Fuel Pump low voltage

So, what are the causes of low voltage to the fuel pump? I searched but couldn't find anything for actual causes of it.

The reason I ask is that I recently turned my jeep into a submarine and my fuel pump ended up dying for whatever reason as I was leaving the trail. Got towed home, put in a used fuel pump I had lying around and it started right up and would idle for 5-10 min. fine and then die. It would restart, but die immediately. After sitting for a couple hours it would start and idle then die, and then start and die immediately, same thing. I started getting codes for low fuel pump voltage and short circuit in relay.

I swapped relays and put in a brand new Bosch fuel pump with new filters and cleaned the gas tank last night (had some sand in it) and re-installed everything. I started it and let it idle for a good 10-15 min. and it didn't die yet. I didn't try driving it b/c I didn't want to risk breaking down on the side of the road. I'm still getting the codes for low voltage and short circuit though.

I'm going to d/c the battery and clear the codes and let the jeep run some more tonight, maybe the codes are just still stuck in the memory.

Anyway, long story short, I want to find out what to check that would casue low voltage at the fuel pump. I've checked the harness at the pump and it all looks good.

It's a 98, 4.0, AW4.

You need to be able to test voltage with a multimeter to verify low voltage at the fuel pump before you have a shot at finding where it is. Sometimes the fault codes aren't really what they sound like. And test your fuel pump ground wire for voltage drop (while running). Set your meter to volts, but one probe on the ground wire and the other probe on a GOOD ground. Should read 0V, but more that 1V is certainly excessive.

where exactly should I test the voltage at/put the probes? To check voltage with the jeep running, everything is connected and the pump is in the tank, so I'm not sure where I can attach the probes to check the voltage.

You test everything at the connector to the fuel pump, like by probing in with a partly straightened paper clip, or pierce the wire with a safety pin (and seal it back up later). The black wire is the ground, and the dark green / white should be the 12V in (based on 2000 wiring diagram). The other two wires are for the fuel gage.

ok, so I checked the voltage at the pump, it was getting 11V. Is that enough? My battery was kind of low at the time, could that be why it's not 12V? I cleared the codes by unplugging the battery too.

Also, after installing the new fuel pump yesterday, I let the jeep idle for about 15-20 min. and it ran fine. If I held it revved up and let it drop down to idle, it would stumble a little bit before resuming normal idle.

Well, I decided to take it out for a quick test drive. Drove it about .25 miles in a loop around my community and as I pull in the driveway it starts sputting and wanting to die. If I put it in Park it would idle very low, if in gear, it would sputter out. It died shortly after.

The low voltage code hasn't come back up yet. I don't know why it keeps dying on me though. It's very frustrating! It seems to idle fine, but if I drive it, it ends up dying within a few minutes.

Any ideas???

Could be your alternator is not charging the battery, a usual consequence of submarine service.

Charge the battery fully, and then check to see if the alternator is putting out enough of a charge.

yeah, i just charged up my battery completely and I'm going to go through my alternator cables soon.

Would the battery not charging cause it to die if the battery was too low on voltage?

Check your cables and ground. Then test battery voltage with key off. Should be 12.5V. Next, start it up and see if voltage goes up to about 13.5. If not, the alternator is toast. Auto parts place can test it. Smaller chance that the voltage regulator (which unfortuantely is built in to the PCM) went bad. Low battery could make it die.

winterbeater said:

Check your cables and ground. Then test battery voltage with key off. Should be 12.5V. Next, start it up and see if voltage goes up to about 13.5. If not, the alternator is toast. Auto parts place can test it. Smaller chance that the voltage regulator (which unfortuantely is built in to the PCM) went bad. Low battery could make it die.

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x2. I have seen cars so low on voltage they would start and run at idle, but if you turned on the fan or lights they would die. About 9v and less and the electronics won't function correctly.

alright cool, I'm hoping that's my problem. I'm going to replace the alternator cable this week because it is badly corroded and see what happens. Thanks for the input!

well, I think I might have fixed it. I replaced the cable that was corroded that ran from the alternator to the PDC and charged up the battery and cleaned the terminals. I fired it up and the voltmeter was back up at around 14V. I let it idle for a few minutes, took it out for a quick test drive, and let it idle for a few minutes more when I got back. It didn't die or sputter or anything. So, maybe it was just the battery too low on voltage. Thanks for the advice guys!

Glad you got a handle on it.