There should not be voltage to the pump when the engine isn't running. In the event of a crash that ruptured a fuel line, that would allow raw fuel to be pumped onto the ground creating a fire hazard. Chrysler Engine Computers turn on the fuel pump relay or automatic shutdown relay when it sees engine rotation, (cranking or running) by the pulses from the crankshaft position sensor and the camshaft position sensor. If either of those sensors fail, you won't have spark either so it's important to check for fuel pressure AND spark, and not get stuck on the first thing you find missing. Newer GM vehicles use a similar system although I'm not as familiar with it as I am with the Chrysler products. On some older GMs, they used a tap on the oil pressure sending unit to turn on the fuel pump relay. In a crash, the engine would stall if the fuel line was broken. That means no oil pressure so the fuel pump would stop running. Both systems are very effective and relatively simple to diagnose, ... Most of the time. The main thing to remember is to do any diagnosis while a helper is cranking the engine, not just with the ignition switch turned on.
Tuesday, November 8th, 2011 AT 5:50 PM