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13 btn OBC and the wiring harness

Posted by Earendil 
March 10, 2009 07:11PM
Well, after about 10 ebay purchases, I finally get burned. Well, perhaps not burned, but mislead.
I bought a 13 button OBC with "Wiring harness". Well when I hear wiring harness I think connectors with wires in between.
I am now the owner of an OBC and 8 inches of wire. Does anyone have any pictures of which wires are which? I'm not afraid of my soldering iron, but I'm having trouble finding pictures that identify the wires, or a diagram that has the color coding listed.

I did have a 6 button OBC in my car, so the wiring harness is in place for that. It would be great to match as many of those wires as possible.
There were 9 wires for the 6 button, and there are 16 wires for the 13 button.

In addition, I don't have the turn stalk, nor do I care about the security system. It doesn't really even need to be plugged into the little speak to produce a chime.

If a picture would help, I could do that.

Thanks for any help or resources you may know of.
This is what I had intended to go off of:
[zoso.no-ip.org]
March 10, 2009 07:53PM
Quote
Earendil
Well, after about 10 ebay purchases, I finally get burned. Well, perhaps not burned, but mislead.
I bought a 13 button OBC with "Wiring harness". Well when I hear wiring harness I think connectors with wires in between.
I am now the owner of an OBC and 8 inches of wire. Does anyone have any pictures of which wires are which? I'm not afraid of my soldering iron, but I'm having trouble finding pictures that identify the wires, or a diagram that has the color coding listed.

I did have a 6 button OBC in my car, so the wiring harness is in place for that. It would be great to match as many of those wires as possible.
There were 9 wires for the 6 button, and there are 16 wires for the 13 button.

In addition, I don't have the turn stalk, nor do I care about the security system. It doesn't really even need to be plugged into the little speak to produce a chime.

If a picture would help, I could do that.

Thanks for any help or resources you may know of.
This is what I had intended to go off of:
[zoso.no-ip.org]

You will need the full OBC wiring harness if you want the trip and mileage parts to work. I suppose you could track down the extra connections you need to make and wire them individually though.

Go to this site and grab one of the wiring diagrams...
[bmwe30cc.org]

The wirung and colour scheme are all there along with where the wires connect smileys with beer
March 10, 2009 08:04PM
There are also wiring diagrams here, in a little more organized and easier-to-find format:

http://www.armchair.mb.ca/~dave/BMW/e30/

__________
Dave
'91 325iX
March 10, 2009 08:49PM
Thanks guys!
I've actually been able to match about half the wires. However, I don't really want to get even a single wire wrong for fear I may fry something :-)
I'll give the diagrams a looksee, and my car again.

The process is not helped by the previous owner having installed an alarm system, re-ran some of the speaker wires, as well as, no joke, installed what appears to be a bunch of wires that go to nothing.

Archeo, so what you're saying is that I need the full wiring harness because some of the connectors aren't tapping into enough pins?
For example, I have 6 wires coming out of my yellow IS connector, and perhaps there should be more?
If that's true... Dang...
March 10, 2009 09:30PM
Quote
Earendil
Thanks guys!
I've actually been able to match about half the wires. However, I don't really want to get even a single wire wrong for fear I may fry something :-)
I'll give the diagrams a looksee, and my car again.

The process is not helped by the previous owner having installed an alarm system, re-ran some of the speaker wires, as well as, no joke, installed what appears to be a bunch of wires that go to nothing.

Archeo, so what you're saying is that I need the full wiring harness because some of the connectors aren't tapping into enough pins?
For example, I have 6 wires coming out of my yellow IS connector, and perhaps there should be more?
If that's true... Dang...

When I had the 13 button unit in my car, I ran it by making a temporary connector that went between the 6 button connector and the 13 button OBC.

This gave me the same functionality as the 6 button unit but with no input/output for the OBC relay; fuel rate, fuel level, fuel tank low or start input. The wiring for those signals is not in the stock 6 button harness sad smiley
March 31, 2009 01:33AM
Thanks to John Yust I was able to get a proper wiring harness, and after soldering a few new bulbs in place, I now have a working 13 button OBC!

I now know that I drive an average (and depressing) 13 mph in my daily life, and get 10mpg. Stop and go traffic, not to mention hills are a real killer on that mpg!

Thanks for all the help everyone.

