Multifunction steering wheel controls
12-12-2002, 12:21 AM
#1
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Thread Starter
Multifunction steering wheel controls
Does anyone know the interface used by the buttons on the multifunction steering wheel?
Doing a steering wheel and airbag retrofit in older cars is rather pricey, but instrument clusters themselves are relatively cheap.
I was thinking it should not be that hard to rig up a set of buttons to set the time and toggle between NAV display and the normal trip meter. The question is, what is the interface?
I have never seen the wiring harness from the switches or contact ring. The switches cost over $200 (retail). How many wires come off the switches? I know they plug into the back of the instrument cluster, but how many wires is that?
I read somewhere online that the switches may be based on a resistance network. This would require the fewest wires (two). I highly doubt it is a CANbus interface.
Thanks.
Doing a steering wheel and airbag retrofit in older cars is rather pricey, but instrument clusters themselves are relatively cheap.
I was thinking it should not be that hard to rig up a set of buttons to set the time and toggle between NAV display and the normal trip meter. The question is, what is the interface?
I have never seen the wiring harness from the switches or contact ring. The switches cost over $200 (retail). How many wires come off the switches? I know they plug into the back of the instrument cluster, but how many wires is that?
I read somewhere online that the switches may be based on a resistance network. This would require the fewest wires (two). I highly doubt it is a CANbus interface.
Thanks.
12-12-2002, 12:31 AM
#3
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Thread Starter
Are you saying there is a CAN controller built into the switches themselves?
I wouldn't put it beyond MB, but wouldn't it make more sense to just run the wires up to the instrument cluster for decoding, and THEN push those signals onto the interior CAN?
I wouldn't put it beyond MB, but wouldn't it make more sense to just run the wires up to the instrument cluster for decoding, and THEN push those signals onto the interior CAN?
12-12-2002, 03:03 PM
#4
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I think it probably not a resistance network, decoded to hi-low... but I need to do more research. Does the steering wheel register more than one button press at a time? I believe it does. Also, there are not enough lines to decode a resistance network.
There are only three wires going from the contact ring into the console (for multifunction use, two more are for SRS in single-stage systems, four more in dual-stage).
Now, from those three wires, the kombi decodes at least 9 different button presses... each button, plus the horn. Three wires only gives you seven states (the eighth possible being "off"). So, it seems more likely there is power, and some serial data stream.
-s-
There are only three wires going from the contact ring into the console (for multifunction use, two more are for SRS in single-stage systems, four more in dual-stage).
Now, from those three wires, the kombi decodes at least 9 different button presses... each button, plus the horn. Three wires only gives you seven states (the eighth possible being "off"). So, it seems more likely there is power, and some serial data stream.
-s-
12-13-2002, 02:25 AM
#5
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Thread Starter
Firstly, thanks for the insightful reply. It is useful to know the wire count from the contact ring.
I did not realize there were 8 buttons (plus horn), but this still does not preclude the use of a resistance based system.
You are correct in stating that if a binary type system is used, 9 discrete buttons would not be possible with just three lines. Assuming the steering wheel itself had a ground path, three wires could give you 8 combinations (000, 001, 010, 011, 100, 101, 110, 111).
However, I feel a resistance network would be quite possible, as only two wires are required (bare minimum) if you assume A and R1 are in the combi, B is sent out to the steering wheel, and C comes back to the combi's ground. If I were designing this (and to make this example simple using 3 wires instead), I would have one power wire, one ground wire, and a third wire for the analog voltage measurement.
Here's a simple diagram. A, B, and C are wires from the cluster to the combination instrument cluster.
Please forgive the poor diagram. But as you can see, just by placing resistors that when switched on add in parallel, you are able to alter the effective resistance, and change the voltage. The number of switches you could monitor is only limited by the resolution of your A/D convertor.
If R1=100k, and R2=500k, R3=250k, R4=125k, etc., then the voltage measured between B and C will be as follows based on each key pressed:
5.0V, no keys pressed
sw1=4.17 V
sw2=3.57 V
sw3=2.78 V
sw4=1.92 V
sw5=1.19 V
sw6=0.68 V
sw7=0.36 V
sw8=0.18 V
Multiple key presses will not be readily predictable, however.
