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AFM offset information

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  • AFM offset information

    Hi guys. I was messing around with the testbench I'm building and I did some experiments with the afm offset resistors. Here's the deal.

    The afm offset resistors are in place to create a voltage offset on the incoming afm voltage. In other words the ecu actually reads the afm voltage minus X offset voltage. In the case of my 88t ecu (and probably most z31 ecus) the offset is 2.0volts. So if the afm actually reads 6.5volts at the pin the ecu is only seeing 4.5volts.

    The purpose of the afm offset voltage is to bring the usable voltage output range of the afm down to the usable ecu input voltage range. The stock z31 afm outputs around 2.5 volts at idle and can read up to at least 7 volts. The ecu reads between 0 and ~5 volts. Without the voltage offset the ecu would only be using a voltage range of the afms min voltage (2.5v) and the ecus max (5v). Obviously this combination isnt good for high power air flow measurement and the lower half of the ecus voltage range is simply wasted. With the voltage offset in place the usable afm voltage is now expanded. 2.5volts out of the afm actually reads .5v on to the ecu and 7 volts at the afm reads 5 volts at the ecu. Now both the ecu and afm are using the majority of their measuring range.

    The reason for removing/altering the resistors when switching afms is to remove or alter the offset. If the cobra or z32 afm only reads .5 volts at idle and you introduce a -2.0v offset the ecu wont read any voltage. This of course causes limp mode. By altering the offset so the new afm voltages read close to between 0 and 5volts you can get as much of the usable metering range of the afm as possible.

    There are three resistors that we can play with that make up the afm offset circuit. There is R201 which is the small surface mount resistor and the two standard resistors mounted on the posts next to it. Based on my testing it appears R201 introduces an offset of roughly 1.58volts by itself. One of the resistors on the posts introduces an additional .4volt offset and the second resistor does about a .03volt offset. The two resistors on teh posts are likely there to trim the offset voltage for any variation encountered through the rest of the circuit. By playing with these resistors the offset voltage should be easy to modify.

  • #2
    I'm guessing this is to use larger injectors with the stock ecu?

    or am i way off
    used to have one of the first high horsepower NA to turbo cam'd 7000rpm pavement pounding z31's....now in process of building an LQ9 LS1 6.0L Z31 with all the goodies

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    • #3
      Well its more for those that are reprogramming the ecu to use a higher flow rate afm. This is usually accompanied by larger injectors but they arent directly related.

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      • #4
        cool
        used to have one of the first high horsepower NA to turbo cam'd 7000rpm pavement pounding z31's....now in process of building an LQ9 LS1 6.0L Z31 with all the goodies

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        • #5
          good info!! , thank you for taking the time to post this stuff. it can help some of us when we are making decisions in which way to go with the afm
          mumble jumble..

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          • #6
            learn somthing new everyday

            Thanks, that helps explain the whole afm swaps alot more

            Terrible idea putting those wheels on...

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            • #7
              Thanks for posting the info, it's really nice to know that they drop it a static 2.0v for the entire curve. I can add your information here to the maf swap page on redz31.com if that's ok?

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              • #8
                wait a minute here, if this is the case, can't we pull-up the vOut on the other MAFs by 2volts, and accomplish the same thing?
                vg33et -blew some chunks outta it, then gave it all away.
                2009 370z touring/nav/sports

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                • #9
                  grahfz wrote: wait a minute here, if this is the case, can't we pull-up the vOut on the other MAFs by 2volts, and accomplish the same thing?
                  Explain why on earth you would want to do that?

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                  • #10
                    I've been up for a bunch of hours...

                    but wouldn't it be the same effect as pulling the resistors?, essentially zeroing out the difference?
                    vg33et -blew some chunks outta it, then gave it all away.
                    2009 370z touring/nav/sports

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                    • #11
                      grahfz wrote: I've been up for a bunch of hours...

                      but wouldn't it be the same effect as pulling the resistors?, essentially zeroing out the difference?
                      It should be the same effect, I'm just wondering (off tangent from your question maybe) why someone would do it that way. Grounding down a DC circuit is one thing, how would you accomplish a static 2.0vdc increase in the circuit without a bunch of work, and how would it be any easier/better than removing the resistors? In either case you would still need to have the correct VQmap in place...

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                      • #12
                        well, getting 5v is pretty easy with .90c voltage regulator, should be MORE than capable of keeping a nice,clean voltage with anything your car's going to be operating at. You can either figure out the resistors to drop this down to 5v, or you can make it adjustable with a pot... hell, you could steal a TPS out of most cars and use that. whatever.. and you wire this in as a 2v pull-up. Shouldn't be too hard... for about 60-70$, I could have three boards made and populated with components, all you'd have to do from there is take your signal wire, cut it, solder it into each end of the board, and be done with it... or I go and cut up some maf's and cut the harnesses out of some junk cars, and have a plug and play solution.

                        I'll be cutting my resistors out though ;D

                        it's just another option, I guess.
                        vg33et -blew some chunks outta it, then gave it all away.
                        2009 370z touring/nav/sports

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                        • #13
                          hmm $70 vs. free

                          you make life hard, cat's have hairballs, we have grahfz

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                          • #14
                            I was just saying that for ~70$ I could have THREE boards fabbed and ready to go. Nice, pretty circuit boards. http://www.expresspcb.com

                            or I could do them by hand.. but I don't bother with that anymore.

                            the other option is to just solder them all together and heatshrink.... about 3$ total ;D

                            Matter of fact, I've got a nice little project box with three filtered 5v regs w/ zener protection on it fabbed up in the garage already. I was going to use it to power some extra sensors for the LM1, but then other things opened up for me.

                            Like I said, I'm going to snip them myself, but why not at least leave the idea to someone who may have some hairbrained reason to actually go through with it?
                            vg33et -blew some chunks outta it, then gave it all away.
                            2009 370z touring/nav/sports

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