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Old 09-29-2008, 12:57 PM   #1
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Default CAS function verification

So I took a test light to my CAS wires, cause I had my MS hooked up for like 5 minutes this weekend, and it flooded. So I assumed the CAS wasn't giving a signal, thus the MS wasn't firing the coils. So I took a test light to my CAS today and I got nothing for any signal wires. I guess I wired it wrong...probably backwards...
Looking at the connector while it's clipped onto the CAS, think of looking down at it from the firewall, on the wire entry side, I wired it like this...
Left: CMP
Middle Left: CKP
Middle Right: 12v
Right: Ground

My colors for some odd reason don't match the colors on the reference pictures you guys sent me, so I had to play the guessing game. This sensor was bought from Flyin Miata with the pigtail...I was told it was off a '97.

Last edited by thirdgen; 09-29-2008 at 05:21 PM. Reason: no responses due to lack of info
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Old 09-29-2008, 05:25 PM   #2
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I guess I'm on my own with this one???
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Old 09-29-2008, 05:29 PM   #3
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just looked at the CAS on one of my 94s and the colors looking at it from the front of the car from left to right are

black GROUND
white w/red +12V
white CKP
yellow w/blue CMP

do those match the diagrams posted before?
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Old 09-29-2008, 05:37 PM   #4
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Yeah, those match the wires posted before...but my wires are a different color. Although...I do have my wires hooked up in that exact same way. If I take a test light, clip it to a ground, and stab a CMP or CKP wire...I should get a flash, right?
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Old 09-29-2008, 05:42 PM   #5
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i have no idea. i just hook the **** up and it works usually.
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Old 09-29-2008, 05:57 PM   #6
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Probably not.

See, the CAS uses what's known as an open-collector output configuration. This means that it provides a closure to ground (via the "open" collector pin of a transistor) to signify activity.

The MS (like the stock ECU) applies a pullup to the lines going to the CAS. In other words, we apply a positive voltage, through a resistor (to limit the current) to the output of the CAS. We then measure the voltage on that line to determine what's going on at the far end. When the CAS is "off" then the voltage is equal to pullup voltage. When the CAS is "on", then it closes to ground and the voltage goes to zero. The resistor is there to limit the amount of current that flows through the CAS to ground. Were it not there, the output driver would pop.

So your reasoning is sound, except for one thing: the current-limiting resistor. If you followed the DIYAutoTune directions for modifying the MS1, then you have a 470 ohm resistor between +5 and the CMP line, and a 470 ohm resistor between +12 and the CKP line. (there's some additional crap in the CKP circuit causing voltage drop. Diodes and such... I don't like the way that circuit gets implemented, but alas.)

Ok, here's the brass tacks:

On the CMP line, the 470 ohm resistor means that line can source, at maximum, 10.6ma. On CKP, you could pull 25ma down the line.

Now, I don't know what kind of test light you're using, but it's entirely possible that this just isn't enough juice to light it up. You really need a piece of test equipment with a high-impedance input (one which does not draw significant current) to do this sort of measurement.



Oh- one last thing. You said it flooded, right? The MS wouldn't be firing the injectors (except for the priming pulse) if the CAS weren't working. You see any RPM indication either in the log or on the MT gauge when cranking? Should be 200 RPM or so on a good battery.

Last edited by Joe Perez; 09-29-2008 at 06:16 PM. Reason: spelling.
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Old 09-29-2008, 05:58 PM   #7
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LOL, nice! Too bad my car is an NB and I had to create wires that never existed...I'm sure I'll get it sooner or later. What I really need to do is just hook the MS back up, and watch for a tach signal on Megatune...
I was just wondering if there is another way to verify signal outputs. The less I stand on my head to do the wire switch, the better. Why didn't Mazda put the ECU behind the seat on the NB's like the NA's were?
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Old 09-29-2008, 06:02 PM   #8
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WOW...as I was typing my short paragraph of a reply...Joe Perez came through once again. So the test light theory is out...that's good, cause I'm 99.9% positive it's hooked up right.
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Old 09-29-2008, 06:09 PM   #9
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Try this; take the CAS out but leave plugged in. Turn ignition to run and spin the CAS with your fingers... Watch Megatune tach and see if it indicates anything.. You'll even be able to hear the injectors firing and fuel pump start up. Or pull your fuel relay and crank the car with it installed, see if it indicates RPM.
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