I'll consider this another BEN forum success story smileys with beer
March 31, 2009 01:52AM
In case anyone was wondering what I meant by wires everywhere, here you go.
Most of the speak wire was restrung, but the idio...I mean previous owner, spliced the wires 2 sometimes 3 times, like so...

Stereo---------splice------------splice------------wire to speaker

So I spent 20 minutes turning all those into a single splice with some good electrical tape instead of the scotch tape he used.
And this picture doesn't begin to explain whatever is supposed to be going on under and to the left of the steering column...holy crap.






Edited 1 time(s). Last edit at 03/31/2009 01:52AM by Earendil.
March 31, 2009 03:14AM
Quote
Earendil
Thanks to John Yust I was able to get a proper wiring harness, and after soldering a few new bulbs in place, I now have a working 13 button OBC!

I now know that I drive an average (and depressing) 13 mph in my daily life, and get 10mpg. Stop and go traffic, not to mention hills are a real killer on that mpg!

Thanks for all the help everyone.

I'll consider this another BEN forum success story smileys with beer

that sounds waaaay too low, even if you are constantly flooring it and stopping. if your car is running well, the OBC might need a different coding plug to output proper values.


March 31, 2009 03:15AM
Quote
Earendil
In case anyone was wondering what I meant by wires everywhere, here you go.
Most of the speak wire was restrung, but the idio...I mean previous owner, spliced the wires 2 sometimes 3 times, like so...

Stereo---------splice------------splice------------wire to speaker

So I spent 20 minutes turning all those into a single splice with some good electrical tape instead of the scotch tape he used.
And this picture doesn't begin to explain whatever is supposed to be going on under and to the left of the steering column...holy crap.



woowwwww. maybe you should just run new speaker wire?


March 31, 2009 11:01AM
Quote
daniel
Quote
Earendil
Thanks to John Yust I was able to get a proper wiring harness, and after soldering a few new bulbs in place, I now have a working 13 button OBC!

I now know that I drive an average (and depressing) 13 mph in my daily life, and get 10mpg. Stop and go traffic, not to mention hills are a real killer on that mpg!

Thanks for all the help everyone.

I'll consider this another BEN forum success story smileys with beer

that sounds waaaay too low, even if you are constantly flooring it and stopping. if your car is running well, the OBC might need a different coding plug to output proper values.

Thanks for the concern Daniel, but I have the correct coding plug. The OBC tells me I have the correct engine, and the speedometer is spot on. If I take the car out on a road at 50mph and set the cruise control I get about 24mpg which is right about at my estimated average (which is more like 22-23 at 75mph).

Sadly, I just have about 1 mile and 2 stop lights, not to mention 1st-2nd gear navigation of a college campus as a major part of my daily routine. It's also a relatively good hill to get up to campus. That's why, despite the speed limit being 35mph, I can only muster an average of 13mph.

As for the wiring, that is basically the end result. The previous owner did have the wherewithal to string new speaker wire, it was just the way that he decided to attach it to the head unit that sucked. I took that newish speaker wire and spliced it directly to the head unit wiring harness. It's all quite a bit cleaner now, though I forgot to take picture when I was done, so I have no proof smiling smiley
March 31, 2009 11:17AM
If you are only getting 10 mpg then something is definitely wrong. My truck averages 18 and it has a V8. I think you should compute your mileage the old fashioned way by filling your tank and seeing how far you go and fill up again and divide the mileage by how many gallons you put in. My OBC tells me that I only average 22.8 mpg but when I do the real calculation it is more like 27.5! I think that the coding plug in the computer must not have the right differential info as that will effect the calculations. If you are really only getting 10 mpg then you have a serious engine problem like only running on three cylinders or a large fuel leak. I would start looking for the fuel leak if your car is running good. You might have a seam weld in the gas tank that is bad or a fuel line problem. You had better find it or your car will catch on fire!
March 31, 2009 12:29PM
Quote
wodcutr
If you are only getting 10 mpg then something is definitely wrong. My truck averages 18 and it has a V8. I think you should compute your mileage the old fashioned way by filling your tank and seeing how far you go and fill up again and divide the mileage by how many gallons you put in. My OBC tells me that I only average 22.8 mpg but when I do the real calculation it is more like 27.5! I think that the coding plug in the computer must not have the right differential info as that will effect the calculations. If you are really only getting 10 mpg then you have a serious engine problem like only running on three cylinders or a large fuel leak. I would start looking for the fuel leak if your car is running good. You might have a seam weld in the gas tank that is bad or a fuel line problem. You had better find it or your car will catch on fire!

