M120 CMP Hall Effect Sensor - PeachParts Mercedes-Benz Forum

oldtrucker · #1 01-28-2015, 01:48 PM

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?

Lincolnman · #2 01-28-2015, 05:58 PM

Quote:

Originally Posted by oldtrucker

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?

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.

Lincolnman · #3 01-28-2015, 06:09 PM

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?