You have the best understanding of how this system works except for one detail. The reason no one eliminates the computer first is because it is by far, the least likely thing to cause this problem. The cam and crank sensors are the most common causes. Where most people get wrapped around the axle is they find the missing spark or the missing injector pulses and stop there. They get stuck in the first system they find dead and troubleshoot that.
The clue here that your problem is different is that missing one second ASD relay when the ignition switch is turned on. While you didn't mention it, I think I can infer from your troubleshooting steps that there is 12 volts feeding the ASD relay coil, because you understood how to ground the other side.
I think where I would go next is to first check for the 5 volts that runs the cam and crank sensors. Unplug them if the 5 volts is missing, then cycle the ignition switch off and back on to see if it comes back. When that line is shorted, the computer shuts that power supply down to protect it. I never checked to see if that affected the ASD relay during that first second.
Next, there will be four ground wires for the computer. Two are for sensor signals and two for the high powered stuff like injectors and coils. Check all of them for open circuits. There could be some resistance in one of them that is too small to measure, but a voltage drop test might show up as the RESULT of that resistance. Unfortunately that can be hard to see with a digital voltmeter. A plain old oscilloscope would show even the tiniest voltage drop during engine cranking.
Given that symptom of no ASD operation at key-on, it is entirely possible the computer is at fault, but before you potentially waste time and money, you might consider doing what we did with the big "bucket brigade" integrated circuits in tvs. Use the pinout diagram of the computer connector and measure the voltage on every pin. Look for a missing or incorrect voltage. In particular, there will be at least three 12 volt feeds. One is there all the time, one with the ignition switch turned on, and one that comes from the ASD relay to tell the computer it did indeed turn on. That last one will only be there, obviously, when the relay is on.
Next, unplug the connector and measure each pin for resistance. This can take quite a while because you'll have to find each one in the diagrams to see what it goes to and what to expect for resistance. I understand that it is much easier to just pop in a different computer, ... If you have one, but testing voltages and resistances could save you the expense of getting one if it isn't the cause of the problem. Sorry I can't help with the years and models that might interchange. I can say with reasonable certainty that a '95 and older will be different because those cars didn't have oxygen sensors after the catalytic converters.
Also check for the 5 volts that runs the throttle position sensor and map sensor. If that supply is missing, it could come off the same circuit in the computer that runs the ASD relay. The computer can shut that down too if there is something shorted. There are a lot of safeguards built into the computer so chances are a different one won't be damaged if something in the car is shorted.
Thursday, December 9th, 2010 AT 3:12 AM