Q&A Forum
Hi, can somebody help me with engine RPM settings ?
I´m using signal from ignition coil primary and have everything wired like in this picture: 
I´m using this optocoupler https://grobotronics.com/pc817-2-channel-optocoupler-isolation-board.html?sl=en
and my computer(and yourdyno box) cannot read any RPM. Am I missing something in settings ? A tried everything.
Hi, check that the optocoupler is strong enough to bring the voltage down. It has a 500Ohm pullup inside the box.
I had better luck hooking to the negative side of the coil
It must be something in software what I´m missing. If I set the rpm2 input to Load cell2 it reads RPM from that pickup. But no matter what combination of setting I have I cannot make it read(maybe just show) engine RPM directly, just calculate it from load cell 1 RPM.
@palo31 Engine RPM is always equal to the brake (or roller) RPM * Gear ratio. This is the main RPM in the system, where power etc is calculated from.
If you have a direct engine RPM measurement, it can be used to set the gear ratio. You can also call it something useful like Motor RPM or Direct Engine RPM or whatever and add this as a gauge in the Run window.
Ok, so it was a combination of several diferent things.
Thank you Jostein !!! Like you said, my optocoupler was "weak", or better said it has 3kOhm resistor in series with output. I removed it and RPM reading was better. Second was settings in software- I make my gauge in run window with direct eng. RPM.
But still my RPM reading is very erratic and all over the place. Can it be noise in 25cm cable from optocoupler to bluebox ?
I doubt it is noise. More likely the voltage is still at the edge (check with an oscilloscope if you can). If needed you may be able to run the input diode of the optocoupler harder (more current). You can also turn off the RPM Spike filter as a test