09-29-2008, 04:57 PM
Join Date: Sep 2005
Total Cats: 1,528
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 05:16 PM.