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Discussion Starter · #1 ·
What are the voltage requirements for the fuel pump to kick on?

Everything was working before winter, and now the fuel pump won't prime when I go to key-on position. I confirmed 0 psi at the fuel rail. Filter is new and no blockage detected back to the tank.

The fuel system fuse is ok, so is the Dome Fuse. I swapped relays with the ASD and Starter Relay and get the same result. I have ground at the pump confirmed via the fuel gauge and also installed a better grounding location with metal to frame contact.

I did take the pump assembly out of the tank and bench-tested it with a 12v source. It came on sporadically. I'm not sure if this confirms the pump or if something else is going on electrically. It did come on, but that was using 12v directly. I thought maybe the voltage gets dropped down when going to the pump and it might not be enough to get it running. So that is why I would like to know what exact voltage the pump operates.

I had this problem before months ago, now it's back and have no idea why. The tank was -completely- dry for about 5 months. I'm talking about that I took the tank outside upside-down and dumped it out to clean it. So I'm wondering if this damaged the pump somehow b/c it sat dry for months.

I did take a crap load of multimeter readings at the Relay center for Fuel-Pump and Auto-Shutdown, if anyone can help me understand that please let me know I'll post it below.
 

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Discussion Starter · #2 ·
<<<Fuel Pump Relay>>>

Key-Off:

87- 0 Volts
86 - 0 Volts
85 - 0 Volts
30 - 12.10 Volts

Key-On:

87 - 0 Volts
86 - 11.7 Volts
85 - 7.15 Volts
30 - 12.08 Volts


<<<Auto-Shutdown Relay>>>

Key-Off:

87- 0 Volts
86 - 0 Volts
85 - 0 Volts
30 - 12.10 Volts

Key-On:

87 - 3.5 Mili-Volts
86 - 11.8 Volts
85 - 7.15 Volts
30 - 12.05 Volts
 

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958 Posts
It sounds like you're covering all your bases. For sure. How did you bench test your pump? Just rip it out and hook it to a battery? Probably the best way. I'd say if its sporadic, replace it. Sounds like that's your last line of defense.
 

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Discussion Starter · #4 ·
I took the pump assembly out and hooked it up to the battery in the Jeep. I touched the positive jumper to the hot-wire terminal on the pump, and then used a jumper from the negative terminal on the battery and touched it to the pump housing / grounding wire.

By sporadic I mean that sometimes that worked, and sometimes it didn't. I would hold the jumpers in place and the pump sometimes took time to get up and running, but it did come on.

I wasn't sure if the 7.5 volts coming from the 85 terminals was enough to get it running.
 

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<<<Fuel Pump Relay>>>

Key-Off:

87- 0 Volts
86 - 0 Volts
85 - 0 Volts
30 - 12.10 Volts

Key-On:

87 - 0 Volts
86 - 11.7 Volts
85 - 7.15 Volts
30 - 12.08 Volts


<<<Auto-Shutdown Relay>>>

Key-Off:

87- 0 Volts
86 - 0 Volts
85 - 0 Volts
30 - 12.10 Volts

Key-On:

87 - 3.5 Mili-Volts
86 - 11.8 Volts
85 - 7.15 Volts
30 - 12.05 Volts


87 is the power output on the relay.

Your bench test pretty much confirms what these readings suggested - a possible bad ground in the fuel pump.
 

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Definitely sounds like its time to swap the pump out. Good info man. Wish everyone could troubleshoot like that.
 

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Discussion Starter · #7 ·
That's a good diagram I got to save that one lol. Thanks for the help guys, it means a lot. I will prepare to swap out the fuel-pump this weekend and report back with the results. You can imagine how disappointed I was to get everything back together and then have the same problem as I did in the Fall when I first dropped the tank.

As JeepRich can testify, I was having a horrible time months ago trying to figure out the same problem. That time, it turned out to be that one of the power wires from the ECM (Yellow #6 if I recall) was ever-so-slightly pulled out from under the relay supply center. I would've never found it if I didn't get an interpretation of my multi-meter results, so thanks again for doing it this time as well. Hopefully, this will be a simple R&R lol *fingers crossed*
 

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Discussion Starter · #8 ·
Conclusion:

So after taking the pump off of the assembly, I wanted to bench test just the pump by itself with 12v, and it worked every time. So before I shelled out $80 for another pump, I wanted to test the pump housing alone outside the circuit system. I wanted to eliminate the housing wiring from being the source of the problem.

Basically, I had an open-line between the outer housing and the wiring in the assembly. The wire terminal that hooks to the assembly structure that also gives the pump it's ground feed was coked up with debris and corrosion. As the prongs on my Multi-Meter were hooked up, I jiggled the wiring, and when the wire moved it gave me my OL indication. So the movement of the fuel + the degradation of the terminal must have been the problem.

I figured that was why the pump was working on the assembly sporadically. So I replaced the terminal, hooked everything back up, and the pump now primes at key-on and runs continuously while at idle. I have good PSI around 32 on the fuel rail.

Something so small giving me such a big problem. Electrical problems are no fun that's for sure. Thanks everyone for the help. I hope this will help someone else in the future. I also hope this will be the last time in a long time that I have to drop the tank by myself lol.
 
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