If it's cranking normally but won't run, first check for spark and fuel pressure. If either ONE is missing, troubleshoot that system. If both are missing suspect the automatic shutdown (ASD) relay isn't turning on. That is typically because a signal is missing from the crankshaft position sensor or camshaft position sensor. If you don't have a scanner, the best way to monitor the ASD relay is to test the voltage at any injector, the ignition coil, or either small wire on the back of the alternator. You should see 12 volts there for one second after turning on the ignition switch. That proves the circuit is okay and the computer has control of the relay. What is important is that voltage must come back during cranking. If it does not, one of those two signals is missing. Check for a diagnostic fault code to see if there's one to identify which signal is missing. If there's no related code, then you'll need a scanner to see which of those two signals is listed as "present" and which is listed as "no".
Be aware that testing fuel pressure can be misleading. The fuel pump will run for that one second after turning on the ignition switch but if the ASD relay doesn't turn on during cranking, the pump won't be running then either. That is an important clue, but that one-second burst at key-on will give the illusion you have fuel pump operation when in fact, you don't. The better test is to measure the voltage. Every injector will have one wire that is the same color. That is the wire to measure the supply 12 volts on during cranking.
Wednesday, August 1st, 2012 AT 6:19 PM
