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Thread: Control module voltage on VCM scanner vs measured voltage

  1. #1

    Control module voltage on VCM scanner vs measured voltage

    Been dialing in my fuel injector offset tables but dont know if I should use the 13.8 control module voltage displayed on VCM scanner or use measured voltage of 14.5v on my volt gage for the off set table ? not sure why I'm seeing a difference in voltage.

  2. #2
    Senior Tuner kingtal0n's Avatar
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    computer probs has things like diodes which protect the circuits but drop voltage to the sensitive electronics

    If you can check the voltage directly at the injectors?

    That said, voltage will change over time. Alt output drops as accessories kick on, fans for example will induce a voltage drop.
    Also temperature, higher/hotter = less voltage from and to electronics
    So your cold injector voltage is likely higher than hot.

    So don't take the voltage at face value, always expect it to wander. Thats why those graphs exist.
    If you have especially huge injectors it might help to 'flat line' the latency in some places. Some injectors this works better than others.

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    Isnt the control module voltage what the pcm actually references? Or am i wrong?
    1997 30th SS. Torqhead 24x, TFS heads, 223/235 cam, 4l80e, S60 D1SC 14psi

  4. #4
    During any log the control module voltage will stay pretty much dead on at 13.8v no matter the load of accessories on. maybe .10 of a volt variance. 14.2 to 14.5 running volts measured through the 12v cig lighter port. Im about ready to throw in the towel in on these injectors anyway and get some good injectors with with some solid reliable data. They do make good power and are very good on mpg its just getting them dialed in for idle and mid throttle is getting old.

  5. #5
    Went back to basics and found on the recently installed Ebay fuel pressure regulator the vacuum diaphragm wont hold a vacuum with a vac pump. disconnected and plugged the FPR vac line and it now idles much smoother but VE way off. It seemed like it was a vacuum leak but it always passed the brake clean spray down test. Fuel pressure drops down to 50lbs at idle with the line on and is 58lbs with it off. Thinking about keeping the vac line capped off see how it does with a sloped IFR and static 58lbs fuel Pressure before I get another regulator.

  6. #6
    Senior Tuner kingtal0n's Avatar
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    Quote Originally Posted by anniversaryss View Post
    Isnt the control module voltage what the pcm actually references? Or am i wrong?

    Hey- That is a very interesting point you bring up. I wonder how far offset the voltage curve for injectors is compared to actual injector voltage.


    In any case. I've tuned around 80/100 cars without injector data, just fine. It is useful when attempting to 'plug in' numbers blindly (essentially that is what injector data is in the first place since as we just found out- there is no way for the computer to know the actual voltage at the injectors). For HPtuners this can be critical for novices since there are so many maps which depend on minute injector on-time adjustments at low revolution speeds (low rpm) however for a stand-alone often the difference is negligible once the VE map is tuned, since massive injector data 'failures' result with very obvious disparities in low-rpm commanded injector on-times (the number in milliseconds will seem to get larger as the rpm approaches zero which doesn't make any sense)

    good thinking, keep thinking like that it will serve you well

  7. #7
    Figured out some things today, I'm tuning to the wrong AFR because we use E10 here in Washington state. Unplugged my FPR and now running a static 58lbs fuel pressure. Scaled down the IFR from a static 41.5 to a sloped 38.6 to 41.5 IFR table and its seems much better. going to rescale the entire VE 6.5% and continue logging at 14.22 Stoic and see how that goes.
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