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Old 03-30-2007, 10:44 PM   #1
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Question CAS Sensor Problem: Poor CKP signal Please Help



Ok, I think I am narrowing in on an issue that has prevented me from running megasquirt like 4 months ago. My initial problem was that after gettign megasquirt running, the tach signal would spike and it eventually died to never return again for the megasquirt. However it has been running fine on the stock ecu so i figured it had to be the megasquirt and so i bought another kit, built it very very carefully ... but it just wont pick up RPM. So i did some testing on the car with a multimeter and a spare CAS and here is what I found out:

Testing the connector with no sensor, both cars read 12v for white/red, and 5v for ckp/cmp wires so there should be no difference there.

When hooked up to my red 90 and using the blk/ltgrn wire for a ground:
White/red is 12v
white (ckp) is nothing, but shows a little tiny bit of voltage when the wheel is spun by hand
Yel/blu operates as expected and outputs either 0 or 5 volts depending on wheel orientation

Then I hooked up the sensor to my 97 to see if the sensor acted the same, but this time the White wire operated as it should and gave me 0 or 5 volts.

So Finally I am left with this question, what can I do to fix this? is there a possibility of it being more complex than simply the wire is grounding itself out somewhere? Perhaps I will try running a separate wire to see if that does it.
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Old 03-30-2007, 11:15 PM   #2
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Ok after working on it a bit more, I discovered that While the ecu was plugged in, the wires Tested 0v or 5v depending on the CAS wheel orientation. With the megasquirt hooked up, CMP works, but not CKP. With the ecu completely unplugged, CKP did not work and I didnt test CMP but will assume it wouldnt. So now I am suspect of the pullup voltage, which diyautotune has reccomended to be 12v, however the stock ecu uses 5v.

Now I think I have a pretty good understanding of how the CAS sensor works. It has four wires: GND, 12v, and two output wires. The ecu sends 5v to each of the two output wires, and as the wheel turns, it blocks the optical sensor and breaks the circut to the ground, bringing the voltage to 0. when the light hits another hole in the wheel, it reconnects the sensor and immediately brings it back up to 5v again. I believe that If I change the 12v pullup to a 5v, that I will have an rpm reading in megatune.

I will report back with the results.
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Old 03-30-2007, 11:17 PM   #3
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lol nobody seems to have much to say about this. I dont usually make lengthy posts like this, but this is important to me because it possibly challenges the www.diyautotune.com instructions. But if I am wrong in anything I say please speak up.
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Old 03-30-2007, 11:26 PM   #4
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I'm sorry mate but I don't think that I know anything that would be any help to you at all.

I did the board mods for the CAS (and that was a loooooong time before they appeared on the diyautotune site) and it worked perfectly from the word go.

I feel for you but I'd guess that Jerry would be the only person who could help you out.
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Old 03-30-2007, 11:48 PM   #5
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phew ... guess even being careful you can make mistakes. I did my board mods by mounting the resistors onto the proto area, and I must have been a little bit anxious to get going .... well i forgot to finish the connection between the resistor to D1, forfeiting my pullup voltage and resulting in no CKP. Fixed the problem and now I have rpm.

Lol, i guess everything happens for a reason and I have a much greater understanding of what is going on down there. I never realized what the pull up was doing exactly, and today I do. I still dont understand why we dont use a 5v pullup, however checking at the CAS plug revealied that The CKP pullup voltage was only 5.8v with the CMP being 5.0v (like stock) Its definitely something to look into.
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Old 03-30-2007, 11:53 PM   #6
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Try it out-- but keep in mind the pullup isn't there for the CAS -- The pullup is there so the MegaSquirt can 'pullup' that 5v signal into a 12v square wave signal that the MegaSquirt expects to see. It won't see the signal without the pullup for sure.

I'm thinking there may be something up with your CAS, and that the stock ECU is either A) getting by without the signal (if it's the signal from the single window I could see this) or B) getting enough of a signal to interpret it.

What could be wrong with the CAS-- one thought is your CAS could have an issue where it's sending a signal but is very easily overpowered by the pullup and not able to pull it down enough to create the square wave signal properly. The way this works is the pullup pulls the signal high, the CAS pulls the signal low (to ground) and together they chart out a square wave with peaks and valleys, with the valleys at 0v and the peaks at or near 12v. If the CAS isn't able to pull it down to 0v or close to it, then you've just got a solid 12v line, and there's nothing to trigger the ECU. You could power your stereo from it though . j/k, don't try that.

So that's one possiblity-- without putting a scope on the signals coming out of the CAS it's hard to say if that's the issue. All I can do is guess.

Another possibility is if the wheel decoder settings aren't right in your ECU, or if something else isn't right with it...
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