There is a transistor in the circuit that I will try to diagrahm out and put it here too. I know this is not a replica of the original part installed in your Benz's, but the basic concept is there. Now that I have this, I can finish the rest of the schematic, the one problem I see with this setup is the capacitor in the circuit is only 100MF at 16Volt. The vulnerability here should be obvious! This is in the original part and not on this map yet. Give me time! I had a hard time finding this much so far and you can bet that MB did not want to give their circuit!:D:book:

That is not the schematic for the RPM sensor!
 

I hate to tell you this but what you found is not the schematic for the RPM sensor! Tomas already posted the same diagram in a post above, which is from National Semiconductors data sheet for the LM1815.

Here is the pinout for the LM1815

http://www.national.com/images/pf/LM1815/00789301.pdf

I suggest you work from this pinout and not the one you found.

I hate to tell you this but this is not the schematic for the RPM sensor! Some people just like to jump the gun. :rolleyes:

Maybe $7 in Sweden? Over here it's probably under $2.:)

This is not the schematic for the Mercedes Benz part and i stated that, but this IC is a analog amplifier and this schematic is for a typical setup for this type of circuit. I would welcome help drawing out the circuit from those who have it apart as I'm working on this same part as well and drawing as I go. However some components cannot be identified, but so far those are capacitors and I'm sure that this mathematical problem can be worked out. I'm still working on it and am about 1/2 way through. didn't mean to mislead, just got excited.:book:

funola,

Why not get one from a wrecking yard or a used one from a supplier of used MB parts?

P E H

See the first post in this thread. ;)

Funola,


OK U like a challenge.

P E H

Well, this part has a high failure rate and Funola got me thinking about how to figure out the schematic and build a better one. I've been at this all day but I'm getting close to being done!

The part that burned to a crisp on mine looks like it is a resistor and it is at the +12V power input. The solder connections at the resistor also failed. The high failure rate of the RPM sensor (I like Tach amp better), based on disecting it and looking at the way it is built is IMO due to heat and mechanical stress. The engine compartment gets pretty hot, with the insulative property of the silicone potting compound, the components inside is overstressed. If the RPM sensor is mounted lower instead of where it is it would be in a cooler environment and may have a higher MTBF. 2 of the pins on mine were pushed in half way and is due to the design of the connector. Basically the pins are only held in place by the silcone sealant.

I have not drawn out the schematic yet. My eyes are kinda tired right now. Maybe tomorrow. Knightrider, if you made a schematic, take a pic and post it and we will compare.

I suggest removing this diagram since it is not representative of the Tach amp aka RPM sensor.

I'm still working on it, but my eyes are kinda tired too. I thought about taking down the post of the schematic I found, but 300SD81 is the one who is hosting it and there are some using it to figure this all out and I cannot remove someone elses post, it has been useful. we'll get together tomorrow! I'm really tired. I've been at this all day!:o:dizzy2::book:

guys -

an LM1815 IC is used in automotive applications mainly to turn a VR signal into a square wave.

I am pretty sure, the pickup & sensor on the engine are variable reluctance (magnetic), but the tach is looking for a 12v square wave (typically found on an ignition coil in a gasser - it's triggered by negative pulses) so the "tach amplifier" is most likely to convert the sine wave into a square wave (so they didn't have to design a new tachometer perhaps?)

Someone should put a shop scope on both lines (coming from VR sensor, and going to dash tachometer) and I bet there's a sine wave on the engine sensor, and a square waveform on the dash tach signal line.

I sold my '83 300D turbo last year, moved to southern CA, and I am shopping for a replacement for a DD. So i've been back on the forums doing some reading... i do aftermarket engine management for a living and I am familiar with LM1815 circuits, and other VR signal converters.

If I am right, you should be able to wire up a cheap GEneral Motors "HEI Ignition Module" between the sensor and dash tach, and get the tach working.

-scott

OK, I need tach help, too. I do not believe my problem in the tach amp, as I have several, and there is no difference with any of them. What are the odds that they are all bad?

I'd like to start with the magnetic pickup and check that. Does anyone have a systematic diagnostic procedure that wll let me check the mag sensor, and its harness? How about a method to check the tach indicator in the dash? I hate to "fix" components that are good.

My symptons are easy--the tach is dead. It never even flickers. Needle stays on the "0"mark regardless.

It is unlikely that your Tach amps are all bad. Start by taking measurements at the Tach amp socket and report your readings

1. key off. Resistance across pins 8&9. Mine is 80 ohms. I believe is the mag pickup coil resistance.

2. key on (glow light on). across pins 2&6 (2 is gnd). 12V on mine. This I believe is power to the TA

3 key on (glow light on). across pins 2&3 (2 is gnd) . 6V on mine.
This I believe is coming from the Tach in the cluster.

These readings are from a working tach in my 83 300DT