I just added a 78 F150 to the collection and it is a really nice truck overall it just has a couple small things (I hope). Its a short WB and bed and its got a 351m with a c6 4x4 ford 9" rear. The fuel gauge reads empty all the time. Im going to replace the tank so i figured id replace the sending unit as well. Rock auto has a good price for both. How do i determine if its the sending unit and not the gauge panel or wiring?

 

This is a really common problem, of which there are two really common causes:

1) Float is punctured, and has filled with gas and sunk to the bottom.

2) Ground out at the frame rail is gone.

While it's highly unlikely that anything with the gauge is bad, you can test it by grounding out the sending unit lead. If the gauge pegs to FULL with the key on, then everything upstream of the tank is good. Chances are it won't happen because it's next to impossible to seek a good ground on a grimy, rusty frame. Try against the bottom of the bed sheetmetal instead.

You can bench test the sending unit by checking the resistance with the float arm at the FULL and EMPTY positions. The resistance should be 10 and 70 ohms, respectively.

Be wary of sending units from Rock Auto, as the Dorman and Spectra brands have the wrong size outlet (5/16" instead of 3/8"). JBG and DC have correct sizes. There is a same thread on this exact same topic a few posts down from yours. This question comes up several times a week. Link to that thread: https://www.ford-trucks.com/forums/1...el-sender.html

 

Thank you for the advice. What is DC? JBG has it for like $55 in their catalog i have. Is it worth it to spend the extra money? I have the rear tank. I have to drop the tank anyway and replace the vent hose because it is cracked and leaks when the tank is completely full. Are there any discrepencies with the tanks from rockauto?

 

And also do the sending units from JBG come with the seal and lock ring amd float etc?

 

DC stands for Dennis Carpenter; they are a high-end restoration parts vendor that uses a lot of original Ford tooling. Having bought the same sending unit from both JBG and DC, however, I suspect they sell the same (Taiwanese) sending unit. However, DC's is more expensive.

When you say "extra money," are you referring to the fact that JBG's is $55, while the Rock Auto units are on the order of $40-something? There's no choice, the Rock Auto parts are wrong. As I said in my previous post, the Dorman and Spectra parts have the wrong size fuel outlet. Sending units from JBG (and DC) will have the seal, lock ring and float. However, all floats on modern sending units are poor quality, so check them carefully. Read through the link I posted previously for details on my experience with them.

I have bought both Dorman and Spectra tanks from Rock Auto (my '79 has the exact same tank as yours). Both manufacturers stick the bare tank in a large corrugated cardboard box with little protection. The Dorman tank showed up sticking out of the box, with the seems folded over and heavily damaged, with several large dents. I sent it back and bought a Spectra tank from Napa (even though it's cheaper from Rock Auto) thinking the shipping would be better, but the Spectra tank from Napa was just as beat up (although it was a higher-quality tank). I sent that one back, and this time ordered another Spectra from Rock Auto, taking the risk. This tank showed up in fair enough shape, although it did have one small dent that I opted to smooth over with some JB Weld to be safe.

I don't have the Spectra tank in yet so I can't speak to its fit, but it looks like it should work just fine. Spectra is a pretty popular brand and the tank seems to be pretty solid (shipping problems aside).

 

Thank you and i was referring to the tank itself. It looks like the same tank from several vendors. And i believe im gonna order the spectre tank from rockauto and hope its not too roughed up. Whats your advice for the float? If mine has not sunken to the bottom should i reuse that? Or are they bad too? I am going to drop the tank tomorrow and try to see whats going on. So when i ground the lead wire to the truck the gauge should shoot to full?

 

While the tanks appear to be the same, there are subtle differences. For example, the Dorman tank is coated in some sort of material (which was flaked off in some areas of the tank I received), whereas the Spectra tank appears to be galvanized steel. That's another reason I would vote for the Spectra tank.

I don't recommend re-using a 30+ year old float, as the solder deteriorates with age. Be weary of replacement floats however, as they do not use as much solder as the factory originals. Check the replacement over thoroughly. I replaced my sending unit with a JBG unit 4 years ago, only for the float to puncture a week or two later (the actual sending unit was fine). All modern aftermarket sending units use the same poor-quality float. Now that I'm finally fixing it, I'm going with one of the "LifeCell" floats that DC sells. Instead of two soldered brass halves, it's one piece of brass with a composite material press-fit into the brass half.

That's what's supposed to happen, but if it doesn't happen, that doesn't necessarily mean the gauge is bad at first. As I said before, it's tough to get a solid ground out at the frame rail.

 

Ok thank you for all your help. How much trouble is it to replace the pigtail for the sender

 

I have not dropped the tank yet and like i said i just bought this truck so im not sure what is going on yet. Will find out more tomorrow

 

It's fairly simple. The pigtail is available from LMC, except the signal lead is unterminated, and not long enough to reach the female bullet connector from the truck. You can extend it and install a bullet connector, or cut the matching piece you need from the original pigtail and splice it to the replacement pigtail.

 

The reason i asked about the gauge was because my oil pressure and temp gauge dont work either. I dont know if its just a coincidence or if theres something wrong with the gauge cluster. The temp sender is there and hooked up but its only like 5 dollars so i am gonna give it a shot after the tank. Im getting ready to drop the tank and try to ground the sending unit and see if the fuel gauge shoots up because i bought aftermarket temp and oil psi gauges so the fuel is the only real concern at the moment