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Old 06-13-2019, 03:35 PM   #1
Malcolm206
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MegaSquirt losing crank signal

I recently installed a new 2.9L M20 with Microsquirt v3.0. I'm using the stock 60-2 toothed wheel and having an issue with the crank position sensor at higher RPMs. Once I hit around 3500 RPM, the engine acts like it's bouncing off a rev limiter/fuel cutoff. The tooth logger shows I'm completely losing the signal and counts as a loss of sink. I bridged the shunt resistor in the case, moved the CPS bracket closer to the wheel, and swapped out CPS sensors with another used one.

Any ideas what I should try to check next?

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Old 06-13-2019, 10:36 PM   #2
justinpirie
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MS can be so frustrating sometimes! The CPS might be a symptom not the cause...

Stupid first question... how good are your grounds?


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Old 06-14-2019, 12:19 AM   #3
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MS can be so frustrating sometimes! The CPS might be a symptom not the cause...

Stupid first question... how good are your grounds?


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They appear to be totally fine. All new wire with a solid ground at the original wiring point near the battery. Thereís no distortion in the signal. I may try moving it to ground on the block just to confirm thatís not the issue.


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Old 06-14-2019, 06:11 AM   #4
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Does the Microsquirt have variable VR sensor inputs? On my MS3X 3.0 board I had to turn the VR trim pots all the way one direction (I think CCW, not completely sure) and then that same issue went away.
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Old 06-14-2019, 09:01 AM   #5
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Does the Microsquirt have variable VR sensor inputs? On my MS3X 3.0 board I had to turn the VR trim pots all the way one direction (I think CCW, not completely sure) and then that same issue went away.


Microsquirt doesnít have variable VR sensor inputs. I bridged the 25k resistor on the back of the board to try and fix this issue, but it had no effect.


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Old 06-14-2019, 09:06 AM   #6
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Try making the sensor air gap tighter, this happens to me a lot on installs. The stock ECU doesn't much care if the gap is too large, but the MS does. The two 13mm screws that hold the CPS bracket can be loosened a little and the bracket has wiggle room.
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Old 06-14-2019, 09:15 AM   #7
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Try making the sensor air gap tighter, this happens to me a lot on installs. The stock ECU doesn't much care if the gap is too large, but the MS does. The two 13mm screws that hold the CPS bracket can be loosened a little and the bracket has wiggle room.


It seems to be as tight as it will go without modifying the bracket, but the CPS sensor still seems to have a decent air gap between it and the wheel.


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Old 06-14-2019, 09:29 AM   #8
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Odd, perhaps the bracket is bent/damaged. Of all the m20's we have worked on the bracket adjustment is enough to make the CPS actually touch the relluctor wheel.
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Old 06-17-2019, 06:38 AM   #9
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First try different resistors (you did already). Next replace the CPS with a new OEM metal CPS and set the air gap properly (1.0mm). MS is picky about how good the signal is, much more than Motronic. If those both fail to fix the issue, start tracing and testing your CPS wires in the engine harness.
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Old 06-17-2019, 08:35 AM   #10
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Odd, perhaps the bracket is bent/damaged. Of all the m20's we have worked on the bracket adjustment is enough to make the CPS actually touch the relluctor wheel.


My mistake, when I was tightening the bolts on the bracket I wasnít holding it firmly enough and it would rotate away from the wheel. I now have it set to 0.88 mm away from the wheel, which didnít change the issue.


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Old 06-17-2019, 08:39 AM   #11
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First try different resistors (you did already). Next replace the CPS with a new OEM metal CPS and set the air gap properly (1.0mm). MS is picky about how good the signal is, much more than Motronic. If those both fail to fix the issue, start tracing and testing your CPS wires in the engine harness.


Iíve tried three different used CPS units, and the system still cuts out at three similar but slightly different RPMs. Iím currently waiting for my new CPS unit.

Has anyone had luck adding a 10k, 17k, 20k, etc. resistor in line for the sensor, or is this issue most often stemming from MegaSquirt being more particular about a properly operating CPS sensor?


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Old 06-17-2019, 08:50 AM   #12
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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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Old 06-17-2019, 08:50 AM   #13
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I had to add a resistor in line to get mine to work. From memory I think it was 10k.


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Old 06-17-2019, 11:40 AM   #14
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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.
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Old 06-17-2019, 11:46 AM   #15
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^ Is your crank wheel set up for rising edge or falling edge? The tooth log should be looking like this:

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