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Thread: DIY: Correcting the OBC MPG

  1. #1
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    DIY: Correcting the OBC MPG

    Adjusting the On-Board Computer (OBC) MPG

    NOTE - Some of this information is widely available on the Internet, but not necessarily all in one place. The Seattle Circuits Fuel Correction Circuit mentioned below, while advertised in the Roundel, has not been mentioned in a DIY article that I know of.



    Why would I want to adjust the OBC MPG?
    (a few reasons):

    1. Factory set value is not accurate
    2. Larger fuel injectors installed
    3. Etc.



    What is KVBR?

    The OBC uses a “fuel consumption factor” or “KVBR” to adjust the mpg reading. The factory range of the KVBR is 750 to 1250, with the factory setting at 1000 (allowing for some increase or decrease in mpg).



    What value of KVBR do I need?


    First, you need to determine your ACTUAL MPG (fill your tank, reset your trip mileage indictor, drive to near empty, fill up and then divide the mileage on your trip indicator by the number of gallons required to re-fill the tank) and your OBC INDICATED MPG (reset the OBC mpg when you fill your tank the first time and record the value just before you fill up your tank for the second time).

    To calculate the KVBR:

    Divide your ACTUAL MPG by INDICATED MPG times 1000 = KVBR or

    [ACTUAL MPG] / [INDICATED MPG] x 1000 = KVBR

    If your KVBR is between 750 and 1250 (factory range), you can adjust it using the OBC or instrument cluster method.

    If your KVBR value is greater than 1250 (larger fuel injectors), the factory adjustment will not be adequate to correct for this situation. One solution is to adjust the fuel injector signal read by the OBC. A commercial circuit is available to accomplish this and is described below.



    How do I adjust the OBC KVBR within the factory range?

    The KVBR can be adjusted in two ways (thanks going to Ron Stygar for posting the OBC method) - either via the OBC itself or via the instrument cluster. To adjust the KVBR via the OBC, please see here:


    OBC METHOD






    CLUSTER METHOD

    The KVBR value can also be adjusted within the factory range by entering into the test mode of the instrument cluster. Test 20 is used to adjust the KVBR.

    See this link for how to enter the instrument cluster test mode:

    http://forums.bimmerforums.com/forum...t=cluster+test

    Once in the correct test, adjust the KVBR value to your calculated KVBR value.



    How do I adjust the OBC mpg when my calculated KVBR is outside of the factory range?

    One method to do this is to leave the KVBR value set at the factory default (1000) and alter the fuel injector signal to the OBC. This requires a circuit that can alter the duty cycle of the fuel injector signal read by the OBC. A commercial circuit for doing this is available at Seattle Circuits:

    http://www.seattlecircuit.com/fuel_signal.htm

    This circuit has a variable resistor that allows the duty cycle to be adjusted to compensate for larger fuel injectors. The circuit is very small (a little larger than a quarter) and can be easily connected and powered by wires available behind the OBC. It requires a switched power, ground and fuel injector signal - all available from the OBC connector.



    Once installed, the Fuel Signal Correction Circuit is adjusted over time – requires patience to dial it in – I recommend adjusting it after burning a tank of gas and comparing actual to indicated. I have mine set at 9 o’clock position and will then move to the 12 o’clock position and hopefully will bracket the correct mpg. From there I will be able to more accurately dial in the indicated mpg of the OBC.



    How do I install the Seattle Circuit Fuel Signal Correction Circuit?

    The Seattle Circuit Fuel Correction Circuit comes nicely packaged with instructions and lead wires for both the switched power connection and the ground connection. You’ll be using your factory fuel injector signal wire for the other two connections (fuel injector signal wire IN and fuel injector signal wire OUT). Be sure to follow the instructions to make sure the wire from the DME is connected to the signal wire input and that the wire to the OBC is connected to the signal wire output.



    The following installation is valid for OBC part number 62-13-8-377-996 (for Z3 up to 04/1999).
    This particular OBC is found on the following (click links for more detailed information):

    E36: Details on E36
    E36 318ti Compact

    Z3: Details on Z3
    Z3 Z3 1.9 Roadster
    Z3 Z3 2.5 Roadster
    Z3 Z3 2.8 Roadster
    Z3 Z3 2.8 Coupe



    As long as the fuel injector signal wire can be determined the circuit should work fine on other OBC part numbers as well.



