Fuel Gauge reads below empty
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Freshman User
Joined: Feb 2018
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From: Petaluma
Fuel Gauge reads below empty
Hello, I have a 1985 F-150 with a 4.9L inline 6. The fuel gauge on my truck always reads below empty when I have plenty of fuel. The first and only time I filled it up, it read full. I only put in 5-6 gallons. When switching between tanks, sometimes it will change, but currently it says they are both below E. When driving it for about 10 minutes a couple days ago, I have had it show 3/4 full, then after 6-8 hours I drove it back home and it showed 1/4 full. The bottom of the truck is pretty rusty, but other than that it is pretty nice. My dad believes it is the wire that is not working due to rust or the switch that changes it between tanks is not working. What do you guys suspect is the problem, the gauge, the switch, the wire from the gauge to the sending unit, or the sending unit itself based on the description?
Lead Driver
Joined: Jul 2009
Posts: 6,477
Likes: 19
From: Sheffield, AL
The sending units may have gone bad [33 years old]. With someone in the truck and the key on [no need for the engine to be running], pull the single wire from the sender terminal [top of gas tank] off and using a "jump wire", briefly touch the frame. The gas gauge should read FULL. Repeat for the 2nd tank.
The floats sometimes crack of develop a whole and get filled with gasoline and stay on the bottom rather than float on top of the gasoline.
The floats sometimes crack of develop a whole and get filled with gasoline and stay on the bottom rather than float on top of the gasoline.
Moderator
Joined: Jan 2001
Posts: 56,984
Likes: 2,738
From: Virginia
Sending units go bad, and I have had the dash switch get flakey. Take the dash switch and aggressively flip it from front to rear tanks about 10 times in quick succession. See if that makes them read ok for awhile. It fixed mine for about a month and then it did it again. After a couple more times of this happening, I just abandoned the rear tank. I never needed it anyway.
Logistics Pro
Joined: Jun 2017
Posts: 3,963
Likes: 512
From: Edmond, OK
I have this problem also.
Thanks for the tip Dave. I'll have to try that on my Bullnose.
My tanks are also a PITA to fill since the vents are broken. I think the best bet is to drop the tank and replace the fuel sending unit making sure all of the connections and ground are secure. It's on my MAW to do list...
Take the dash switch and aggressively flip it from front to rear tanks about 10 times in quick succession. See if that makes them read ok for awhile.
My tanks are also a PITA to fill since the vents are broken. I think the best bet is to drop the tank and replace the fuel sending unit making sure all of the connections and ground are secure. It's on my MAW to do list...
Moderator
Joined: Jan 2001
Posts: 56,984
Likes: 2,738
From: Virginia
I have this problem also.
Thanks for the tip Dave. I'll have to try that on my Bullnose.
My tanks are also a PITA to fill since the vents are broken. I think the best bet is to drop the tank and replace the fuel sending unit making sure all of the connections and ground are secure. It's on my MAW to do list...
Thanks for the tip Dave. I'll have to try that on my Bullnose.
My tanks are also a PITA to fill since the vents are broken. I think the best bet is to drop the tank and replace the fuel sending unit making sure all of the connections and ground are secure. It's on my MAW to do list...
There is a lot of air coming out of the tank when you putting a lot of fuel in it at the station. All that air coming up the fill pipe needs to be kept away from the fuel going in, or you get splashing and gulping. I think Ford knew they had a little problem, because on the later trucks they switch hose duties, and instead of the large pipe feeding fuel and the small pipe exhausting air, they switch it around to where the small pipe fed the fuel and the large pipe exhausted the air. I think they work better and could possibly be retro fitted to a early truck.
FTE Legend
Joined: Nov 2015
Posts: 30,912
Likes: 4,123
From: Angier, NC
So you know what that plastic tube looks like inside the hose assy.
Because I am doing a custom metal floor on my flare side I have to try and use different filler necks, tubes and hoses to get it all to work.
Dave ----
Because I am doing a custom metal floor on my flare side I have to try and use different filler necks, tubes and hoses to get it all to work.
Dave ----
Tuned
Joined: May 2013
Posts: 396
Likes: 5
From: Ottawa, Canada
Checking the sending unit is easy. Get a multimeter and set it to ohmeter. Disconnect the tank wires and then check the 2 prongs on the tank, 75ish ohms is empty and 10 is full. If you get some unexpected number that's your issue.
The guage cluster sends a speratic voltage to the sending unit averaging 5v. With the key on you can check the two wires for DC voltage, but the voltmeter will scatter between 0 and maybe 10v meaning its working.
The guage cluster sends a speratic voltage to the sending unit averaging 5v. With the key on you can check the two wires for DC voltage, but the voltmeter will scatter between 0 and maybe 10v meaning its working.
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Moderator
Joined: Jan 2001
Posts: 56,984
Likes: 2,738
From: Virginia
Checking the sending unit is easy. Get a multimeter and set it to ohmeter. Disconnect the tank wires and then check the 2 prongs on the tank, 75ish ohms is empty and 10 is full. If you get some unexpected number that's your issue.
The guage cluster sends a speratic voltage to the sending unit averaging 5v. With the key on you can check the two wires for DC voltage, but the voltmeter will scatter between 0 and maybe 10v meaning its working.
The guage cluster sends a speratic voltage to the sending unit averaging 5v. With the key on you can check the two wires for DC voltage, but the voltmeter will scatter between 0 and maybe 10v meaning its working.
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Elder User
Joined: Apr 2011
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From: South Carolina
On these older trucks, there is a small plastic vent pipe that sits in a clip inside the large fill pipe where you put the station nozzle. This small vent pipe likes to fall out of that clip, and then it gets in the way of the fill station nozzle and keeps cutting it off.
There is a lot of air coming out of the tank when you putting a lot of fuel in it at the station. All that air coming up the fill pipe needs to be kept away from the fuel going in, or you get splashing and gulping. I think Ford knew they had a little problem, because on the later trucks they switch hose duties, and instead of the large pipe feeding fuel and the small pipe exhausting air, they switch it around to where the small pipe fed the fuel and the large pipe exhausted the air. I think they work better and could possibly be retro fitted to a early truck.
There is a lot of air coming out of the tank when you putting a lot of fuel in it at the station. All that air coming up the fill pipe needs to be kept away from the fuel going in, or you get splashing and gulping. I think Ford knew they had a little problem, because on the later trucks they switch hose duties, and instead of the large pipe feeding fuel and the small pipe exhausting air, they switch it around to where the small pipe fed the fuel and the large pipe exhausted the air. I think they work better and could possibly be retro fitted to a early truck.
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