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MegaSquirt losing crank signal

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  • Austrianvespaguy
    replied
    So yes there are various combinations that can probably made to work, but I now completely understand these circuits and can assure you that using a single inline resistor with the Microsquirt VR conditioning circuit is just plain WRONG (even if it 'works'). The reason for this is the MAX9926 chip uses differential input, meaning that it's looking at BOTH the VR+ and VR- signals relative to each other and NOT relative to ground (aka single-ended input, which is how the LM1815 works). So putting extra resistance on one signal and not the other will skew the zero-crossing reading, which is bad. The shunt resistor between the two wires keeps both signals 'even' but still drops the peak voltage generated at higher RPM speeds.
    Also the MAX9926 chip is really the best of the bunch; so if you set it up correctly with the shunt resistor, it WILL work for everybody with that setup, and it will take out the touchiness of individual sensors, air gaps, etc. It's a really robust circuit; not that you want to bother, but with the shunt resistor I'll bet you it would also work perfectly with your OLD sensor too!

    EDIT: And Malcolm was really on the right track at first by bridging the SB2 on the board to use its built-in shunt resistor, but I suspect that 5k resistor is intended for 'other' trigger wheels and the 60-2 wheel needs the 10k resistor as indicated in the Microsquirt manual. Re-reading Malcolms last post, I think he has done things correctly by putting in a 10k shunt resistor in place of the 5k, but hopefully he'll confirm this for us!
    Last edited by Austrianvespaguy; 06-19-2019, 01:10 PM.

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  • Digitalwave
    replied
    There are a variety of resistors that will "work." I doubt this will solve everyone's problem, as there are a lot of reasons it could be happening.

    FWIW once I replaced my CPS with a new metal one it fixed my issue. I can and have used a 10k, 18k, 20k, and 25k resistor and all of them worked fine with the new CPS. My MS is Microsquirt-based.

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  • Austrianvespaguy
    replied
    I HAVE SOLVED THIS FOR EVERYONE!!!
    But the solution varies depending on exactly which Megasquirt setup/VR condition circuit you have, so make sure you understand this! @Malcolm206, as I understand it you're using MICROsquirt v3, and if that's correct, you are not done yet! So here's the rundown:

    1.) If you are using an LM1815 chip for as your VR conditioner (found as an optional extra on some PCBs such as the DIYPNP), then you want the 18k resistor in series with the VR+ sensor wire (to limit the current that the IC needs to sink).

    2.) If you are using MEGAsquirt 3.0 or 3.57, then you want the 10k resistor in series with the VR+ sensor wire (to filter out noise at higher RPMs).

    3.) If you are using MICROsquirt, then you want a 10k SHUNT resistor that goes BETWEEN the VR+ and VR- sensor wires! (<- This really solved my problem and is the proper setup for the MAX9926 chip on the Microsquirt board)! See page 47 of the Microsquirt Maunal here!
    http://www.msextra.com/doc/pdf/Micro...rdware-3.3.pdf

    Malcolm and DigitalWave, I'd suggest you double check which VR condition circuit you are using; if you have the white Mirosquirt daughter card that looks like this:
    http://www.microsquirtmodule.com/mic...V2.2_front.jpg

    THEN YOU NEED THE SHUNT RESISTOR IN SOLUTION 3!!!

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  • Austrianvespaguy
    replied
    Well a little more info for the general knowledge-base here; I'm using the DIYPNP board w/Microsquirt card, and it has TWO different VR signal conditioners on it. The Microsquirt uses a MAX9926 chip, and the DIYPNP board has an extra optional LM1815 chip. The MAX9926 wants the ~10k resistor on the input for noise reduction, and the LM1815 wants the ~18k ohm resistor on the input to lower the current that it needs to sink. I made the small mistake of installing the 18k resistor with the MAX9926 circuit, and I think it was blocking too much of the signal. Just switched down to the more proper 10k, and things are certainly improved. I'm not closing the book on this one just yet, only tested briefly and think I still dropped once or twice at redline, so might go with an even lower resistance, but looks like I'm on the path to improvement now!

