Good observation that you're missing fuel AND spark. Most people get sidetracked on the first system they find with a problem.
The coils, injectors, fuel pump or pump relay, alternator, and oxygen sensor heaters ALL get their power from the Automatic Shutdown (ASD) relay. That relay only turns on two times. First, it turns on for less than two seconds as soon as you turn on the ignition switch. This insures fuel pressure is up and ready for starting. You might hear the short burp from the pump if you listen closely. Second, the relay turns on when the engine computer sees engine rotation, (cranking or running).
If you measure the voltage at the coil or coil pack, or on the small terminals on the back of the alternator if they're easier to get to, you will see voltage for two seconds when you turn on the ignition switch. That's your proof the ASD relay and engine computer are good. If the voltage doesn't come back during cranking, suspect a defective crankshaft position sensor on the back of the transmission bell housing on the 3.3L, or a defective optical pickup assembly in the distributor on the 3.0L, or Hall Effect switch in the distributor on the 4 cylinders. The Hall Effect switch is a common failure item; it's very rare to have a failure of the optical pickup.
The ASD relay is Chrysler's very effective and reliable answer to Ford's miserable inertia switches. In the event of a crash that ruptures a fuel line, raw fuel pumped on the ground would be a severe fire hazard. Without fuel pressure, the engine will stall. No pulses will come from the crankshaft position sensor, optical pickup, or Hall Effect switch, (depending on the engine), so the engine computer knows the engine has stopped, and it turns off the ASD relay. That removes power for the fuel pump so no fuel gets pumped onto the ground.
You should also be aware of the engine computer issues. Chrysler had very little computer trouble in the '90s. Replacing it should be the last thing on your mind. Even if you put the original one back in, by unplugging it, all the stored information was lost. Short and long term fuel trims were memorized and used to adjust fuel metering under a multitude of conditions and various sensor readings. It can take a few miles of driving after the engine is warmed up to relearn short term fuel delivery modifications. It will take days to relearn the long term fuel trims. You should not notice any problems while this is happening.
The computer also knew the characteristics of the sensor readings and their relationship to each other. You might notice some minor performance issues while these values are being relearned. Of greatest importance is relearning minimum throttle. Until you do this, the engine will not idle on its own without you holding your foot on the accelerator just a little. To relearn it, drive at highway speed with the engine warmed up, then coast with your foot off the brake and gas pedals for at least 7 seconds. This will present the conditions that trigger the computer to put the throttle position sensor's reading in memory. Every time it sees the same value from now on, it will know it's time to be in control of idle speed. This same procedure must be done when replacing or disconnecting the battery, (or running it totally dead).
Friday, December 4th, 2009 AT 3:57 AM