The ODBII system sends an engine RPM signal to Pin 9 of the Toyota OBDII plug. This signal changes in frequency according to RPM. Does anyone know if there is a chart that matches the frequency of the RPM signal to actual RPM? Or, does anyone know what the forumula would be to calculate the RPM value from the frequency changes?
Mapping out each signal frequency to actual RPM would take too long and would be too inaccurate.
(no, the answer is not to buy an aftermarket tach. This is a pet project that my friend is working on for school)
Do you have a frequency meter on a multi-meter or something? What are you using to test?
And yes, it should be that simple. Hold the RPMs steady for a while if you are going to let something like a multimeter figure out the frequency.
-Charlie
The multi-meter we're using measures frequency. At first, we thought that the tach signal would be voltage based (more RPM, more voltage), but then we found out that it's frequency based.
RPM at idle was like 21 hertz (Mhz? I don't remember which) .... and at 5000 RPM it was about 105 i think.... and we also took readings at other points, but none of it really makes much sense because as RPMs climb, the frequency doesn't climb at the same rate.
The other thing is that we don't have a reference point other than 0 RPM, but that's not very useful.
Like I mentioned earlier, we could just go and record the frequency at different RPM levels and generalize a chart, but we were hoping to get a much more precise reading. (My friend is hoping to build his own tach out of some stepper motors he has left over from his other work term projects and we were hoping to get to an accuracy of +/-20 RPM.
For a 4-cylinder car, the tach signal should be the RPM/30 in Hz... That's if the ECU sends out one pulse to the tach for every firing of a cylinder.
4 cylinders = 2 spark plugs firing per motor revolution
1 rpm = (1/60) rps
so
30 RPM = 1 Hz
Go back and re-take those measurements with the motor being constant at whatever RPM for a while. Notice that the idle measurement comes right out to 630RPM, right where you'd expect it. The 5k measurement you took is like 3100RPM - you just rev'd the motor, right? It takes a long time for a frequency meter to read on a multimeter. Hold the motor at 4k or something for 5 or 10 seconds to get a true measurement. Come back w/ the results.
-Charlie
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For a 4-cylinder car, the tach signal should be the RPM/30 in Hz... That's if the ECU sends out one pulse to the tach for every firing of a cylinder.
4 cylinders = 2 spark plugs firing per motor revolution
1 rpm = (1/60) rps
so
30 RPM = 1 Hz
Go back and re-take those measurements with the motor being constant at whatever RPM for a while. Notice that the idle measurement comes right out to 630RPM, right where you'd expect it. The 5k measurement you took is like 3100RPM - you just rev'd the motor, right? It takes a long time for a frequency meter to read on a multimeter. Hold the motor at 4k or something for 5 or 10 seconds to get a true measurement. Come back w/ the results.
-Charlie
Actually, I'm an idiot...i forgot to tell my friend that there are two sparks per cycle...apparently, it made a different, but I'm not sure how.... and we didn't hold at the RPMs long enough to get a totaly stable reading.
Either way, we're well under way to making our own DIY shift light and tach set. woot!
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