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  • Rpm

    Hi...I need help. I am trying to use fb to get rpm value. I came accross a few problems...the value I get varies....I have a couple of questions.

    - I read in a thread that we might need to get VSS or the rpm signal after PCM. Anyone know where I could tap this 2 signals? I am getting the rpm signal before it goes into the ECU of the this the vss or it is the signal after PCM?

    - I am still very confused by the calculation of the F2V board....from what I understand we need to change the values of R1 and C1 to the requirements of the car...but how about R2 and C2? I am powering the F2V board from FB using 5volts. Can anyone help?


  • #2
    If the signal you are getting is a train of pulses, then it is the correct signal. If it is the raw firing wire, then you might be getting a pulse every time a piston goes down, not every time the shaft rotates.

    As for modifying the board, you dont have to. It will work at a lower resolution. However if you want a higher resolution over a smaller range, then you modify the components. I would get it working without modification before attempting to "improve" it.
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    • #3
      You need to identify how many pulses per revolution your ignition system is putting out. I was able to get the signal from tachometer wire in my cars dash. It was the speedometer that I had to reference before the PCM. What car are you working with?


      • #4
        Hi, Thanks for the replies....I'm working on a 91 lexus LS400 v8 engine. I've managed to get it working...studied the circuit diagram and got the signals from the ecu. The voltage is stable and response to engine rev. However I doubt the calculations and values are accurate. How may I know the number of pulses my car is putting out?

        May I know the way to calculate others as well? For example water temp and fuel level, if I get values smaller then 5v i'd connect it straight to FB. Do I just use scale to get the limits?



        • #5

          I have identified the value and scales for many GM sensors. I'm unfamiliar with Toyota sensors.

          I made a water temp sensor out of the temp sensor from the Fusion Brain store. That enabled me to use the built in function. Most pressure, temp and fuel level sensors are resistive. These are utilized by creating voltage divider circuits. The ohm values have to be researched and a logic created in MDX to return useful output. Best to pick one gauge sensor and start googling. I learned the OEM sensors are not the precision scientific instruments most people credit them to be. Don't get caught up trying to figure out why your oil pressure is off 2 or 3 PSI from your calculations.

          Fuel level is easy. Track down the sending wire from your tank. Run the car out of gas and measure the resistance. Dump in a gallon and measure it again and so on. Your fuel sender ohm outputs are typically well documented to get you in the ballpark with a voltage divider circuit.


          • #6
            I noticed the resistance for fuel level is different when the engine is started and before. Which resistance value should we use?

            If the voltage across the sensor is far less then 5volts, the value is around 0.5v then do we still need a voltage divider? If I understand correctly voltage divider is only required when the voltage is more then 5volts.

            Another thing, for resistive sensors ex. the fuel level sensor. We could just measure the voltage across the resistance right? Do we need to use the voltage source from FB?



            • #7
              Use the resistance from the running engine.

              The voltage you are measuring is the difference through all the wiring back to the battery positive. Voltage drops in an entire circuit. Yes you need a voltage divider to get correct readings. Even if a value is under the 5v you are at the risk of a spike up to 12V. If powered from the FB 5v it can never go over 5V and damage it.


              • #8
                When engine is started the resistance value is 18K (fuel level sensor), how do I choose the resistor value to match the 18K resistance. I'd have to get the range of the sensor's resistance but what value of resistor do I use to match that range? Should I use the max resistance value?

                Can I power the fuel level sensor from FB's 5volts? There is already a voltage across the sensor, is it save? How can I do it?

                Anyone know how can I get the range for other sensors like the stock water temp sensor range? When the car is started, the temp is normally at the middle. Can we use scale? Seems very difficult to get the min and max for temp as the min and max is very hard to duplicate.


                • #9
                  Start googling each sensor to find the range. I looked around for a few minutes and found Toyota fuel level senders seem to be 4ohms full, 110ohms empty. I'd try a 51ohm resistor for R1. This should give you a range of 0.36V full and 3.4V empty using the FB 5v for Vin. You will still need to do some tuning to compensate for other variables in your wiring. Not sure why you are measuring 18K.

                  Using the FB 5v is always safest. You don't have to worry about voltage spikes wrecking your FB.


                  • #10
                    Thanks for the information and help. I'm still puzzled, I saw wires going in to the fuel tank so I assumed its the sensor wires. I measured parallel (to R2) to the wires when engine is turned off. Getting lower resistance value, when the engine is turned ON it goes up to 18K. Probably got the wrong wires as I can see there are 2 sets of wires there, maybe it was the fuel pressure. Anyway what I am still confused is when I measure voltage parallel (to R2) to the wires I get voltage readings. Can I still supply it with 5volts from FB? or I'd have to isolate the sensors before I can supply 5volts to it. Kindly refer to the image below. Again thank you for the help.


                    • #11
                      Your car most likely has the fuel pump in the tank. Go to an auto parts website like Autozone and lookup your fuel sender. The specs on it will tell you the resistance range. You can do this for oil pressure, water temp, etc. You might even be able to find the number of pulses from your VSS.



                      • #12
                        Found this wiring diagram for the Lexus LS400. Should I be tapping the resistor? If yes how should I do it? There probably will be a current flowing thru the resistor already. How could I do a voltage divider?


                        • #13
                          That's the fuel pump circuit - not fuel level.


                          • #14
                            I've attached the doc of the circuit....I can't seem to find the sensor for fuel level. Only found fuel pressure on the upper right corner of the diagram. Anyone have any ideas what type of sensor a 91 Lexus LS400 uses? Should be resistive type but couldn't find it.

                            Another question, I saw the fuel pressure sensor seems to be connected to a solenoid. Does this means it is returning variable frequency signal and not variable electrical signal? I'd need a frequency to voltage converter board right?

                            Attached Files


                            • #15
                              That's engine control stuff.

                              You need the gauge wiring (dash console etc).
                              That's assuming you want the fuel (tank) level?

                              For engine speed there is the engine speed sensor, left or right dizzy, ignitor, sensor etc.