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  • Austrianvespaguy
    replied
    Ah, I didn't realize it was mounted to the rubber part of the pulley, good call! I have checked it's resistance (575 ohms at the sensor connector) but I'll verify at the 55 pin connector also. My gap is pretty small, about 0.5mm and no I don't really see much runout, but I do see what I think is a very small amount in the tooth logger.

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  • Digitalwave
    replied
    Originally posted by Austrianvespaguy View Post
    Here's my tooth log. You can see the extra long gap in the middle that's then only followed by 57 instead of 58 teeth. Also, this problem seems to worsen as engine temperature increases, which I really can't explain. Revs fine to redline dead cold (though I don't like to do that!) Here it's about 190 and cutting out around 6000 RPM, and fully warm around 210 it cuts out at 4500-5000 RPM.
    Hmm, attachment not working, so here's a link to the screenshot:
    https://drive.google.com/open?id=1uC...Yogu-rwSgjyIWz
    On a final note, is the small wave along the normal teeth just a bit of runout on the wheel itself?
    Since yours is temperature dependent I would consider that it could be the toothed wheel itself. It is mounted on a rubber damper that degrades over time. When your car is running does it look like the toothed wheel has a lot of run out? Have you tried other/new CPS's? Have you ohm tested yours both at the sensor and at the 55 pin connector?

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  • Austrianvespaguy
    replied
    Good stuff! Glad we were able to get you're solved! Now if I can just figure out my issue. . .

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  • Malcolm206
    replied
    I was under the impression that the resistor on the board was 25k, but after reviewing the schematics realized it was actually only 5k. I added a 10k resistor and that seems to have fixed the issue with the CPS.


    Sent from my iPhone using Tapatalk

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  • Austrianvespaguy
    replied
    Here's my tooth log. You can see the extra long gap in the middle that's then only followed by 57 instead of 58 teeth. Also, this problem seems to worsen as engine temperature increases, which I really can't explain. Revs fine to redline dead cold (though I don't like to do that!) Here it's about 190 and cutting out around 6000 RPM, and fully warm around 210 it cuts out at 4500-5000 RPM.
    Hmm, attachment not working, so here's a link to the screenshot:
    https://drive.google.com/open?id=1uC...Yogu-rwSgjyIWz
    On a final note, is the small wave along the normal teeth just a bit of runout on the wheel itself?
    Last edited by Austrianvespaguy; 06-17-2019, 07:19 PM.

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  • Austrianvespaguy
    replied
    Oh and to the OP, your graphs certainly look like your problem is not seeing the missing teeth gap properly (sorta opposite my problem), but for this the 18k (or 10k) resistor in series with the VR+ wire should be the first fix you try if you haven't already!

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  • Austrianvespaguy
    replied
    My screenshot is of the composite log instead of the tooth log, so that's why it looks a little different. The red graph along the bottom is the de-sync counter. Next time I fire up the car I'll grab a tooth log for comparison, but basically it'll show the missing teeth section as being too tall of a bar and then only one too few short bars in between.

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  • Digitalwave
    replied
    ^ Is your crank wheel set up for rising edge or falling edge? The tooth log should be looking like this:

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  • Austrianvespaguy
    replied
    Interestingly I'm currently struggling with the exact same problem on my DIYPNP setup (which includes an 18k resistor in-line with VR+). From the composite log file, I can see that the problem stems from the system only seeing 57 instead of 58 teeth and then getting thrown off, but I can't for the life of my figure out WHY/HOW it's missing a tooth, as all the timings look pretty good! Also strange is that sometimes, it all works fine up to my 7000 RPM redline limit, but others (like now) the cutout happens right around 5000 RPM. I *think* it might be a bit temperature dependent, but not entirely sure on that one yet.
    Attached Files

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  • justinpirie
    replied
    I had to add a resistor in line to get mine to work. From memory I think it was 10k.


    Sent from my iPhone using Tapatalk

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  • Digitalwave
    replied
    I tried a bunch of different resistors and didn't find much if any change. You do need a 10-20k resistor based on most of my research. You can read my experience here: https://www.r3vlimited.com/board/sho...d.php?t=418713

    Have you tested the resistance of the CPS @ the 55 pin connector inside the car? It should be 540 ohms +/- 10%.

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