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pull up resistors and sharing sensors for parrallel install.

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  • pull up resistors and sharing sensors for parrallel install.

    I have my sm4 wired in with the inputs only to check that I get tach/trigger/sync signals and I do, I have to share the coolant temp sensor as there is nowhere suitable to install another for the autronic, it reads about 15deg low, to remove the pull up resistor on the sm4 ECT input is it a case of switching it off in the software, and how, or does a resistor need to be removed from the board?

  • #2
    You can't disable the coolant pull-up however you could possibly do a Linearisation table if you can measure the value of the other pull-up resistor. Unplug both ECU's and check if you can measure a resistance between the other ECU coolant input and 5v TPS supply.

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    • #3
      thanks Ian, is it not possible to remove the resistor from the board completely, I understand this is not recommended and may have warranty issues if my unit was new but it is an old 1.07 unit I had upgraded, I also have access to some very skilled electronics people who could remove it for me if you or Richard could identify which one it is. I also considered replacing the coolant signal to the oem ecu with a fixed value resistor but given that the oem ecu will be controlling the fbw throttle it may cause cold start idle up issues. If the sm4 does not use coolant temp in the VE calculation would it be possible to just Tune around it by basically just running my cold start and other coolant temp controlled outputs etc at a lower temp to account for the variance. set cold start cut out at 50deg and so on?
      Last edited by JohnnyIntense; January 26th, 2017, 11:23 AM.

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      • #4
        You can remove the resistor but you will still need to measure the other ECU pull-up resistor to check if it the same value (2200 ohms) as the SM4. If it is different you will need to do a Linearisation to have temperatures read correctly.

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        • #5
          thanks, my plan was remove the pull up and check the sm4 temp vs the oem temp with a scan tool and then make a table if required. do you happen to have the resistor number handy by chance, failing that i'll just track it back on the input circuit until I find one bridged to sensor +5v. I need the factory ecu to read temp correctly so it activates the rad fans and idles up when cold etc via the fbw throttle. does that sound reasonable?

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          • #6
            for those in a sensor sharing situation like me the pull up resistor for ECT is R42, the ect reads 5deg high in the autronic ie when the engine is 90deg the sm4 sees 95deg but most importantly it is now correct in the factory ecu that controls my rad fans, cold start dbw idle up etc, without removing the tps pull up the tps voltage is a touch lower but can be easily recalibrated on the oem lexus ecu and set manually in the SM4, voltage goes from 0-3.9v on my lexus when the tps signal is shared so I left it in place.

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            • #7
              Did you measure the pullup value in the OEM ECU?

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              • #8
                no, not yet, I was just happy that it started and ran after my oversight that left me scratching my head and bothering you for a few days re: the power issue. the factory lexus dead head fuel system runs 40-55psi varying fuel pressure so I can"t even drive it to check anything else until I install my fuel system and can set a base pressure that suits the injector parameters. I was hoping to do a bit of driveability/low boost tuning on the stock fuel system with just the upgraded intank pump while I got the other systems ready to go.

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