    The OBC connector Pin 4 is a white wire with a black stripe and carries the fuel injector signal from the DME to the OBC. This wire is cut and the Fuel Signal Correction Circuit is inserted inbetween the DME and the OBC. This allows the duty cycle of the fuel injector signal to be adjusted solely for the OBC (does not interfere with DME operation of the fuel injectors). Switched power can be obtained from Pin 10 (violet wire with a white stripe) of the OBC. Ground can be obtained from Pin 6 (brown wire with an orange stripe) of the OBC.



    The most difficult part of the installation is removing the OBC from the console. I popped out my alarm panel first (easiest to remove) and then used the hole to remove my A/C switch and finally arrived close enough to my OBC to use an L-hook to apply pressure to the rear of the OBC to leverage it out.

    The wire connector on the back of the OBC uses a lever lock and is easy to remove. The pin diagram below is oriented as if you are looking directly at the wire connector (front side where the pins insert, not the back side where the wires come out of). Note the notch orientations.









    Last edited by Monolith; 07-31-2009 at 11:24 AM.

  2. #2
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    Great job Monolith, I'm glad you were able to have it working: now I have no more excuses not to go with the custum tuning!!!!

    Were you able to figure out matematically the angle at which set the knob or are you using a trial-and-error method?



    ZetaTre
    2003 BMW Z4 2.5i
    2009 BMW X5 xDrive35d
    2006 Ducati 999S
    2012 Ram 2500 Laramie Longhorn 6.7 Cummins

  3. #3
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    Is this the ad in Roundel?

    Either way, good work.

  4. #4
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    Quote Originally Posted by LPCapital View Post
    Great job Monolith, I'm glad you were able to have it working: now I have no more excuses not to go with the custum tuning!!!!

    Were you able to figure out matematically the angle at which set the knob or are you using a trial-and-error method?
    With enough ratios (new fuel injector rate/old fuel injector rate), you could do it mathematically. I just started with a 9 o'clock setting and then will move to a 12 o'clock setting to attempt to bracket the correct value. It's going to take a while since the weather here has been nothing but rain for a while and into the immediate forecast.

    Quote Originally Posted by tmaher View Post
    Is this the ad in Roundel?

    Either way, good work.
    Yes, Seattle Circuit has an ad in the Roundel. Steve makes a couple of different circuits. (www.seattlecircuit.com)
    Last edited by Monolith; 07-30-2009 at 06:39 PM.

  5. #5
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    Nice, I was just looking for that.
    Car is in the shop right now, but will definately check this out next week.

  6. #6
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    Quote Originally Posted by Monolith View Post
    With enough ratios (new fuel injector rate/old fuel injector rate), you could do it mathematically. I just started with a 9 o'clock setting and then will move to a 12 o'clock setting to attempt to bracket the correct value. It's going to take a while since the weather here has been nothing but rain for a while and into the immediate forecast.
    IIRC Nick uses 30lbs injectors while the stock are 19lbs. I also know that you gotta take 2/3 of the MPG reported (this makes sense): in my mind this means you have to extend the cycle signal by 50% (in the sense that a 19lbs would have to stay open 50% longer than a 30lbs to spray the same amount of fuel)... I haven't look in detials at the instruction you sent me, but it appears that at 0 there's a 30% cycle extension while at 100% the cycle is extended by 150%.
    If I did the math right you should get a 50% signal increase when the dial it's at about 17% of it's range. That's of course if the signal extension grows linearly with the position of the varaible resistor...



    ZetaTre
    2003 BMW Z4 2.5i
    2009 BMW X5 xDrive35d
    2006 Ducati 999S
    2012 Ram 2500 Laramie Longhorn 6.7 Cummins

  7. #7
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    Nice info there. Not that I need it (I have the analog clock), but I just love all these great info threads.

  8. #8
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    Ron Stygar just pointed out in the other forum that the OBC itself can adjust the KVBR value. I've asked Ron if I can include his info in the DIY and will update it when I get his okay.

