[FIXED] --1998 Nissan Frontier - Low Fuel Light Always On

I'm hoping someone can enlighten me on what turns on a low fuel light. Squirrels chewed the wiring harness to my in-tank fuel assembly. It has six wires running to it. I took the bed off of my truck and hoped to simply do a wiring repair to reconnect the wires, but there was nothing much left of two of them into the assembly, so I bought another one. The chewed wires caused a check engine light because the computer was looking for a reading from the fuel temperature sensor and it wasn't seeing it.

I bought a napa C0548M fuel pump. Normally don't like aftermarket, prefer OEM, but dealer parts wasn't open and Napa was. Installed the pump and check engine light is out. So, that's good. However, my low fuel light is on permanently. Is something stuck? What turns it on? Anyone know? Did I get a pump that doesn't support that function so it's on automatically? The fuel gauge does work properly. I can't figure out how the light works and I want the stupid thing to work. Any help is much appreciated. Thanks,

Jim

This a section of the OEM diagram for reference:



Unfortunately I couldn't find one diagram that shows the pump, the level sensor AND the low fuel indicator all in one. But this is the part we're really interested in. The fuel level sensor and the low fuel indicator share a ground.

With this circuit design, you're down to either a short to ground in the light green wire between the instrument cluster and pump assembly, or there's something wrong with the pump assembly.

Easiest thing would be to unplug the fuel pump, turn the key on and see if the light goes out. :silly: If it does not, you're down to finding the short to ground.

Tyler,
Thanks so much. I much appreciate your explanation. I have the OEM service manual and just couldn't locate that diagram. I did read where it said that if the light has power and ground, it turns on. I went out and turned on the ignition and the light came on. I unplugged the pump at the harness connector under the bed of the truck and turned on the ignition and it was NOT on. Sooo... confirming that in my case you would suspect a bad replacement pump? Just doesn't seem like it should happen, but it's happening. Bummer.

Depending on what Napa says, I may just unplug the light. The gauge works and I use the tripmeter as backup. I wanted to be able to picture the circuit, and you answered that question for me. I would love to know how the internals of the pump ground out when the level is low, but there are only so many hours in a day. Thanks again for the explanation. Helps my brain.

Jim

JimT wrote:

Tyler,
Thanks so much. I much appreciate your explanation. I have the OEM service manual and just couldn't locate that diagram. I did read where it said that if the light has power and ground, it turns on. I went out and turned on the ignition and the light came on. I unplugged the pump at the harness connector under the bed of the truck and turned on the ignition and it was NOT on. Sooo... confirming that in my case you would suspect a bad replacement pump? Just doesn't seem like it should happen, but it's happening. Bummer.

Sure looking like a problem with the replacement pump, yeah. If you really want that 110% confidence, you could use a jumper wire connected to ground on the light green wire at the fuel pump connector. If you can make the low fuel light come and go with the jumper, then you're on the right track.

Depending on what Napa says, I may just unplug the light. The gauge works and I use the tripmeter as backup.

I was actually gonna suggest that. :lol: You could just clip the light green wire in an easily repairable location and put some heat shrink tubing over both ends. If/when someone decides to fix it, the wires are still there.

I would love to know how the internals of the pump ground out when the level is low, but there are only so many hours in a day.

I tried to figure out the exact mechanism based on Google pictures? It's been awhile since I changed one of these fuel pumps. :blush:

Something I did notice while on RockAuto is that the low fuel light is apparently an option?



What's interesting is that on both the with and without light options, all six pins in the connector are occupied. :huh: According to the pictures, anyway.

Do you know what the part number was for the Napa pump you got?

RE: part number...
yes... their part number is C0548M.

JimT wrote:

RE: part number...
yes... their part number is C0548M.

From NAPA:



If I had to take a WAG, I'd say that on the w/o light versions have the light green circuit grounded to the pump frame, or tied into the level sender ground. The w/ light versions route the light green circuit across whatever it uses to switch for low fuel level. I don't actually know, I'm just speculating.

Either way, I think you've got the wrong pump. FWIW, NAPA does offer a w/light option from Dephi:

Eureka! I think your WAG is 100% correct. I think I'm going to look into pulling the light green wire out of the connector and heat shrinking it. You don't know how satisfying it is that this makes sense inside my head. I care more about the concept than the actual light. Thanks so much for your time.

I'm calling this fixed. I depinned the wire from the connector and shrink wrapped it. Light out (and non-functional), but that's okay with me. Thanks for all of the help.