Rather than typing for hours again, here's a copy / paste version of a reply I've sent many times. This person mistakenly wanted to replace the fuel pump relay. Take from it what will help you.
... What exactly is the symptom? If the engine runs, the relay is working. Relays give extremely little trouble, but it's the first thing do-it-yourselfers run to when they think they're going to fix a problem. If you believe the fuel pump is not running, (and you may be right), you are absolutely not done with your diagnosis. You must also check for spark.
The ignition system, (spark plugs), and the fuel system are both very reliable and trouble free. It's the trigger system that gives most of the problems. To get you started, (I don't know your level of troubleshooting ability), you can measure voltage at the fuel pump, coil, injectors, or the alternator field, (the two small wires on the back of the alternator). You're looking for 12 volts. Alternately, you could remove the fuel pump relay and / or the Automatic Shutdown (ASD) relay, pop their covers off, reinstall them without the covers, and watch what happens when a helper runs the ignition switch. Both of those relays are controlled by the same circuit of the engine computer, so both relays will turn on at the same time. Simply feeling them with your fingers isn't a good test because there will be other relays clicking so it will be confusing. If you watch these relays with the covers off very closely, you will see a shiny metal lever move just a tiny amount.
Under proper operation, when the helper turns the ignition switch to "run", you will see those two relays click on, then click back off in less than two seconds. They are supposed to turn back off, but that's where novices get confused when they find the fuel pump not running. If you listen by the tank, you'll hear the fuel pump run for that one to two second burst. There's your proof the fuel system including the relay is working. Time to look elsewhere. ( I always wanted to use that word)!
Those two relays will turn on again when the engine computer sees engine rotation, (cranking or running). Have your helper crank the engine and watch the relays. If they do not turn on during cranking, suspect a defective crankshaft position sensor.
This is Chrysler's extremely effective and reliable answer to Ford's miserable "inertia switch". In the event of a crash that causes a ruptured fuel line, the pump could keep running, pumping raw fuel onto the ground. Major fire hazard. With a ruptured line, there will be no fuel pressure. With no pressure, nothing will spray from the injectors, so the engine stalls. When pulses stop coming from the crankshaft position sensor, the engine computer turns the fuel pump and ASD relays off so power to the fuel pump, coil(s), and injectors is removed. No more fuel dumped onto the ground.
In the rare event you find that both relays are indeed turning on while cranking, but the engine doesn't run, that's when you need to troubleshoot the fuel or spark problem. A likely suspect then would be the fuel pump. Sometimes banging on the tank will get it started, and once it's running, it will continue to run fine until you stop the engine. This problem is due to worn brushes in the pump motor. Another problem they had in the mid 1990s was pump bodies warping in extremely cold weather causing the impeller to drag. It was very common to have a truck towed in for a no-start condition, and the mechanic would push it into the shop at the end of the day so it would be warm when he started working on it the next morning. But by morning, it had warmed up in the shop and would start right up. Nothing to troubleshoot then because obviously everything is working. It became a common enough issue that with that set of symptoms, we knew to change the pump. I suspect that batch of pumps was replaced many years ago, but I could envision that happening to someone who bought a used truck from down south, or who lived there, and now that truck is up here in WI. ...
OK, back to a live human being. Your fuel pump should only run for one to two seconds when you turn on the ignition switch, then it will turn off. That's when you're doing your testing. You need to do the testing while cranking the engine. If the pump runs for two seconds, but not while cranking the engine, suspect a defective crankshaft position sensor. You will also find you don't have voltage to the coil either.
Monday, December 7th, 2009 AT 4:26 AM