Mercedes-Benz Forum banner

M120 CMP Hall Effect Sensor

3K views 11 replies 3 participants last post by  Lincolnman 
#1 ·
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
 
#4 ·
Maybe this'll help. This is from my Star Service manual on the camshaft sensor function. It's not much and not even for your engine, but maybe it'll help.

It doesn't look exactly what I said it would be, since it doesn't seem to be timing itself on a missing tooth, but instead it's some electronic timing involving magnets.

Honestly it isn't much and I prefer detailed manuals.
 

Attachments

#6 · (Edited)
thisisntchris87,
I cannot open your PDF. I updated Adobe Reader, but it just goes to the opening screen and does nothing with it. Photoshop could not open it, either. It told me that "This file cannot be opened because the security handler used to secure this file is not available in Photoshop. Please unsecure this file in Acrobat."
So I have no idea what you posted.
I do appreciate the effort, but like the repair manual I bought for the car on a DVD from eBay, it just isn't working. You didn't charge me $40, though.

MAVA,
I don't know about dumb, but maybe confused (as am I by your puzzling response). I don't know what you thought I was asking, but it had nothing to do with being concerned about not having time to fire every cylinder. To clarify - I am trying to determine the TYPE of sensor that is used for the CMP (Camshaft Position Sensor), and also the type of trigger that it utilizes. I have been informed that I must have been mistaken and it is not a hall effect, but rather a magnetic (inductive) sensor. Now I need to know if the sensor has a magnet that is reading the tooth or teeth on the trigger, or whether it has a "flying magnet(s)" on the trigger itself and the sensor reads from that.


Now I also want to know if the whole CMP system from an early unit can be chucked and replaced with the later style (if need be) or would it not fit because they changed the size of the sensor boss or something silly.
 
#7 · (Edited)
thisisntchris87,
I cannot open your PDF. I updated Adobe Reader, but it just goes to the opening screen and does nothing with it. Photoshop could not open it, either. It told me that "This file cannot be opened because the security handler used to secure this file is not available in Photoshop. Please unsecure this file in Acrobat."
So I have no idea what you posted.
I do appreciate the effort, but like the repair manual I bought for the car on a DVD from eBay, it just isn't working. You didn't charge me $40, though.
I forgot that these PDFs have DRM and such, sorry about that. I uploaded the pages as images this time and as you can see it's just two pages. I hope this is worth it and that it helps you out. You want GF07.04-P-5035-02D.

Further more, in some of my documentation for the M120 engine, I am reading that the left and right camshaft sensors are indeed hall-effect sensors. They're listed that way in GF07.61-P-0001-02FU, which I have attached. Those two pages are an engine fuel injection and ignition diagram for the M120 engine and the sensors are listed as B6/2 and B6/3.

EDIT: Gahh, why does this website compress images till they're useless?

GF07.04-P-5035-02D:
http://i.imgur.com/Vs92zwE.png
http://i.imgur.com/TIwIiGT.png

GF07.61-P-0001-02FU:
http://i.imgur.com/OjE8al5.png
http://i.imgur.com/D5AUJML.png
 
#8 · (Edited)
Thank you, that is a start. You obviously have better information than what I had.
I see there are notations on the page referring up to 1997 and other notes about 1997 and later - but no difference noted between early and late for the CMPs.
Does anyone know for certain which type they are for the early style (if they are even different)?
It does look like it is probably a flying magnet multi tooth wheel - so a hall sensor (I think), so that is a start.
I read somewhere that at idle it should read .8-1.5 volts, but does anyone know if it has a built in pull-up resistor?
Still confused as I think that mine had only two wires. I need to drive back out to the shop and double check it.
 
#9 ·
Some information from another MB forum - can anyone tell me anything about this theory? It was just a theory - the person who posted it admitted to no prior knowledge of this particular setup and engine.

Quote:
Originally Posted by oldtrucker

"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.
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.

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.

The distance between the magnet and the Hall is important."
 
#10 ·
It was pointed out in another forum that there is a small metal ring on the mount for the sensor - most likely a ground through the engine. That would make this a three wire Hall effect sensor.

Now I just need to know the trigger wheel arrangement.
 
#11 ·
Posts like these makes me wish for more extensive documentation. While my Star Classic service manual is good, it doesn't get into excruciating detail like this is requiring. I happen to like reading technical manuals.

What you're asking is more than what the service manual was designed for. Not saying what you're asking is wrong, but more of the fact that these sensors are expected to be just tossed and replaced, not reversed engineered.
 
#12 ·
Oh, I get that. I have bought factory manuals for a number of vehicles and some are really not too much help. As much as I am a Ford guy primarily, their factory service manuals are garbage, but the GM manuals are awesome.

I looked at getting the factory manual for the Mercedes, but $600 seems steep.

I think I can figure out the sensor at this point, now my bigger concern is the trigger wheel. I think I have to physically pull it out of the motor and look.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top