Would this not contradict what I said above, that the OBC matches my previously estimated (old fashioned way) MPG of 23, and that the OBC gives a highly accurate MPH reading, both on freeway and off? It would seem to me that driving on the freeway would only exaggerate the error, and make it more apparent.
Though, on second thought, I really hope I have a fuel leak or only running on 3 cylinders, because hot damn, if I can get 23 mpg on the freeway under those conditions, I can't wait to see what I get after I fix the problem ;-)

Really though, I think this can be attributed more ti driving style and conditions. Out of my driveway I have to make a hard 180 turn into busy 4 lane traffic that goes 40mph, so I end up flooring flooring my poor car every morning just to keep from being rear ended by an SUV. Oh yeah, and that road is entirely uphill. I watched the live MPG meter this morning and the sucker came indistinguishably close to 0 sad smiley

But we'll see. Those were the averages after 2 days, I'll see what she says after 4 days smiling smiley

Oh, and again, the coding plug is correct, and the OBC things it's correct by holding down the 100 and 1 keys (or is it 1000 and 1?).
The only way it could be wrong, aside from circuit malfunction, is if the car isn't as stock as I think it is...
March 31, 2009 01:35PM
Quote
wodcutr
If you are only getting 10 mpg then something is definitely wrong. My truck averages 18 and it has a V8. I think you should compute your mileage the old fashioned way by filling your tank and seeing how far you go and fill up again and divide the mileage by how many gallons you put in. My OBC tells me that I only average 22.8 mpg but when I do the real calculation it is more like 27.5! I think that the coding plug in the computer must not have the right differential info as that will effect the calculations. If you are really only getting 10 mpg then you have a serious engine problem like only running on three cylinders or a large fuel leak. I would start looking for the fuel leak if your car is running good. You might have a seam weld in the gas tank that is bad or a fuel line problem. You had better find it or your car will catch on fire!

the differential doesnt influence the MPG readouts. the readout is calculated by reading the signal being sent to the fuel injectors. if you have different-than-stock fuel injectors, then the reading will be off. your OBC should be reading correctly unless you got the OBC from another car.

case in point: driving on the highway, my OBC tells me i am getting around 35MPG, when i am really getting around 30. this is because the injectors in my motor are larger than the injectors in the M20.


March 31, 2009 01:37PM
if it really is all uphill, then 10mpg is reasonable.

just for kicks you should do a test to see how accurate it is. reset it, then drive around for like 1/4 of a tank, maybe even less, and see if your calculation matches the readout.


March 31, 2009 02:38PM
Quote
daniel
Quote
wodcutr
If you are only getting 10 mpg then something is definitely wrong. My truck averages 18 and it has a V8. I think you should compute your mileage the old fashioned way by filling your tank and seeing how far you go and fill up again and divide the mileage by how many gallons you put in. My OBC tells me that I only average 22.8 mpg but when I do the real calculation it is more like 27.5! I think that the coding plug in the computer must not have the right differential info as that will effect the calculations. If you are really only getting 10 mpg then you have a serious engine problem like only running on three cylinders or a large fuel leak. I would start looking for the fuel leak if your car is running good. You might have a seam weld in the gas tank that is bad or a fuel line problem. You had better find it or your car will catch on fire!

the differential doesnt influence the MPG readouts. the readout is calculated by reading the signal being sent to the fuel injectors. if you have different-than-stock fuel injectors, then the reading will be off. your OBC should be reading correctly unless you got the OBC from another car.

case in point: driving on the highway, my OBC tells me i am getting around 35MPG, when i am really getting around 30. this is because the injectors in my motor are larger than the injectors in the M20.

I believe the OBC calculates mileage based on inputs from the injectors, the rpm and the speed sensor in the diff.
If it was just injectors, then you would never get an accurate reading since mileage is a measure of fuel consumed over distance. The injectors only record fuel consumed over time.