I did not realize there were 8 buttons (plus horn), but this still does not preclude the use of a resistance based system.
You are correct in stating that if a binary type system is used, 9 discrete buttons would not be possible with just three lines. Assuming the steering wheel itself had a ground path, three wires could give you 8 combinations (000, 001, 010, 011, 100, 101, 110, 111).
However, I feel a resistance network would be quite possible, as only two wires are required (bare minimum) if you assume A and R1 are in the combi, B is sent out to the steering wheel, and C comes back to the combi's ground. If I were designing this (and to make this example simple using 3 wires instead), I would have one power wire, one ground wire, and a third wire for the analog voltage measurement.
Here's a simple diagram. A, B, and C are wires from the cluster to the combination instrument cluster.
Code:
R1 R2
A o-----/\/\/\------o---------/\/\/\----sw1--------------o C
+5V B | | GND
| R3 |
|---/\/\/\----sw2----|
| |
| R4 |
|---/\/\/\----sw3----|
| |
| Rn |
|---/\/\/\----sw4----|
If R1=100k, and R2=500k, R3=250k, R4=125k, etc., then the voltage measured between B and C will be as follows based on each key pressed:
5.0V, no keys pressed
sw1=4.17 V
sw2=3.57 V
sw3=2.78 V
sw4=1.92 V
sw5=1.19 V
sw6=0.68 V
sw7=0.36 V
sw8=0.18 V
Multiple key presses will not be readily predictable, however.
12-13-2002, 03:53 AM
#7
Dema,
Yes, it could easily be CANbus from the steering wheel to the instrument cluster.
CANbus is 3 wires: power, ground, and a single bi-directional data line.
Another way to do lots of buttons is by way of a keyboard scanning matrix. Say you want an 4x4 keypad (16 buttons). You use 4 output lines and 4 input lines. Each button connects a row to a column. Output 1110, 1101, 1011, 0111 in sequence. Read the input bits each time. If you read a 0, then you know that the button corresponding to that row and column was pressed.
If they are using CANbus, they could use a keypad scaning program to scan the buttons and send the data over the CANbus.
The horn is probably somewhat low-tech, old style and reliable. Maybe they switch away from those things for cost reasons though.
Yes, it could easily be CANbus from the steering wheel to the instrument cluster.
CANbus is 3 wires: power, ground, and a single bi-directional data line.
Another way to do lots of buttons is by way of a keyboard scanning matrix. Say you want an 4x4 keypad (16 buttons). You use 4 output lines and 4 input lines. Each button connects a row to a column. Output 1110, 1101, 1011, 0111 in sequence. Read the input bits each time. If you read a 0, then you know that the button corresponding to that row and column was pressed.
If they are using CANbus, they could use a keypad scaning program to scan the buttons and send the data over the CANbus.
The horn is probably somewhat low-tech, old style and reliable. Maybe they switch away from those things for cost reasons though.
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12-13-2002, 07:07 AM
#8
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Originally posted by oktane
I did not realize there were 8 buttons (plus horn), but this still does not preclude the use of a resistance based system.
You are correct in stating that if a binary type system is used, 9 discrete buttons would not be possible with just three lines. Assuming the steering wheel itself had a ground path, three wires could give you 8 combinations (000, 001, 010, 011, 100, 101, 110, 111).
I did not realize there were 8 buttons (plus horn), but this still does not preclude the use of a resistance based system.
You are correct in stating that if a binary type system is used, 9 discrete buttons would not be possible with just three lines. Assuming the steering wheel itself had a ground path, three wires could give you 8 combinations (000, 001, 010, 011, 100, 101, 110, 111).
But, remember, with a binary decoder, 8 states equals 7 button presses (the 8th state being OFF).
After looking through WIS, I have determined the following:
1) there are 10 buttons decoded... the two horn buttons are separate and fed into separate encoders
2) it is NOT CANbus (the listing of all CAN related items lists the Kombi, but not the steering wheel). the Kombi decodes the output from the wheel
For instance, the horn buttons on non-multifunction steering wheels are simply passed through the contact ring. In the case of my C43's original wiring, the horn wire goes directly into the ignition switch. In systems with the multifunction steering wheel (my C43 today, for instance) the horn buttons are encoded, then passed to the Kombi. The horn output is now to be found on N1 on connector 2 of the kombi. This wire is now passed to the ignition switch, and sounds the horn.
