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Old 12-30-2009, 02:12 AM   #1
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Default MS issue/question/CAS going bad?

After getting MS in and running in my 93 I have found a strange occurance. There is no obvious cause for this problem but it seems to occur the most while cruising at highway speeds in 5th gear.

What will happen is the car will lurch randomly, When the car lurches the Odometer will quickly fall and then rise back to its previous reading as if nothing had happened. It will sometimes never happen, and other trips it will happen multiple times. During my most recent log the car lurched at least 10 times during my 15 minute highway commute.

Name:  CasSpike.jpg
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This spike in the middle of the image shows a RPM of 9265. The other spikes show similar numbers. Any ideas on what my problem might be?
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Old 12-30-2009, 03:08 AM   #2
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The log you have shown is indeed suggestive of a noisy CAS signal.

I also find the lean spikes in your AFR at the right side of the image intriguing.
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Old 12-30-2009, 03:18 AM   #3
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Its a practically untuned fuel map, tuning as I go. And If I remember correctly It might have been the car leaning out for "engine Braking" not sure though,
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Old 12-30-2009, 11:39 AM   #4
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Bump for more feedback and another question. Is that much noise from the Map sensor normal? Its plumbed off of the back of the intake mainifold.
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Old 12-30-2009, 11:41 AM   #5
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have you placed a 0.1uf cap between JS8 and ground?
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Old 12-30-2009, 11:56 AM   #6
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Dont believe so, what would that accomplish?
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Old 12-30-2009, 12:06 PM   #7
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It'd clean up the CAS signal.

I had a random miss for years on the high_res code, (but not the standard code weirdly) which bugged the hell out of me.

Till I fitted the cap.
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Old 12-30-2009, 12:08 PM   #8
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Ill Give that a try then! thanks for the input.
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Old 12-30-2009, 12:24 PM   #9
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Quote:
Originally Posted by Gryff View Post
Dont believe so, what would that accomplish?
filters the second trigger for noise...

Quote:
Originally Posted by found soul
The real resolution for my issue at least, was noise on the 2nd trigger input signal coming from the CAS. You're need a really high resolution scope to capture the noise, the noise blips I captures were roughly 4us (4 millions of a second) wide, but it was enough to present an additional, or in some cases simply a mis-timed rising/falling edge to the CPU which triggered it either at the wrong time or an additional time causing a misfire. I could see this clear as day in the scope traces once I cranked the sample rate up high enough to get a look at it.

Long story short we needed to filter the noise out without filtering the signal out or negatively effecting it. Math happened and we started trying a few capacitor values and landed at .1uf between the processor input and GND. A much smaller value seemed to solve the issue, but the .1uf did as well and provided some leeway in case the noise ever worsened, and it had no negative effect on the signal at any RPM this engine will ever see.
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Old 12-30-2009, 05:39 PM   #10
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He has one of your builds Brain. I thought yours had the cap already in place.
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Old 12-30-2009, 06:07 PM   #11
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If I built it, yeah it should.
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Old 12-30-2009, 06:42 PM   #12
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I haven't had a chance to take a look at his to see where all the noise is coming from in Gryff's setup, but it effects all sensor inputs; Not just map.

He has a cap in place between JS8 and ground so the problem comes from somewhere else.

Attached is a little better detail as to what it is doing
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MS issue/question/CAS going bad?-spike.png  
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