Also, the diff can make a large difference in mileage for a given speed. You will get significantly better fuel economy with a 2.73 diff in top gear at 60mph than with a 4.10 diff.
March 31, 2009 03:38PM
Quote
Archeo-peteriX
Quote
daniel
Quote
wodcutr
If you are only getting 10 mpg then something is definitely wrong. My truck averages 18 and it has a V8. I think you should compute your mileage the old fashioned way by filling your tank and seeing how far you go and fill up again and divide the mileage by how many gallons you put in. My OBC tells me that I only average 22.8 mpg but when I do the real calculation it is more like 27.5! I think that the coding plug in the computer must not have the right differential info as that will effect the calculations. If you are really only getting 10 mpg then you have a serious engine problem like only running on three cylinders or a large fuel leak. I would start looking for the fuel leak if your car is running good. You might have a seam weld in the gas tank that is bad or a fuel line problem. You had better find it or your car will catch on fire!

the differential doesnt influence the MPG readouts. the readout is calculated by reading the signal being sent to the fuel injectors. if you have different-than-stock fuel injectors, then the reading will be off. your OBC should be reading correctly unless you got the OBC from another car.

case in point: driving on the highway, my OBC tells me i am getting around 35MPG, when i am really getting around 30. this is because the injectors in my motor are larger than the injectors in the M20.

I believe the OBC calculates mileage based on inputs from the injectors, the rpm and the speed sensor in the diff.
If it was just injectors, then you would never get an accurate reading since mileage is a measure of fuel consumed over distance. The injectors only record fuel consumed over time.

Also, the diff can make a large difference in mileage for a given speed. You will get significantly better fuel economy with a 2.79 diff in top gear at 60mph than with a 4.10 diff.

i didn't mean to say it was ONLY the injectors. the speed sensor on the diff is used, but the differential ratio does not directly influence the readout. the diff ratio does affect RPMs and therefore fuel economy, but it does not directly affect the OBC. this is why there are only a few coding plugs, as far as i know one per engine type, not one for each differential ratio (as there were many offered on the e30 over the years).


March 31, 2009 04:08PM
Quote
daniel
Quote
Archeo-peteriX
Quote
daniel
Quote
wodcutr
If you are only getting 10 mpg then something is definitely wrong. My truck averages 18 and it has a V8. I think you should compute your mileage the old fashioned way by filling your tank and seeing how far you go and fill up again and divide the mileage by how many gallons you put in. My OBC tells me that I only average 22.8 mpg but when I do the real calculation it is more like 27.5! I think that the coding plug in the computer must not have the right differential info as that will effect the calculations. If you are really only getting 10 mpg then you have a serious engine problem like only running on three cylinders or a large fuel leak. I would start looking for the fuel leak if your car is running good. You might have a seam weld in the gas tank that is bad or a fuel line problem. You had better find it or your car will catch on fire!

the differential doesnt influence the MPG readouts. the readout is calculated by reading the signal being sent to the fuel injectors. if you have different-than-stock fuel injectors, then the reading will be off. your OBC should be reading correctly unless you got the OBC from another car.

case in point: driving on the highway, my OBC tells me i am getting around 35MPG, when i am really getting around 30. this is because the injectors in my motor are larger than the injectors in the M20.



I believe the OBC calculates mileage based on inputs from the injectors, the rpm and the speed sensor in the diff.
If it was just injectors, then you would never get an accurate reading since mileage is a measure of fuel consumed over distance. The injectors only record fuel consumed over time.

Also, the diff can make a large difference in mileage for a given speed. You will get significantly better fuel economy with a 2.79 diff in top gear at 60mph than with a 4.10 diff.

i didn't mean to say it was ONLY the injectors. the speed sensor on the diff is used, but the differential ratio does not directly influence the readout. the diff ratio does affect RPMs and therefore fuel economy, but it does not directly affect the OBC. this is why there are only a few coding plugs, as far as i know one per engine type, not one for each differential ratio (as there were many offered on the e30 over the years).

Not sure how you have convinced yourself of that...but the speed sensor is driven directly off the big crown gear in the differential. I fail to see how that isn't direct input to the OBC confused smiley
March 31, 2009 04:13PM
Quote
Archeo-peteriX
Quote
daniel
Quote
Archeo-peteriX
Quote
daniel
Quote
wodcutr
If you are only getting 10 mpg then something is definitely wrong. My truck averages 18 and it has a V8. I think you should compute your mileage the old fashioned way by filling your tank and seeing how far you go and fill up again and divide the mileage by how many gallons you put in. My OBC tells me that I only average 22.8 mpg but when I do the real calculation it is more like 27.5! I think that the coding plug in the computer must not have the right differential info as that will effect the calculations. If you are really only getting 10 mpg then you have a serious engine problem like only running on three cylinders or a large fuel leak. I would start looking for the fuel leak if your car is running good. You might have a seam weld in the gas tank that is bad or a fuel line problem. You had better find it or your car will catch on fire!

