First off 1993 S600, engine only - no car.
I have been searching the internet high and low to find this information but it seems to be a great mystery somehow. The CMP (Camshaft Position Sensor) on the Mercedes M120 is apparently a hall effect sensor, but I need to know what sort of signal generator it uses. From what I can tell without dismantling anything it runs off of the distributor's "intermediate" gear, but I don't know how. Is it a single tooth, a missing tooth, a 50/50 wheel, or something else?
Anyone ever been this far into one of these to find out? Are they different on the later units, I know they changed the sensor slightly and perhaps (like the flex plate) they changed the tooth count as well?
Thanks in advance,
Chris

If 3 wire it is a hall effect, if 2 wire it is a magnetic pickup.

A hall effect transistor conducts when a magnet is brought close to the transistor. Some systems use a fixed magnet and a shutter wheel to alter the magnetic field.

Generally, you need to apply + 5 volt DC to one terminal, ground to another then you will get a just under 5 volt square wave out as the shutter / magnet is moved across the transistor.

If 2 wire, it is a coil of wire wrapped around a steel core. If there is a magnet in the core, it will sense shutter cuts, gear teeth. If no magnet' it needs a flying magnet to trigger.

In either case, you will get an AC signal that varies voltage with speed.


Given the V12 uses 2 inline 6 computers, I'd have a look at a early electronic FI M104 for information.

In the effort to make this information searchable for future individuals, this is taken from my messages inbox.

Quote:

Originally Posted by Arthur Dalton S600 was not available in year 1993. That would be a 600sel. So, that would be an inductive sensor. If the car was a S600, it can be a Hall effect sensor.....but not in '93 You sure on what yr/mod. the engine came out of ?? A hall would be a 3 wire sensor. An inductive would be a 2 wire. Inductive 2 wire will be sine wave [ ac ] Hall 3 wire will be square wave. [ on/off ]

...and my response...

Thank you, Arthur. I do not know what year it was from, all I know is that it was from an early W140 (came with a 4 speed transmission).
I can look again, but I think that it was two wire - one center pin and a shroud.
So from what you are saying they did change the style from the early engines to the later ones?
Do you have any idea if they interchange from a mechanical standpoint?
To whit - if I wanted to go to hall effect can I change the sensor and trigger and will it all bolt in place?

...also in the name of full disclosure, a magnetic / inductive sensor can be two or three wire - some systems use a shield in the jacket that is an additional ground to prevent noise in the system that presents itself as a third wire. I do not believe that Mercedes is one of these systems - I think that they just use two wires.

So getting back on track I think I have a magnetic or inductive sensor. That still leaves me with the question that I do not know if the magnet is fixed in the sensor or flying on the distributor's intermediate gear. I also don't know how to calibrate for it. Does anyone even know if it reads from a ferrous trigger? I mean, I think it must - but perhaps there is something (more) that I don't know.

Interesting post e.g. reverse engineering!

What I write here is pure therory and don't quote me on it.
Also, I have little to no experience with this engine type.

If I had my fingers in the design, I would use a hall effect sensor. sort of a transistor that I can switch on and off with a magnet.
(Mercedes is using this technology on the wheel speed sensor, although it is somewhat easier on the wheels because we don't care in what position the wheel is. All we want to know is, if the wheel is still moving for ABS purpose.)
This type would require some voltage to be applied.
The induction sensor is probably not the way to do it, since there are other sources of interference that could lead to misreadings. Also, an inductor has the disadvantage to generate a negative pulse and Voltage and Current are out of phase for a brief moment every time it gets triggered, causing unwanted harmonics (Interference). It can be knocked down, but in a vehicle, we don't want to overdo it, if there is a better solution.

The cam shaft has probably teeth or magnets (magnetized theeth number depends on how accurate it needs to be) at the end with one of them being slighty larger than the rest of it.
This would allow for a start position sense and then in conjuction with a fixed number of pulses, the exact position of the shaft could be determined by counting the teeth from the start pulse.
I can imagine that there are at least two cam shafts, left (6) and right (6) cylinder, I suppose.
The design is probably the same if there are two or more cams.