    See here:
    http://www.bimmerfest.com/forums/sho...69&postcount=3

    EDIT - Thanks Ron for the okay. DIY has been updated.
    Last edited by Monolith; 07-31-2009 at 11:26 AM.

  9. #9
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    With 30lb injectors, the 10:30 position is within 0.65 mpg of actual. The variable resistor has a sweep of 310 degrees with the min at about 25 degrees and the max at 335 degrees (in case someone wants to calculate position in degrees to map out larger injectors).

  10. #10
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    Quote Originally Posted by Monolith View Post
    With 30lb injectors, the 10:30 position is within 0.65 mpg of actual. The variable resistor has a sweep of 310 degrees with the min at about 25 degrees and the max at 335 degrees (in case someone wants to calculate position in degrees to map out larger injectors).
    Thanks!!!!



    ZetaTre
    2003 BMW Z4 2.5i
    2009 BMW X5 xDrive35d
    2006 Ducati 999S
    2012 Ram 2500 Laramie Longhorn 6.7 Cummins

  11. #11
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    Any idea about how to correct PPP, when it appears?

  12. #12
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    The KVBR formula is inverted. Start with the indicated mileage and divide it by the actual mileage x 1000

    After the dealer replaced my battery, I had to reset the radio code and lost the mpg calibration in the OBC. Thanks for posting the calibration procedure.

  13. #13
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    Cool

    Quote Originally Posted by Z3 Willy View Post
    The KVBR formula is inverted. Start with the indicated mileage and divide it by the actual mileage x 1000
    The formula is correct, but you are also correct.


    These are German cars, so they use the metric system, where fuel consumption is typically (always?) measured in liters / 100km (I'll refer to it as “lkm"). “Mileage” on the other hand is measured in miles per gallon, which you get with the conversion formula 235.22 * (1 / amount_in_lkm).

    The important part here is that it's the inverse - as fuel consumption goes up, lkm goes up, but mpg goes down. So if you've got your values in mpg, the KVBR formula needs to be inverted. But most of the world is metric, and for them, the original formula is correct.

    In any case, you'll know that the result you calculated is correct if you are adjusting the KVBR upwards for higher-than-indicated fuel consumption.

  14. #14
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    Quote Originally Posted by Monolith View Post
    Adjusting the On-Board Computer (OBC) MPG

    NOTE - Some of this information is widely available on the Internet, but not necessarily all in one place. The Seattle Circuits Fuel Correction Circuit mentioned below, while advertised in the Roundel, has not been mentioned in a DIY article that I know of.



    Why would I want to adjust the OBC MPG?
    (a few reasons):

    1. Factory set value is not accurate
    2. Larger fuel injectors installed
    3. Etc.



    What is KVBR?

    The OBC uses a “fuel consumption factor” or “KVBR” to adjust the mpg reading. The factory range of the KVBR is 750 to 1250, with the factory setting at 1000 (allowing for some increase or decrease in mpg).



    What value of KVBR do I need?


    First, you need to determine your ACTUAL MPG (fill your tank, reset your trip mileage indictor, drive to near empty, fill up and then divide the mileage on your trip indicator by the number of gallons required to re-fill the tank) and your OBC INDICATED MPG (reset the OBC mpg when you fill your tank the first time and record the value just before you fill up your tank for the second time).

    To calculate the KVBR:

    Divide your ACTUAL MPG by INDICATED MPG times 1000 = KVBR or

    [ACTUAL MPG] / [INDICATED MPG] x 1000 = KVBR

    If your KVBR is between 750 and 1250 (factory range), you can adjust it using the OBC or instrument cluster method.

    If your KVBR value is greater than 1250 (larger fuel injectors), the factory adjustment will not be adequate to correct for this situation. One solution is to adjust the fuel injector signal read by the OBC. A commercial circuit is available to accomplish this and is described below.



    How do I adjust the OBC KVBR within the factory range?

    The KVBR can be adjusted in two ways (thanks going to Ron Stygar for posting the OBC method) - either via the OBC itself or via the instrument cluster. To adjust the KVBR via the OBC, please see here:


    OBC METHOD






    CLUSTER METHOD

    The KVBR value can also be adjusted within the factory range by entering into the test mode of the instrument cluster. Test 20 is used to adjust the KVBR.

