Fuel Gauge
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New User
Joined: Aug 2009
Posts: 15
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From: Arlington
Fuel Gauge
Im sure this problem has been covered but i couldnt find anything that covered my question. I just replace sending unit on my truck (75 F-100 19 gallon rear tank) gauge is working now but I put between 10 and 12 gallons in it and its reading between a 1/4 and 1/2 a tank. My question is do these tanks have a reserve when the gauge says empty like newer cars?
Senior User
Joined: Feb 2010
Posts: 197
Likes: 0
From: Salem,Oregon
my gas guage is so off it showed i had half a tank and i ended up running out of gas goin down the road..
i dont even look at it anymore.. lol its so far off at all times that i just take a flashlight and look into my tank..lmao and its not showing the same readings either..it goes up and down and all sorts of things.. idk
i dont even look at it anymore.. lol its so far off at all times that i just take a flashlight and look into my tank..lmao and its not showing the same readings either..it goes up and down and all sorts of things.. idk
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
That sounds like a ground problem downstream of the sending unit; or a bad sending unit. You can check the sending unit rheostat with a multimeter - it should read about 10 ohms full and 70 ohms empty.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
The sending unit grounds to the crossmember underneath the bed. The frame itself grounds to the cab, which in turns grounds to the block. That means the return path for the sending unit is crossmember -> frame -> cab -> firewall -> engine block. The engine itself, of course, is connected to the battery via the negative battery cable. There is a lot of room for resistance through all of these junctions (which floats the sending unit from true ground, resulting in a false-low reading).
Senior User
Joined: Feb 2010
Posts: 197
Likes: 0
From: Salem,Oregon
i see a wire that goes from my sending unit to the side of my cab.
the other wire goes to the top of sending unit and goes to the dash...
the other wire goes to the top of sending unit and goes to the dash...
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
My post was referring to the under-bed tank that the OP has. You must have the tank behind the seat, in which case the sending unit grounds to the cab itself (which grounds to the block at the firewall).
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