I'm going to have a go at decoding the wires at some point in the future. If the darn airbags weren't so expensive (I was just quoted $640 for one at discounted price), I would rip it apart to see what makes it tick.
-s-
12-13-2002, 04:58 PM
#9
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Thread Starter
Scorchie, I see your point now about the 8th state being OFF (no keys pressed).
I wonder why MB decided to separate each horn button. I guess they figured it would be easier to stay consistent with their design scheme of one button to one ouput state.
As for the switches, I didn't know they were integrated into the airbag. I think maybe it is for the C-steering wheel but not the CLK. They show up as a separate part in the EPC I believe.
Next time you are behind your kombi, maybe you could use a multimeter with one lead grounded and probe around at each of the three pins to see their status. I bet you would definitely see a power and ground, and on the third wire either a varying analog voltage with keypress or a fluctuating voltage more indicative of a serial stream (as people have mentioned here.)
Thanks again all for your help.
I wonder why MB decided to separate each horn button. I guess they figured it would be easier to stay consistent with their design scheme of one button to one ouput state.
As for the switches, I didn't know they were integrated into the airbag. I think maybe it is for the C-steering wheel but not the CLK. They show up as a separate part in the EPC I believe.
Next time you are behind your kombi, maybe you could use a multimeter with one lead grounded and probe around at each of the three pins to see their status. I bet you would definitely see a power and ground, and on the third wire either a varying analog voltage with keypress or a fluctuating voltage more indicative of a serial stream (as people have mentioned here.)
Thanks again all for your help.
12-13-2002, 08:26 PM
#10
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I'll put a scope on it eventually (I do know a bit about electronics... hehehe).
However, the colors on the wires indicate that none of them are grounds (they would have a brown in there, and the wires are blue, yellow, black).
Also, my C43 has a CLK kombi/steering wheel (and no airbag right now, I plug in a W203 airbag to use the buttons).
The 2000+ W208/W210 steering wheels/airbags/kombis are the same design. The 2002+ W463 steering wheel/airbag is also this design. The W203 and W209 steering wheels, although implemented with differently shaped parts, are still the same engineering design. All have separately orderable buttons, however the part number for the airbag in the EPC will INCLUDE THE BUTTONS (probably because if you need an airbag, your buttons have been destroyed). Also, the G500 and AMG models have leather covered and stitched airbags, which cost quite a bit more than the "normal" model airbags.
Figuring this out was actually something I was working on currently, so I'll keep you posted with more information.
-s-
However, the colors on the wires indicate that none of them are grounds (they would have a brown in there, and the wires are blue, yellow, black).
Also, my C43 has a CLK kombi/steering wheel (and no airbag right now, I plug in a W203 airbag to use the buttons).
The 2000+ W208/W210 steering wheels/airbags/kombis are the same design. The 2002+ W463 steering wheel/airbag is also this design. The W203 and W209 steering wheels, although implemented with differently shaped parts, are still the same engineering design. All have separately orderable buttons, however the part number for the airbag in the EPC will INCLUDE THE BUTTONS (probably because if you need an airbag, your buttons have been destroyed). Also, the G500 and AMG models have leather covered and stitched airbags, which cost quite a bit more than the "normal" model airbags.
Figuring this out was actually something I was working on currently, so I'll keep you posted with more information.
-s-
12-14-2002, 04:08 AM
#11
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Thread Starter
Well, it's great to hear you have a scope! Sounds like the mystery will be solved afterall. 
If the airbag comes with buttons, $600 wholesale isn't as bad as I first thought. But it still would be nice to play around with the combi without a full wheel/airbagl conversion.
I look forward to hearing what you find out, when you get around to it.
If the airbag comes with buttons, $600 wholesale isn't as bad as I first thought. But it still would be nice to play around with the combi without a full wheel/airbagl conversion.
I look forward to hearing what you find out, when you get around to it.
12-17-2002, 01:53 AM
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A new twist...
Notice that the buttons on the airbag are illuminated, and respond to changing the light level on the dash?
It's gotta be power, and some sort of serial data stream. Grrr.
-s-
It's gotta be power, and some sort of serial data stream. Grrr.