the differential doesnt influence the MPG readouts. the readout is calculated by reading the signal being sent to the fuel injectors. if you have different-than-stock fuel injectors, then the reading will be off. your OBC should be reading correctly unless you got the OBC from another car.

case in point: driving on the highway, my OBC tells me i am getting around 35MPG, when i am really getting around 30. this is because the injectors in my motor are larger than the injectors in the M20.



I believe the OBC calculates mileage based on inputs from the injectors, the rpm and the speed sensor in the diff.
If it was just injectors, then you would never get an accurate reading since mileage is a measure of fuel consumed over distance. The injectors only record fuel consumed over time.

Also, the diff can make a large difference in mileage for a given speed. You will get significantly better fuel economy with a 2.79 diff in top gear at 60mph than with a 4.10 diff.

i didn't mean to say it was ONLY the injectors. the speed sensor on the diff is used, but the differential ratio does not directly influence the readout. the diff ratio does affect RPMs and therefore fuel economy, but it does not directly affect the OBC. this is why there are only a few coding plugs, as far as i know one per engine type, not one for each differential ratio (as there were many offered on the e30 over the years).

Not sure how you have convinced yourself of that...but the speed sensor is driven directly off the big crown gear in the differential. I fail to see how that isn't direct input to the OBC confused smiley

Perhaps because the IC needs to know what diff, and the IC just feeds the OBC a current speed, and not the raw revolutions per time data?
THat way the OBC can just use the injector data vs speed/distance to figure mpg, and save it some math.
March 31, 2009 04:21PM
Quote
Archeo-peteriX
Not sure how you have convinced yourself of that...but the speed sensor is driven directly off the big crown gear in the differential. I fail to see how that isn't direct input to the OBC confused smiley

if direct is going from the big crown to the speed sensor and then to the OBC, then yes it is direct. to me, in this context, direct means a direct connection, like speed sensor to OBC, not big crown to OBC. i hope that clears up my thought process a bit. i don't know exactly what the internals of a differential are like, but i am assuming the difference between a 2.79 diff and a 4.27 diff is the gearing, and not something that requires a different speed sensor.

in the end, what i am trying to say is that having a different diff ratio will not make your OBC inaccurate.


March 31, 2009 04:23PM
Quote
Earendil
Perhaps because the IC needs to know what diff, and the IC just feeds the OBC a current speed, and not the raw revolutions per time data?
THat way the OBC can just use the injector data vs speed/distance to figure mpg, and save it some math.

yes, i think the mpg is determined by the injector data vs speed/distance relationship.


March 31, 2009 04:25PM
Quote
Earendil
Quote
Archeo-peteriX
Quote
daniel
Quote
Archeo-peteriX
Quote
daniel
Quote
wodcutr
If you are only getting 10 mpg then something is definitely wrong. My truck averages 18 and it has a V8. I think you should compute your mileage the old fashioned way by filling your tank and seeing how far you go and fill up again and divide the mileage by how many gallons you put in. My OBC tells me that I only average 22.8 mpg but when I do the real calculation it is more like 27.5! I think that the coding plug in the computer must not have the right differential info as that will effect the calculations. If you are really only getting 10 mpg then you have a serious engine problem like only running on three cylinders or a large fuel leak. I would start looking for the fuel leak if your car is running good. You might have a seam weld in the gas tank that is bad or a fuel line problem. You had better find it or your car will catch on fire!

the differential doesnt influence the MPG readouts. the readout is calculated by reading the signal being sent to the fuel injectors. if you have different-than-stock fuel injectors, then the reading will be off. your OBC should be reading correctly unless you got the OBC from another car.

case in point: driving on the highway, my OBC tells me i am getting around 35MPG, when i am really getting around 30. this is because the injectors in my motor are larger than the injectors in the M20.



I believe the OBC calculates mileage based on inputs from the injectors, the rpm and the speed sensor in the diff.
If it was just injectors, then you would never get an accurate reading since mileage is a measure of fuel consumed over distance. The injectors only record fuel consumed over time.

Also, the diff can make a large difference in mileage for a given speed. You will get significantly better fuel economy with a 2.79 diff in top gear at 60mph than with a 4.10 diff.

i didn't mean to say it was ONLY the injectors. the speed sensor on the diff is used, but the differential ratio does not directly influence the readout. the diff ratio does affect RPMs and therefore fuel economy, but it does not directly affect the OBC. this is why there are only a few coding plugs, as far as i know one per engine type, not one for each differential ratio (as there were many offered on the e30 over the years).

Not sure how you have convinced yourself of that...but the speed sensor is driven directly off the big crown gear in the differential. I fail to see how that isn't direct input to the OBC confused smiley

Perhaps because the IC needs to know what diff, and the IC just feeds the OBC a current speed, and not the raw revolutions per time data?
THat way the OBC can just use the injector data vs speed/distance to figure mpg, and save it some math.

Try holding your foot on the brakes and applying power to the engine...see what your mpg readout is like then.

If you guys want to believe that half the inputs to the OBC are ignored then be my guest eye rolling smiley
March 31, 2009 04:26PM
Quote
Archeo-peteriX
Try holding your foot on the brakes and applying power to the engine...see what your mpg readout is like then.

If you guys want to believe that half the inputs to the OBC are ignored then be my guest eye rolling smiley

wouldnt it be zero, since you arent going anywhere, but using fuel?

i am trying to understand your reasoning but haven't been able to figure it out. can you quickly summarize how you think the process works?


March 31, 2009 04:28PM
Quote
daniel
Quote
Archeo-peteriX
Not sure how you have convinced yourself of that...but the speed sensor is driven directly off the big crown gear in the differential. I fail to see how that isn't direct input to the OBC confused smiley

if direct is going from the big crown to the speed sensor and then to the OBC, then yes it is direct. to me, in this context, direct means a direct connection, like speed sensor to OBC, not big crown to OBC. i hope that clears up my thought process a bit. i don't know exactly what the internals of a differential are like, but i am assuming the difference between a 2.79 diff and a 4.27 diff is the gearing, and not something that requires a different speed sensor.

in the end, what i am trying to say is that having a different diff ratio will not make your OBC inaccurate.

You are dead wrong but I won't belabour the point.

I'm out here sad smiley
March 31, 2009 04:37PM
Quote
Archeo-peteriX
I'm out here sad smiley

seriously? ok then, i guess i will try to find another source to explain to me how this whole system works since you aren't willing to share.


March 31, 2009 06:01PM
I could be wrong but I believe that the OBC has its own coding plug. I think it uses the coding plug in the dash as well and that calculates speed and economy depending on inputs from the other sensors. The sensor in the diff can only sense each revolution, just like the crank sensor, thus with different gearing such as a 3.73 or a 4.10 the revolutions would be different so the computer needs to know what diff is in the car or else the calculations will be wrong. If you swap an OBC from a car with a 4.10 diff into one with a 3.73 diff the mileage calculations will be wrong!
March 31, 2009 08:02PM
Quote
wodcutr
I could be wrong but I believe that the OBC has its own coding plug. I think it uses the coding plug in the dash as well and that calculates speed and economy depending on inputs from the other sensors. The sensor in the diff can only sense each revolution, just like the crank sensor, thus with different gearing such as a 3.73 or a 4.10 the revolutions would be different so the computer needs to know what diff is in the car or else the calculations will be wrong. If you swap an OBC from a car with a 4.10 diff into one with a 3.73 diff the mileage calculations will be wrong!

So the OBC and Instrument Cluster (IC) each figure out the MPG based on direct input from the injectors and the sensor in the diff, and do so independently?

If so, it seems mildly inefficient. So if there is a single coding plug for the 13btn OBC and 325i combination post 88 (according to my research at least), does that mean there was only a single diff offered on the 325i from 88-91?

