Good afternoon all,

I have a 2001 Grand Cherokee Laredo with the 4.0 inline six, 230ish-k miles. While I was out on work travel for a couple months the wife was driving the Jeep after she wrecked her car. She told me it would have trouble starting and battery dying. Well it eventually died completely and would not start when she got off work one day. I was able to drive home for a weekend and replaced the starter thinking that was the culprit. Turns out it wasn't. Got it towed home and it sat for about a month until I was able to come home for good. I have concluded it is a fuel pump issue, but after troubleshooting it for the past 2 days, I cannot figure out the true underlying issue causing the fuel pump not to turn on. So, I will list things I have done to troubleshoot it to narrow everything down and hope someone might have dealt with the specifics I have.

I cut an access panel to get to the fuel pump in the spare tire well.
Original fuel pump would not turn on, no prime or fuel pressure from Schroeder valve.
Checked relay via click test and ohming out with a multi meter and it checked out good.
Jumped pin 30 to 87 to force power to the fuel pump and still would not turn on.
Replaced fuel pump after deeming it has to be faulty and the new pump does not power on either.
Checked voltage at the fuel pump assembly connection and get 12.5 volts with relay bypassed but when I connect the fuel pump, voltage drops to zero.
Connected the fuel pump directly to the battery just for piece of mind and it powered on.
Checked fuel pump ground under the carpet on the driver seat and is good.
Verified battery ground was good as well.
Hooked a wire jumper from positive battery terminal to positive connector on the fuel pump and jumper wire from negative connector pin on the fuel pump assembly and fuel pump powers on.
Repeat previous ^ except reversed, jumper on negative battery terminal to negative pin on fuel pump and jumper wire from positive connector on fuel pump assembly and the fuel pump does NOT power on.
Connect multi meter lead to positive jumper wire from the battery and other lead to the jumper wire from the jumper wire on the negative connector on the fuel pump assembly and read 12.5 volts.
Repeat previous ^ except reversed, jumper on negative battery terminal to one lead and wire jumper from negative connector on fuel pump assembly and still read 12.5 volts. So the circuit is completing to the best of my knowledge.
All fuses are still good.
Attempted to try out a ground wire to a piece of metal jumpered to the negative pin of the fuel pump and positive pin to the jumper wire on the positive connector of the fuel pump assembly and still did not power on.
Even during my frustration the jumper wire from the positive connector of the fuel pump assembly accidentally came into contact with the temporary ground wire i was just testing with and got NO ARC, which is strange considering I put the multi meter on them right after and still reading 12.5 volts.

I have come to a thought that even though I am getting the correct voltage through the harness from the battery, that I am not getting enough amperage. But testing grounds and even making a temporary ground wire directly to the fuel pump and it not working still, that it may possibly be the PCM?

Thanks for anyone that even read all of that and has any info/tips/help.

 

Yes, lots of strange things happen when the PCM doesn't get a full 12.5v. Initially when the key is turned on, the PCM pulls in the fuel pump relay of one second to charge the fuel rail with pressure, then turns it off. Then it waits until it gets a signal from the crank sensor telling it the crankshaft is actually turning for start, then it turns on the fuel pump and ASD relays. The ASD relay controls both spark and injector circuits. SO, with the key on and the initial charge cycle complete, the fuel pump relay will not turn on until you are cranking the engine.

If the battery isn't fully charged, the starter will pull enough amperage so that the PCM doesn't get it's full 12.5 volts and won't function. The PCM has a voltage control in it that changes the 12 volts to 5 volts for the solid state circuits that control everything. If the circuit boards don't get their 5 volts they won't work. The computer uses digital drivers that do the actual switching of the 12 volt circuits they control by either grounding them or un-grounding them. When you turn the key on, the fuel pump, ignition system, and the injectors get 12v supplied to them, BUT REMAIN NOT GROUNDED until the PCM decides which should or should not be grounded.

Here's what you should do. Disconnect the battery and clamp both cables together. Turn on the key and headlights for a couple of seconds to completely drain the capacitors in the PCM. Fully charge the battery completely making sure it's reading a full 12.5 or better voltage. Reconnect the battery and try starting it. Listen close to see if you can hear the fuel pump charge the rail before cranking it. It should start like there never was a problem. Please report back what you found.

 

Thanks for the tips Dave. I tried them and it didn't resolve the issue but I decided to go in deep today after I did a voltage drop test and ended up tracing the Fuel Pump wiring from the pigtail all the way down the door sils to the gas pedal... Low and behold, I disconnect the panel connector by the foot rest and after i took the white cover off I notice the dark green wire (Fuel Pump hot wire) is pinched and damn near severed...Solder sleeved it together and she started! Only took a new starter, new fuel pump and 2 new batteries to locate the gremlin, but I did, lol...Saved me from actually having to drop a couple grand on some beater to get around in. Until next time.

 

Good deal! Glad you found it! Tracing the wiring would have been the next step. Quite often they corrode near the plug at the tank.