-s-
12-17-2002, 01:03 PM
#13
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Is there a ground path through the steering column? I'm guessing no, so we're down to these 3 wires:
A: illumination
B: data
C: gnd
I still think B will be some form of analog encoding. How could you fit a PCB and microcontroller into the airbag? Unless it is mounted just behind the switches, I don't think it's likely...
A: illumination
B: data
C: gnd
I still think B will be some form of analog encoding.
How could you fit a PCB and microcontroller into the airbag? Unless it is mounted just behind the switches, I don't think it's likely...
12-17-2002, 03:20 PM
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Since I'm using a W203 airbag and a W210 steering wheel, I hold the airbag out away from the column while using it (this is just while I am in the garage, to program the kombi).
Ergo, there's no ground path (well a very high resistance on through my butt and the leather of the seat). Everything could work on a complex voltage differential system, but that seems a lot more complex than a set of wires for power, ground, and two-way data.
The airbag is several layers, and the actual airbag floats above the backing metal plate. This is so that the airbag can be pushed to act as a horn switch. Behind the bag, mounted to the plate, you can spy the pc board. I assume there are several microchips on this board based on the traces (they're not really traces, they are actual metal mounted in the plastic). However, I still don't want to take the airbag apart due to concerns with accidental activation.
I'll try and get a picture posted of it later.
Like I said, I'm still working on it... but I don't think we're going to like the answer in any case.
I believe the wheels with the upshift/downshift paddles work with the same three wires, and the kombi will decode this and send it on the CANbus. Then a proper shifter will read this signal and shift the transmission... wow!
-s-
Ergo, there's no ground path (well a very high resistance on through my butt and the leather of the seat). Everything could work on a complex voltage differential system, but that seems a lot more complex than a set of wires for power, ground, and two-way data.
The airbag is several layers, and the actual airbag floats above the backing metal plate. This is so that the airbag can be pushed to act as a horn switch. Behind the bag, mounted to the plate, you can spy the pc board. I assume there are several microchips on this board based on the traces (they're not really traces, they are actual metal mounted in the plastic). However, I still don't want to take the airbag apart due to concerns with accidental activation.
I'll try and get a picture posted of it later.
Like I said, I'm still working on it... but I don't think we're going to like the answer in any case.
I believe the wheels with the upshift/downshift paddles work with the same three wires, and the kombi will decode this and send it on the CANbus. Then a proper shifter will read this signal and shift the transmission... wow!
-s-
12-17-2002, 03:21 PM
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Forgot to add... WIS shows that the switches from one side (including one horn switch) goes into some sort of chip... and the switches from the other side also go into some sort of chip, and the two chips feed into another chip that has the three lines on it.
WIS is usually pretty accurate on things like this, so I don't know what to make of it. Its description was lacking, though.
-s-
WIS is usually pretty accurate on things like this, so I don't know what to make of it. Its description was lacking, though.
-s-
12-18-2002, 11:12 AM
#16
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Well it looks like my guess was totally wrong then! Definitely sounds a lot more complex than I imagined.
I agree that it is probably not a good idea (or worthwhile) to take apart your airbag. If there's logic in there, we are really limited in our options.
I think it will be very hard to determine what the protocal is, whether it is RS-232, i2C, CAN, or who knows what else. We might luck out and have it be a PWM scheme instead of just analog encoding.
I think that is likely to just be wishful thinking though!
What are the chances of finding a deployed airbag for dirt cheap? Is it possible to buy these, or are there restrictions to prevent things like repackaging with duds? That way, we could just strip away the logic to either use it directly in any application, or at least study it in more detail.
I agree that it is probably not a good idea (or worthwhile) to take apart your airbag. If there's logic in there, we are really limited in our options.
I think it will be very hard to determine what the protocal is, whether it is RS-232, i2C, CAN, or who knows what else. We might luck out and have it be a PWM scheme instead of just analog encoding.
I think that is likely to just be wishful thinking though!
What are the chances of finding a deployed airbag for dirt cheap? Is it possible to buy these, or are there restrictions to prevent things like repackaging with duds? That way, we could just strip away the logic to either use it directly in any application, or at least study it in more detail.
