fuel gauge not working
 

i just got this truck and the fuel gauge not working. reading empty. and i know i just filled 1st tank up. it has 2 tank. i put 5 gal in other tank both show empty. someone had put a new gauge down low, that not working either. i do a video in morning so you see what iam saying. . i just had the plugs changed wires belts cap rotter points conds. was going to store it acted like it was bot getting gas. i did not smell any gas, its a 4 speed 360 1974 2 wheel drive.

Sounds like your fuel sending unit or the float inside the tank is stuck or maybe corroded...especially if the truck has been stored or not run in a while. You might need to drop the tank(s) and check but I would do just one at the start and see what you find first.

Probably a bad sending unit, sunk float, or the crossmember has lost ground.

You can ground the sending wire and gauge should peg. If it does, bad sender. If not, either gauge or wire between them is bad. With two tanks, I would suspect the switch. Grounding the sender wire also works testing oil pressure and temp gauge.

Since you have a dual tank set up, I would be surprised if both floats in both tanks were stuck (still possible). Could be a sending unit, but then again you would have one in both tanks and for both to be bad (also highly unlikely. Deffinetly electrical problem, but before you start dropping tanks and replacing guages, I would do what gfw1985 suggested with grounding the guage. If it pegs full then not the guage or a fuse. Tace the wire from the guage to each sending unit. I am guessing you are grounding out since neither sending unit in either tank is working. (Check where the new guage was installed.

i added a video on youtube, YouTube - ‪1974 Ford F-100 Fuel Gauge Not working‬‏

The black wire seems to be the ground and the red lights I believe are indicator lights to let you know which tank is in use. It lookes as if the guy has wired the guages using the wires from the original guage and added the lights, not sure why, because if set up correctly the one gauge should read empty when the other tank is being used. Lookes like the wiring is the issue and I dont think your guases are ground properly or wired correctly. Disconnect the lights you won't need them and wire the gauges directly to the switch. Whenever you see the gauges dip the way the one on the left did when you turn the key it is because you have a grounding issue.

Another option is to connect one guage at a time. I would start with the one on the left and reverse the wires so the guage moves to the right. I am almost positive that the guy crossed the ground with the common and that is why your guage dips.

will try that thank you.

Both my floats had rotted away in my dual tank setup. Popped them in, gauges work great now.

i may have fixed it. 2nd youtube video YouTube - ‪1974 Ford F-100 Fuel Gauge Part 2‬‏ when i turn knob on floor change tank nothen happens. but if first tank reading right i can deal with that for now, thanks for all the help out there,

Going in the right direction. Sounds like you have manual valve in floor, will only change tanks, not gauge. Should be a switch that changes gauge to read other tank.

Looks like the wires were crossed and the ground worked, glad to see it was something simple. As for the guages not working when you switch between tanks, that is because the way it is wired also. The originat switch in the truck is designed for one guage to give a reading for whichever tank you are using. My guess it the install of the second gauge is not working properly because the gauge is not directly wired to the second tank. I would bet that if you switch between tanks the gauge on the left should reflect the gas you have in the other tank. When you switch between tanks, does the guage on the left go down?

The fuel sending unit float is made from two pieces of copper soldered together.

Solder breaks down, gas seeps in, float settles to the bottom of the tank.

COAZ-9202-B .. Fuel Sending Unit Float / Available from Ford.

Fits ALL: 1957/79 Passenger Cars/Trucks, 1961/79 Econolines & 1966/79 Bronco's.
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1973/79 F100/350 with dual fuel tanks: There are two different fuel tank selector valves.

1973/74: The valve is manually operated, mounts on the floor adjacent to the drivers side of the seat.

A switch is mounted within the heater control panel. This is only used to switch the dash fuel gauge from one tank to the other.
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1975/79's use an electric fuel tank selector valve and solenoid assy. This valve is mounted on the left (drivers) side frame rail.

The switch is located within the heater control panel, changes the fuel tanks and the gauges.

Damaged fuel gauge
 

I purchased my 72 ford f100 about a year ago and I hadn't realized that we had basically purchased a truck with engine issues. We took it to the mechanic and $2200 later it runs good. When I took it to my mechanic the fuel gauge was working and I was able to fill up the tank. Well now, it always shows that it has half a tank and I've been stranded because it was actually empty. Another issue is, that I use to be able put in $40 worth of gas and now it only lets me put in $13 at a time. So frustrating! I took it back to the mechanic who says the problem is the tank is small. I know for a fact that that is not true. I was able to add a full tank with no problems. He then shut off my second tank permanently so I am not able to use the second tank. He says it was difficult to find a fuel gauge for my truck so he had to fix the old one. The problem is, it still doesn't work. I'm about to take it back to the mechanic because I hate that I have to gas up several times due to the gauge not working. What can be the problem? I mean, if my mechanic can't fix it, what advice do you have for me? It is just so frustrating! Thanks!