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Old 06-08-2013, 12:21 AM   #1
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Default MS1 tach signal diagnostic(need help)

Hi, I had some issue with my swap last summer and ended up realizing that my problem was probably coming from my megasquirt.

I have a MS1 PCB V3.

My problem :
-I have no RPM in the megatune, but all other sensors are working
-> No RPM = no tach signal.


I finally took the time to test the megasquirt with stim on an oscilloscope at school today.

I probed the tach signal at 3 different area on this simplified tach signal circuit :




This is what I got from pin 24 on the DB37 connector :

*That signal look clean to me.


The second signal I took was the one after R12:

** This is where it gets tricky, I don't know if it's fine, the signal seem to be damped alot....
*** And, I have no way of comparing with another megasquirt to see if it's fine,


The third signal I took, the one between the opto-isolator and directly at the input pin of the MS1 CPU pin 14 :

**** That signal look like crap, it looks like it is full of noise
***** Once again, I have no clue if this is right or not.



So, I would like to know if someone out there can tell me if my second and third signal should be looking like this or not.

I'm thinking maybe :
A) R12 is failing
B) The opto-isolator is failing (probably the case, looking at the noise)
C) The CPU input is dead...


I'm looking forward at any suggestion. thank you!


-Marc
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Old 06-09-2013, 04:59 PM   #2
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Is there anybody that can tell me if the waveform at the CPU should look like this?

I have no idea and I cannot compare...
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Old 06-09-2013, 06:14 PM   #3
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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.
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Old 06-09-2013, 08:17 PM   #4
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I grounded the probe at the power supply ground.

Thanks alot for your time and the answer!

I'll try to sort this out, now that that I have a small idea of how it should look like!
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