    See this link for how to enter the instrument cluster test mode:

    http://forums.bimmerforums.com/forum...t=cluster+test

    Once in the correct test, adjust the KVBR value to your calculated KVBR value.



    How do I adjust the OBC mpg when my calculated KVBR is outside of the factory range?

    One method to do this is to leave the KVBR value set at the factory default (1000) and alter the fuel injector signal to the OBC. This requires a circuit that can alter the duty cycle of the fuel injector signal read by the OBC. A commercial circuit for doing this is available at Seattle Circuits:

    http://www.seattlecircuit.com/fuel_signal.htm

    This circuit has a variable resistor that allows the duty cycle to be adjusted to compensate for larger fuel injectors. The circuit is very small (a little larger than a quarter) and can be easily connected and powered by wires available behind the OBC. It requires a switched power, ground and fuel injector signal - all available from the OBC connector.



    Once installed, the Fuel Signal Correction Circuit is adjusted over time – requires patience to dial it in – I recommend adjusting it after burning a tank of gas and comparing actual to indicated. I have mine set at 9 o’clock position and will then move to the 12 o’clock position and hopefully will bracket the correct mpg. From there I will be able to more accurately dial in the indicated mpg of the OBC.



    How do I install the Seattle Circuit Fuel Signal Correction Circuit?

    The Seattle Circuit Fuel Correction Circuit comes nicely packaged with instructions and lead wires for both the switched power connection and the ground connection. You’ll be using your factory fuel injector signal wire for the other two connections (fuel injector signal wire IN and fuel injector signal wire OUT). Be sure to follow the instructions to make sure the wire from the DME is connected to the signal wire input and that the wire to the OBC is connected to the signal wire output.



    The following installation is valid for OBC part number 62-13-8-377-996 (for Z3 up to 04/1999).
    This particular OBC is found on the following (click links for more detailed information):

    E36: Details on E36
    E36 318ti Compact

    Z3: Details on Z3
    Z3 Z3 1.9 Roadster
    Z3 Z3 2.5 Roadster
    Z3 Z3 2.8 Roadster
    Z3 Z3 2.8 Coupe



    As long as the fuel injector signal wire can be determined the circuit should work fine on other OBC part numbers as well.



    The OBC connector Pin 4 is a white wire with a black stripe and carries the fuel injector signal from the DME to the OBC. This wire is cut and the Fuel Signal Correction Circuit is inserted inbetween the DME and the OBC. This allows the duty cycle of the fuel injector signal to be adjusted solely for the OBC (does not interfere with DME operation of the fuel injectors). Switched power can be obtained from Pin 10 (violet wire with a white stripe) of the OBC. Ground can be obtained from Pin 6 (brown wire with an orange stripe) of the OBC.



    The most difficult part of the installation is removing the OBC from the console. I popped out my alarm panel first (easiest to remove) and then used the hole to remove my A/C switch and finally arrived close enough to my OBC to use an L-hook to apply pressure to the rear of the OBC to leverage it out.

    The wire connector on the back of the OBC uses a lever lock and is easy to remove. The pin diagram below is oriented as if you are looking directly at the wire connector (front side where the pins insert, not the back side where the wires come out of). Note the notch orientations.









    Quote Originally Posted by macnlz View Post
    The formula is correct, but you are also correct.

    These are German cars, so they use the metric system, where fuel consumption is typically (always?) measured in liters / 100km (I'll refer to it as “lkm"). “Mileage” on the other hand is measured in miles per gallon, which you get with the conversion formula 235.22 * (1 / amount_in_lkm).

    The important part here is that it's the inverse - as fuel consumption goes up, lkm goes up, but mpg goes down. So if you've got your values in mpg, the KVBR formula needs to be inverted. But most of the world is metric, and for them, the original formula is correct.

    In any case, you'll know that the result you calculated is correct if you are adjusting the KVBR upwards for higher-than-indicated fuel consumption.
    Great info! I was hunting around for something like this to see what could be done for a VF S/C'ed e38. Thanks Monolith & macnlz!
    "A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible. There are no prima donnas in engineering." - Freeman Dyson

  15. #15
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    I really should fix the OBC on my engine-swapped 2.8, but I have to admit I like getting 35mpg when I floor it around town.

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