12-18-2002, 02:05 PM
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Originally posted by oktane
What are the chances of finding a deployed airbag for dirt cheap? Is it possible to buy these, or are there restrictions to prevent things like repackaging with duds? That way, we could just strip away the logic to either use it directly in any application, or at least study it in more detail. [/B]
What are the chances of finding a deployed airbag for dirt cheap? Is it possible to buy these, or are there restrictions to prevent things like repackaging with duds? That way, we could just strip away the logic to either use it directly in any application, or at least study it in more detail. [/B]
With the recent rains and storms in this area, it's likely, however unfortunate, that there is a greater chance of finding deployed airbags.
More later.
-s-
12-18-2002, 09:32 PM
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Originally posted by scorchie
I'll put a scope on it eventually (I do know a bit about electronics... hehehe).
However, the colors on the wires indicate that none of them are grounds (they would have a brown in there, and the wires are blue, yellow, black).
Also, my C43 has a CLK kombi/steering wheel (and no airbag right now, I plug in a W203 airbag to use the buttons).
The 2000+ W208/W210 steering wheels/airbags/kombis are the same design. The 2002+ W463 steering wheel/airbag is also this design. The W203 and W209 steering wheels, although implemented with differently shaped parts, are still the same engineering design. All have separately orderable buttons, however the part number for the airbag in the EPC will INCLUDE THE BUTTONS (probably because if you need an airbag, your buttons have been destroyed). Also, the G500 and AMG models have leather covered and stitched airbags, which cost quite a bit more than the "normal" model airbags.
Figuring this out was actually something I was working on currently, so I'll keep you posted with more information.
-s-
I'll put a scope on it eventually (I do know a bit about electronics... hehehe).
However, the colors on the wires indicate that none of them are grounds (they would have a brown in there, and the wires are blue, yellow, black).
Also, my C43 has a CLK kombi/steering wheel (and no airbag right now, I plug in a W203 airbag to use the buttons).
The 2000+ W208/W210 steering wheels/airbags/kombis are the same design. The 2002+ W463 steering wheel/airbag is also this design. The W203 and W209 steering wheels, although implemented with differently shaped parts, are still the same engineering design. All have separately orderable buttons, however the part number for the airbag in the EPC will INCLUDE THE BUTTONS (probably because if you need an airbag, your buttons have been destroyed). Also, the G500 and AMG models have leather covered and stitched airbags, which cost quite a bit more than the "normal" model airbags.
Figuring this out was actually something I was working on currently, so I'll keep you posted with more information.
-s-
Did Steve do the upgrade of the multi-buttom steering wheel and Comand retrofit for you??? I am thinking about doing the same thing, do the controls on the steering wheel as well as airbag work properly?? And do you mind give me the parts that i'll need for the airbag and buttoms. I am very confused about what parts i need to get for the same upgrade especially the steering wheel assembly. Plz help!
Thanks
12-19-2002, 06:12 PM
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Originally posted by oktane
If the airbag comes with buttons, $600 wholesale isn't as bad as I first thought.
If the airbag comes with buttons, $600 wholesale isn't as bad as I first thought.
.good luck on decoding airbagbuttons here...greetingz,
12-19-2002, 07:01 PM
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Originally posted by mac99
Scorchie,
Did Steve do the upgrade of the multi-buttom steering wheel and Comand retrofit for you??? I am thinking about doing the same thing, do the controls on the steering wheel as well as airbag work properly?? And do you mind give me the parts that i'll need for the airbag and buttoms. I am very confused about what parts i need to get for the same upgrade especially the steering wheel assembly. Plz help!
Thanks
Scorchie,
Did Steve do the upgrade of the multi-buttom steering wheel and Comand retrofit for you??? I am thinking about doing the same thing, do the controls on the steering wheel as well as airbag work properly?? And do you mind give me the parts that i'll need for the airbag and buttoms. I am very confused about what parts i need to get for the same upgrade especially the steering wheel assembly. Plz help!
Thanks
Since this isn't really an easy install (and it's still going on), I only recommend it for people who don't mind doing a bit of fabrication. Everyone cringed when I started drilling into the roof for the GPS antenna!
The parts are all available from Steve, contact him and he can supply you with what you'll need.
Again, it's not all worked out perfectly, so if you're not adventurous, I wouldn't try it. Also, it helps if you have another car to drive while yours is holed up with issues and errors.
-s-