With a two wire design, the actual wire could be the supply and signal wire at the same time.
Physically, I can see a coaxial connector, center signal/supply and the outer is possibly a shield attached to chassis or computer ground.
The device that receives the signal is probably "low active" meaning there is a supply voltage as soon as the ignition switch is on. As the cam turns the teeths on it will pull the hall receiver to ground, meaning it is active. if a non magnetic part matches the Hall sensor, the voltage will rise to whatever supply it is, and so on. The slightly longer active time (low) will determine the start position of the cam. Now there is most certainly more involved, but I think that is the pricipal of the beast.

I don't believe that one can interchange cam position sensors that aren't designed for this engine!
The distance between the magnet and the Hall is important.
Anything is possible, but at what cost?

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

I don't know, I am getting to where it is less interesting from the fun standpoint and more from the frustrating enigma standpoint...

I would have used a hall sensor (from an engineer's standpoint) as well, but as I research this I keep thinking that although it is referred to as Hall it is only two wire.
I found something saying that they have a circuit built into them to suppress system noise, but it did not go into much detail.

The camshaft itself definitely does not have teeth. There is some sort of intermediate gear that connects the distributor base to the camshaft and that is where I think the sensor reads from. A user of another MB forum posted something that refers to multiple teeth and a flying magnet (so far as I can surmise from what was there), but it lists it as a Hall Effect sensor and there are only two wires connecting to it...

The engine actually has four camshafts, being a DOHC V12.

Yes, it very much looks like a coaxial connector.

I am interested about your theory on a two wire system using the same wire for supply and signal - can anyone chime in on how in the heck I wire that sort of setup? What you described makes some sense to me - the most sense so far. So that would make it the uber-rare two wire Hall Effect?

My question about interchange was from early to late style, but when I looked into it I decided the later sensor was the same but without the integrated lead wire.

Good stuff - this might actually be what I was looking for.

Oh, I realized that I asked about wiring method - disregard that. That is a question for the MegaSquirt forum. Can anyone verify this weird Hall sensor theory?

Lm:

Is the pickup (sensor), be it either inductive or Hall Effect, mounted externally? If so, can you remove it for examination? If it is mounted internally, it would seem that you are left with wire count, and if it is inductive, a check of continuity. Failing the above, disassembly would apparently be in order. The engine, as with many others, may very well use two pickups; one for engine speed and ignition triggering, and a second for injection synchronizing. The first is often from a crank speed source, i.e., flywheel, and the second from a half-speed source, i.e., camshaft or dist. drive.

All that said, it would seem that you will need to see the components to answer your basic question: What is the nature of the triggering wheel/device ("signal generator")?

Oh, boy. Now I feel dumb.
It was pointed out that the sensor's body has a small metal ring where the bolt goes through - most likely a ground to the engine.
That would mean that it is a three wire hall effect sensor.

I still do not know the trigger wheel arrangement, though.

Yes, it is external. I had not removed anything in case there was some sort of adjustment - I did not want to get it out of whack. From what I read the only adjustment possible is shims - so if there are any just put 'em back in place.

There are two (actually at least four) sensors - engine speed is from (two sensors 60* apart reading) the flex plate which is designed as a 60-2 wheel, and then there are (at least) two cam sensors.

I guess I may just have to remove the distributor base from the engine. I had hoped to avoid that...

System noise has to be suppressed in any case, even if the signal looks like a square wave, it still can have some ringing.

If the metal ring is in fact attached to the sensor, then my guess is that the signal level is just reversed e.g. it is "high active". In this scenario one wire would have a static supply as soon as the ignition is on the other (signal) wire would go from low to a little below of the supply voltage when active. The ground would be just what it is a reference.

The supply would come from the ECM. The ECM would set the signal to high. At this point it is in a known state. If the sensor at the cam is pulling the signal down the ECM will recognize the new state.
All level detection circuits would be inside the ECM.
Due to the very low output of the Hall device, in any case an amplification is required, most probably inside the sensor housing or close to the Hall device.
After the sensor goes back to its non triggered state the ECM (still trying to supply the voltage) would go back into a high state. This would go on and on like that. Only two wires would be required.
Granted, this would require a very careful design.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert