13 btn OBC and the wiring harness
Posted by Earendil
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 
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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 
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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
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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 03/30/2009 11:52PM by Earendil. |
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.  Edited 1 time(s). Last edit at 03/31/2009 01:15AM by daniel. |
woowwwww. maybe you should just run new speaker wire?
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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 
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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
But we'll see. Those were the averages after 2 days, I'll see what she says after 4 days
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... |
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.
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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.
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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).
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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 
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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. |
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.
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yes, i think the mpg is determined by the injector data vs speed/distance relationship.
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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 
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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? Edited 1 time(s). Last edit at 03/31/2009 02:27PM by daniel. |
You are dead wrong but I won't belabour the point. I'm out here
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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.
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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
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. |
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.
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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. |
thank you ferd! now will people believe me? jeebus grist.
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