512), but I'll post it here for all to see.

I was looking for techguy to purchase a new tach computer, but his website is down.

So I went hunting for whatever other information I could find, and I stumbled upon this:



But I was also looking into testing the sensor itself. And I'm now confused as to what type of sensor this may be.

1. Inductive sensors seem to have two wires, and a permanent magnet. They produce a sinusoidal wave, and are tested by measuring the resistance between the two wires;


2. Hall sensors have three wires, and they usually include some circuitry in their housing. These are tested by applying electric power (but not too much) to the power and ground wires, and measuring the voltage change as a magnet passes by. They produce square wave outputs.


Our cars' sensors have 3 wires, but I'm dead sure the tab on the balancer is not magnetic. So our sensors can't be Hall sensors, right? Yet they have 3 wires instead of 2, so they're not inductive sensors either, right? I'm also quite sure they don't have magnetic tips.

Stretch mentions that Funola has done some tests on these sensors with a oscilloscope.

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1995 W202 C250 Diesel, OM605 NA engine, 5-speed manual
1986 W123 200, OM617 NA engine, 5-speed manual, rev counter
1989 W126 300SE, 5-speed manual

It is a 2 wire inductive pickup. The 3rd wire is a shield to prevent the wire from picking up stray signals.

Inductive. If you follow the diagram, pin 8 shield is connected to pin 9, one of the inputs (ground). As implemented in the OM617A, it is not a crank position sensor, but a RPM sensor. The sensor on the front of the engine should be around 80 ohms, the one on the transmission bell housing on 85 (and some 84 MY), should be around 1600 ohms.

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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

Thanks

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1995 W202 C250 Diesel, OM605 NA engine, 5-speed manual
1986 W123 200, OM617 NA engine, 5-speed manual, rev counter
1989 W126 300SE, 5-speed manual

this is for 1982 300d 617 turbo. I can't seem to find one.

Kent had one listed, but they are out of stock and he doesn't provide part number.

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Andrew
Montclair, NJ
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1982 300D Turbo
120k
Petrol Blue Green

The subject title of this thread is erroneous. W123 does not have a crankshaft position sensor.

An 82 W123 has a TDC (top dead center) sensor which is mounted on the block by the crankshaft harmonic balancer.

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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

ah - good to know. Well does anyone know the part number for the sensor/wire component that does fit an 82 300?

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Andrew
Montclair, NJ
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1982 300D Turbo
120k
Petrol Blue Green

Item #50
1982 Mercedes 300 D TURBODIESEL ATTACHMENT PARTS,HEATER PLUGS,AND SPEED SENDER UNIT

What's wrong with yours? They rarely go bad. It's just a coil of wire around an iron core embedded in plastic. Measure it with an ohmeter, if around 80 ohms it's good. If open circuit, you need a new one.

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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

Reviving this old thread. Now that I have my compressor removed I am able to remove and inspect my rpm sensor.

Quick back story on why I’m looking at this part. When I got my 82, I had the intermittent tach. I experimented with the cig butt trick and it seemed to help, but it would still go out and then finally stopped working. I inspected the op-amp for any bad solder points, but couldn’t see anything wrong. I happened to find a working op-amp for 40 bucks so I just bought it. Put it in, tach started working again but still has this intermittent behavior.

Reaching down blind with my fingers to examine the pickup sensor, I could feel some kind of shredded plastic bits frayed on the surface, like the top of the sensor had been nicked at one point. Maybe by a tool or who knows? The motor and all the belts, flywheel, fan, pullies- all run smooth and balanced so I don’t think it was that it made contact with the little pin. Pin seems fine too, clearances seem good all around.

Anyway, when I got it out today, cleaned it off, I used an exacto to carefully trim away the frayed plastic bits. I think the bits of plastic were just intermittently interfering with the pickup when they got lodged and unlodged in various positions.

Now that I’ve trimmed them away, I think I can see a bit of the metal pickup inside - look here:

https://photos.app.goo.gl/weqNVRiAxatxdbfv6

So, before I put everything back together and the new compressor in, should I seek out a new sensor, or just go with this cleaned up one? Will the exposed metal make it not work any more?

If it ohms out it should still be good.

I would clean the area well, apply some epoxy so that it's sealed, and give it a try. You have nothing to lose at this point.

If it turns out dead then hit the yard and find another one, there is not a lot that can go wrong with these.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

i was wondering if i should slap some epoxy over the end. didn't want to assume that any epoxy would be as electrically/magnetically transparent as the original plastic though... would some silicone or RTV work?

the only thing I have to lose is installing everything back together and it not working - since to the best of my knowledge this thing is impossible to get to without either the compressor or the vacuum pump off. Right now I have the compressor out but the belt is hanging so I can't start the engine. I'm a newbie with belts so I'm not sure how I could remove this belt without remove the power steering belt, and then I'm into another area. But maybe removing these belts should not scare me at this point in my journey....

Important question then is: Is there consensus that you must use a proper belt tension gauge when putting back belts? Or can this be done by feel?

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Andrew
Montclair, NJ
--------------
1982 300D Turbo
120k
Petrol Blue Green