MS1 tach signal diagnostic(need help)
06-09-2013, 05:14 PM
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No, the waveform at the cpu should not look like that. It should have fairly clean transistions and run close to 5V and ground.
The first trace, before R12, runs between 7V and 3V roughly. It would need to go below about 1.2V for the LED in the optoisolator to turn off. Where did you ground your scope probe? Assuming there's no offset voltage fooling us, the optoisolator is on all the time; it never switches.
The second trace, after R12: what we want to see is the signal transitioning between ~1.2V (one diode drop) and something closer to ground. The fact that it's stuck more or less at 1.3V confirms my suspicion that the input signal is just not getting close enough to ground. Yes, it's going up and down about 50mV, but don't let that fool you. It's not switching.
The first trace, before R12, runs between 7V and 3V roughly. It would need to go below about 1.2V for the LED in the optoisolator to turn off. Where did you ground your scope probe? Assuming there's no offset voltage fooling us, the optoisolator is on all the time; it never switches.
The second trace, after R12: what we want to see is the signal transitioning between ~1.2V (one diode drop) and something closer to ground. The fact that it's stuck more or less at 1.3V confirms my suspicion that the input signal is just not getting close enough to ground. Yes, it's going up and down about 50mV, but don't let that fool you. It's not switching.
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