Since the IC has it's own coding plug as well, it seems like you'd want to store as much of the variable information, like different diff ratios, in only one of the two coding plugs, and it makes sense to me to put it in the coding plug for the IC, since every car has an IC, but not every car has an OBC. At least that's how I would design it, but then I'm not BMW making entire car systems in the late 80's, what do I know smiling smiley

I have a different (and incorrect) coding plug for my IC, I could pop that sucker in there and see if the OBC is at all effected. I also have one for the OBC that is wrong, but I'm not about to rip my dash apart again just yet.
March 31, 2009 08:05PM
Quote
wodcutr
I could be wrong but I believe that the OBC has its own coding plug. I think it uses the coding plug in the dash as well and that calculates speed and economy depending on inputs from the other sensors. The sensor in the diff can only sense each revolution, just like the crank sensor, thus with different gearing such as a 3.73 or a 4.10 the revolutions would be different so the computer needs to know what diff is in the car or else the calculations will be wrong. If you swap an OBC from a car with a 4.10 diff into one with a 3.73 diff the mileage calculations will be wrong!

are you saying that the speedometer will be off then, also, with a different differential, or just the OBC? my car now has a 3.25LSD. it originally had a 4.10 or whatever came in automatic 88 325is's. This would mean my speedo would read 20% lower than I am actually going, which simply isn't true. the ratio of the differential has nothing to do with the speedometer.

am i going totally crazy? i wish Peter would not be done with this thread so I could read some kind of explanation.


March 31, 2009 09:43PM
The speedometer has its own coding plug which figures everything out for the gauge in the cluster and it has to be the right one for each specific diff too. Not all cars came with an OBC so the cluster has to function all on its own. The OBC has its own coding plug that allows it to work as well. Lets conduct an experiment. Go and drive your car and see if the cluster mpg meter is the same as the OBC when you reset it. Get going about 60 on the highway on a flat stretch and your dash meter should read about 40mpg. Then reset the OBC so it will start to calculate the mileage from that point. They should be the same if both coding plugs are correct. Hey I am not a BMW engineer nor do I completely understand the process here but that would explain to me why and how it all works. Both units get the same information from the same sensors, but are independent from each other.
March 31, 2009 10:14PM
Quote
wodcutr
I could be wrong but I believe that the OBC has its own coding plug. I think it uses the coding plug in the dash as well and that calculates speed and economy depending on inputs from the other sensors. The sensor in the diff can only sense each revolution, just like the crank sensor, thus with different gearing such as a 3.73 or a 4.10 the revolutions would be different so the computer needs to know what diff is in the car or else the calculations will be wrong. If you swap an OBC from a car with a 4.10 diff into one with a 3.73 diff the mileage calculations will be wrong!
The speedometer/odometer pickup sensor is triggered by a nine-bladed paddle disk that revolves with the differential as the rear wheels are rolling.

The bladed disk is lying on top of the ring gear in the bottom left of this photo.



The disk can be seen on the right inside this differential case.



The nine blades of this disk always trigger the speedo sensor switch to open and close nines times per axle revolution, regardless of what the diff ratio is. The speedometer and odometer will always read accurately regardless of the diff ratio, as long as the car is moving whether the engine is even running or not. However, different tire sizes of different rolling radius, or even just different inflation pressure, will affect the speedometer accuracy. The diff ratio has no effect at all on the speedo/odo.

The OBC calculates fuel mileage based on the ratio of fuel consumed versus distance travelled. The OBC gets its distance measurement directly from the same speedometer/odometer signal. So again the diff ratio will have no effect on the speed signal.

However, the amount of fuel consumed per mile/kilometer obviously will be affected by the diff ratio as the engine will be running at a different rpm for any particular road speed depending on the final drive ratio. The OBC calculates how much fuel is being burned from the injector cycles and durations. If you spend all your time driving on the highway in 3rd gear with the engine at high revs, obviously you would be consuming fuel at a greater rate than if you were cruising at the same speed at lower revs in 5th gear. So gear ratios do have a large effect on that.

The OBC coding plug tells the computer important things like how many cylinders the engine has, as a six cylinder engine with six injectors will be feeding more fuel at a certain rpm and load versus a four cylinder engine with only four fuel injectors. The coding plug and the OBC do not need to know the diff ratio, as the speed is still being measured accurately regardless of diff ratio, and the fuel flow rate is being determined accurately via the injector cycles at whatever corresponding rpm the engine is turning at.
March 31, 2009 10:23PM
Quote
Ferdinand
The nine blades of this disk always trigger the speedo sensor switch to open and close nines times per axle revolution, regardless of what the diff ratio is. The speedometer and odometer will always read accurately regardless of the diff ratio, as long as the car is moving whether the engine is even running or not. However, different tire sizes of different rolling radius, or even just different inflation pressure, will affect the speedometer accuracy. The diff ratio has no effect at all on the speedo/odo.

thank you ferd!

now will people believe me? jeebus grist.


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