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Long story short, we are running one main tank, and that was replaced as brand new a few years ago, presumably with the fuel pump. Fuel pump doesn't seem great so we're going to replace that. The other issue is the fuel gauge reads empty. When I turn on the key, the gauge goes from far left pegged to just under E. The main tank has at least 10 gallons in it. I see on the sending unit plug there are two posts and teh plug has two posts plug a third wire that goes to the frame. I read you can 'ground' something on these to verify that what's working and what is not, but I after a few posts of research, I don't understand what to ground.
You want to remove the wire from the sender that runs along the frame rail to the front of the truck. Sorry, I can't recall the color, maybe green? Ground this wire, turn the key on, gauge should go full scale pretty quickly.
OK. There seem to be two wires, one goes straight to the frame like 8" away, and the other must travel on to the dash. I see what I need to do. Thanks!
interested in your results. I may have the same issue.
One rear tank. When tank is full gauge is on F as should be. It goes down pretty fast to E. When refilling, I get only about 9 -10 gallons in there, though. So at E there are aboutv 10 gallons left, assuming I have the 19 gallons tank.
Till now I always filled up when it was on E and hadn't kept driving to see what happens going below E.
Patrick it sounds like you sending unit is about to fail. Could be your float might have a pin hole in it and it has some gas in it and making for a incorrect reading. You can buy just the float, or just get a new sending unit. "Disconnect the wire that goes to the fuel sending unit at the tank. Ground that wire to the frame, making sure the frame is clean. Turn the ignition to the "run" position and watch the fuel gauge. If the gauge swings all the way to full, either the sending unit itself is bad or it has a bad ground. If the gauge stays on empty then either the gauge is bad or there is a wiring problem.
To pop the plug off for the test, you can barely get up in there with your head and see the plug. While looking at it, VERY CAREFULLY use a screwdriver to pry it off. AND I MEAN VERY CAREFULLY. It is only pushed on like a 1/8". This is only if you can’t get it with your hands."
Patrick it sounds like you sending unit is about to fail. Could be your float ..."
Hi Rich, thanks for the hint and explanation. I will take a look at there and see whether I can see or reach it. My bed floor is bend down where the tank sits so I am not sure how much space there is or if it already touches the sending unit maybe..?
I grounded the green wire to the frame. Had not to pull it of the sender unit as I have a plug in that green wire where the wire meets the frame rail.
Turned the key on "run" position and the needle went fast to the very right side of the gauge.
Thought maybe that plug could be the issue too, sanded it and sprayed some contact spray in there, Got it even futher in replugged. Turned the key again, but needle just went to E again..
That double brown wire is an wiring I had to do to get orange rear turn signals, so not factory.
The single dark brown wire comes from the sending unit and is screwed to the frame/that crossmember you see there. On that same grounding spot I connected my double brown wire and with that the rear lights work. So, the ground for the sending unit should be good in my case and the sending unit itself is the problem ?
OK. I tried to test this today. Not really much luck. What I know:
When the tank selector is on MAIN, there Voltage at plug on selector valve is 0V. When I flip the switch to AUX, the voltage is 12V at that plug.
So, with that plug UNPLUGGED:
I started testing my main (midship) tank wiring.
OHMS between posts on fuel sending unit = 78 Ohms. There is about 10 gallons of fuel in this tank, maybe 13, not sure, but at least 10.
For the WIRE that goes to the tank, I have continuity between one terminal and the ground, or the fuel tank. Ground is good.
When I short the other terminal to ground, I get no movement in the gauge. The gauge remains just under E, but not pegged dead.
HOWEVER....
While still testing the midship tank, with that wire to the tank valve unplugged, if I flip the switch to AUX, then the midship terminal shorted to ground will cause my gauge to move to Full. If I plug it into the sender, it drops to E.
So, I need to know what the tank selector switch DOES other than apply 12V to the valve. I feel like this switch might be the issue. Obviously the guage works. And obviously the wire is good from midship to the guage. And obviously the sender is good. But I can't figure out why when it's on AUX, the midship wire provides signal to the gauge.
Sounds like you’re getting good readings, but the sender is dead.
Although it’s hard to follow all the back-and-forth, but I think that’s Ford‘s fault. Over the years, if I remember correctly, they flip-flopped between calling the rear tank the main tank, and the midship tank the main tank. probably to do with changing from in-cab tanks, to external tanks only.
So there may be a naming inconsistency here. But there could’ve also been changes made by by anyone doing wiring, as to which one is labeled which.
So I wouldn’t worry about labels at this point, and just go by your readings.
As far as the ohm readings go, 73 is empty and 10 is full. So your gauge is reading what the sending unit is telling it to.
At least on one of them…
On the switch other than supplying the power the the valve, I believe it also tells what sending unit 9switch selected) to send the fuel level to the gauge. 2nd paragraph below.
I’m just frustrated. Everything in the diagrams above checks out. Even my 40 year old tank valve “clicks” when it’s plugged in and the tank switch is changed.
I just can’t figure why the midship tank shows empty when plugged in and grounded and that same wire shows full when it’s grounded and the tank switch is moved to aux. but even then, when the wire is plugged into a sender that is showing 78 ohms, the gauge goes to zero. None of this makes sense.
So far, everything on the truck has simply needed minor stuff. Even the dash lights all worked when I took them out and reseated them. The switch in the dash is clearly working. I don’t know what else to troubleshoot.
the wiring is good. The dash gauge is good. The switch is good. The valve is good. The sender is good. But the gauge doesn’t work when plugged in??
Knowing if 78 ohms on the sending unit relates to FULL or EMPTY would be useful.
ALL fuses in the block test good with a multimeter and continuity. So that’s not a problem.
So think about all the parts in the complete system. I tried to reread thru it all and can not see what you missed. Switch in dash, each tank sending unit. Both sending unit grounds, all associated elec wiring, fuse/fuses. Elec connectors at sending unit. Tank valve.
I know on my 78 F250 with dual tanks, to get the fuel gauge to switch over. (everything else switched over properly) I had to sometimes give the dash switch a few wiggles and giggles. Then it would work right and indicate the corresponding tank.
Based on your graphic, when resistance is HIGH = EMPTY, since I have read 78 Ohms at the sender, and the tank definitely has fuel, that explains why the sender and gauge indicate empty. So, the float is sunken or there is some other issue with the sender. However, it does not explain why the MIDSHIP or MAIN tank gauge only works when the switch is set to AUX. The only way that is possible is if MAIN on this truck is the *** end tank, and Aux is the big tank on the left side in the middle of the truck. If this is the case, then all is well and the sending unit is the issue.
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